// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.

using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Linq;
using System.Threading;
using Microsoft.CodeAnalysis.Semantics;
using Microsoft.CodeAnalysis.Text;

namespace Microsoft.CodeAnalysis
{
    /// <summary>
    /// Allows asking semantic questions about a tree of syntax nodes in a Compilation. Typically,
    /// an instance is obtained by a call to GetBinding on a Compilation or Compilation.
    /// </summary>
    /// <remarks>
    /// <para>An instance of SemanticModel caches local symbols and semantic information. Thus, it
    /// is much more efficient to use a single instance of SemanticModel when asking multiple
    /// questions about a syntax tree, because information from the first question may be reused.
    /// This also means that holding onto an instance of SemanticModel for a long time may keep a
    /// significant amount of memory from being garbage collected.
    /// </para>
    /// <para>
    /// When an answer is a named symbol that is reachable by traversing from the root of the symbol
    /// table, (that is, from an AssemblySymbol of the Compilation), that symbol will be returned
    /// (i.e. the returned value will be reference-equal to one reachable from the root of the
    /// symbol table). Symbols representing entities without names (e.g. array-of-int) may or may
    /// not exhibit reference equality. However, some named symbols (such as local variables) are
    /// not reachable from the root. These symbols are visible as answers to semantic questions.
    /// When the same SemanticModel object is used, the answers exhibit reference-equality.
    /// </para>
    /// </remarks>
    public abstract class SemanticModel
    {
        /// <summary>
        /// Gets the source language ("C#" or "Visual Basic").
        /// </summary>
        public abstract string Language { get; }

        /// <summary>
        /// The compilation this model was obtained from.
        /// </summary>
        public Compilation Compilation
        {
            get { return CompilationCore; }
        }

        /// <summary>
        /// The compilation this model was obtained from.
        /// </summary>
        protected abstract Compilation CompilationCore { get; }

        /// <summary>
        /// The syntax tree this model was obtained from.
        /// </summary>
        public SyntaxTree SyntaxTree
        {
            get { return SyntaxTreeCore; }
        }

        /// <summary>
        /// The syntax tree this model was obtained from.
        /// </summary>
        protected abstract SyntaxTree SyntaxTreeCore { get; }

        /// <summary>
        /// Gets the operation corresponding to the expression or statement syntax node.
        /// </summary>
        /// <param name="node">The expression or statement syntax node.</param>
        /// <param name="cancellationToken">An optional cancellation token.</param>
        /// <returns></returns>
        public IOperation GetOperation(SyntaxNode node, CancellationToken cancellationToken = default(CancellationToken))
        {
            return GetOperationCore(node, cancellationToken);
        }

        protected abstract IOperation GetOperationCore(SyntaxNode node, CancellationToken cancellationToken);

        /// <summary>
        /// Returns true if this is a SemanticModel that ignores accessibility rules when answering semantic questions.
        /// </summary>
        public virtual bool IgnoresAccessibility
        {
            get { return false; }
        }

        /// <summary>
        /// Gets symbol information about a syntax node.
        /// </summary>
        /// <param name="node">The syntax node to get semantic information for.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the semantic info.</param>
        internal SymbolInfo GetSymbolInfo(SyntaxNode node, CancellationToken cancellationToken = default(CancellationToken))
        {
            return GetSymbolInfoCore(node, cancellationToken);
        }

        /// <summary>
        /// Gets symbol information about a syntax node.
        /// </summary>
        /// <param name="node">The syntax node to get semantic information for.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the semantic info.</param>
        protected abstract SymbolInfo GetSymbolInfoCore(SyntaxNode node, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Binds the node in the context of the specified location and get semantic information
        /// such as type, symbols and diagnostics. This method is used to get semantic information
        /// about an expression that did not actually appear in the source code.
        /// </summary>
        /// <param name="position">A character position used to identify a declaration scope and
        /// accessibility. This character position must be within the FullSpan of the Root syntax
        /// node in this SemanticModel.
        /// </param>
        /// <param name="expression">A syntax node that represents a parsed expression. This syntax
        /// node need not and typically does not appear in the source code referred to  SemanticModel
        /// instance.</param>
        /// <param name="bindingOption">Indicates whether to binding the expression as a full expressions,
        /// or as a type or namespace. If SpeculativeBindingOption.BindAsTypeOrNamespace is supplied, then
        /// expression should derive from TypeSyntax.</param>
        /// <returns>The semantic information for the topmost node of the expression.</returns>
        /// <remarks>The passed in expression is interpreted as a stand-alone expression, as if it
        /// appeared by itself somewhere within the scope that encloses "position".</remarks>
        internal SymbolInfo GetSpeculativeSymbolInfo(int position, SyntaxNode expression, SpeculativeBindingOption bindingOption)
        {
            return GetSpeculativeSymbolInfoCore(position, expression, bindingOption);
        }

        /// <summary>
        /// Binds the node in the context of the specified location and get semantic information
        /// such as type, symbols and diagnostics. This method is used to get semantic information
        /// about an expression that did not actually appear in the source code.
        /// </summary>
        /// <param name="position">A character position used to identify a declaration scope and
        /// accessibility. This character position must be within the FullSpan of the Root syntax
        /// node in this SemanticModel.
        /// </param>
        /// <param name="expression">A syntax node that represents a parsed expression. This syntax
        /// node need not and typically does not appear in the source code referred to  SemanticModel
        /// instance.</param>
        /// <param name="bindingOption">Indicates whether to binding the expression as a full expressions,
        /// or as a type or namespace. If SpeculativeBindingOption.BindAsTypeOrNamespace is supplied, then
        /// expression should derive from TypeSyntax.</param>
        /// <returns>The semantic information for the topmost node of the expression.</returns>
        /// <remarks>The passed in expression is interpreted as a stand-alone expression, as if it
        /// appeared by itself somewhere within the scope that encloses "position".</remarks>
        protected abstract SymbolInfo GetSpeculativeSymbolInfoCore(int position, SyntaxNode expression, SpeculativeBindingOption bindingOption);

        /// <summary>
        /// Binds the node in the context of the specified location and get semantic information
        /// such as type, symbols and diagnostics. This method is used to get semantic information
        /// about an expression that did not actually appear in the source code.
        /// </summary>
        /// <param name="position">A character position used to identify a declaration scope and
        /// accessibility. This character position must be within the FullSpan of the Root syntax
        /// node in this SemanticModel.
        /// </param>
        /// <param name="expression">A syntax node that represents a parsed expression. This syntax
        /// node need not and typically does not appear in the source code referred to  SemanticModel
        /// instance.</param>
        /// <param name="bindingOption">Indicates whether to binding the expression as a full expressions,
        /// or as a type or namespace. If SpeculativeBindingOption.BindAsTypeOrNamespace is supplied, then
        /// expression should derive from TypeSyntax.</param>
        /// <returns>The semantic information for the topmost node of the expression.</returns>
        /// <remarks>The passed in expression is interpreted as a stand-alone expression, as if it
        /// appeared by itself somewhere within the scope that encloses "position".</remarks>
        internal TypeInfo GetSpeculativeTypeInfo(int position, SyntaxNode expression, SpeculativeBindingOption bindingOption)
        {
            return GetSpeculativeTypeInfoCore(position, expression, bindingOption);
        }

        /// <summary>
        /// Binds the node in the context of the specified location and get semantic information
        /// such as type, symbols and diagnostics. This method is used to get semantic information
        /// about an expression that did not actually appear in the source code.
        /// </summary>
        /// <param name="position">A character position used to identify a declaration scope and
        /// accessibility. This character position must be within the FullSpan of the Root syntax
        /// node in this SemanticModel.
        /// </param>
        /// <param name="expression">A syntax node that represents a parsed expression. This syntax
        /// node need not and typically does not appear in the source code referred to  SemanticModel
        /// instance.</param>
        /// <param name="bindingOption">Indicates whether to binding the expression as a full expressions,
        /// or as a type or namespace. If SpeculativeBindingOption.BindAsTypeOrNamespace is supplied, then
        /// expression should derive from TypeSyntax.</param>
        /// <returns>The semantic information for the topmost node of the expression.</returns>
        /// <remarks>The passed in expression is interpreted as a stand-alone expression, as if it
        /// appeared by itself somewhere within the scope that encloses "position".</remarks>
        protected abstract TypeInfo GetSpeculativeTypeInfoCore(int position, SyntaxNode expression, SpeculativeBindingOption bindingOption);

        /// <summary>
        /// Gets type information about a syntax node.
        /// </summary>
        /// <param name="node">The syntax node to get semantic information for.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the semantic info.</param>
        internal TypeInfo GetTypeInfo(SyntaxNode node, CancellationToken cancellationToken = default(CancellationToken))
        {
            return GetTypeInfoCore(node, cancellationToken);
        }

        /// <summary>
        /// Gets type information about a syntax node.
        /// </summary>
        /// <param name="node">The syntax node to get semantic information for.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the semantic info.</param>
        protected abstract TypeInfo GetTypeInfoCore(SyntaxNode node, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// If "nameSyntax" resolves to an alias name, return the IAliasSymbol corresponding
        /// to A. Otherwise return null.
        /// </summary>
        /// <param name="nameSyntax">Name to get alias info for.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the alias information.</param>
        internal IAliasSymbol GetAliasInfo(SyntaxNode nameSyntax, CancellationToken cancellationToken = default(CancellationToken))
        {
            return GetAliasInfoCore(nameSyntax, cancellationToken);
        }

        /// <summary>
        /// If "nameSyntax" resolves to an alias name, return the IAliasSymbol corresponding
        /// to A. Otherwise return null.
        /// </summary>
        /// <param name="nameSyntax">Name to get alias info for.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the alias information.</param>
        protected abstract IAliasSymbol GetAliasInfoCore(SyntaxNode nameSyntax, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Returns true if this is a speculative semantic model created with any of the TryGetSpeculativeSemanticModel methods.
        /// </summary>
        public abstract bool IsSpeculativeSemanticModel
        {
            get;
        }

        /// <summary>
        /// If this is a speculative semantic model, returns the original position at which the speculative model was created.
        /// Otherwise, returns 0.
        /// </summary>
        public abstract int OriginalPositionForSpeculation
        {
            get;
        }

        /// <summary>
        /// If this is a speculative semantic model, then returns its parent semantic model.
        /// Otherwise, returns null.
        /// </summary>
        public SemanticModel ParentModel
        {
            get { return this.ParentModelCore; }
        }

        /// <summary>
        /// If this is a speculative semantic model, then returns its parent semantic model.
        /// Otherwise, returns null.
        /// </summary>
        protected abstract SemanticModel ParentModelCore
        {
            get;
        }

        /// <summary>
        /// Binds the name in the context of the specified location and sees if it resolves to an
        /// alias name. If it does, return the AliasSymbol corresponding to it. Otherwise, return null.
        /// </summary>
        /// <param name="position">A character position used to identify a declaration scope and
        /// accessibility. This character position must be within the FullSpan of the Root syntax
        /// node in this SemanticModel.
        /// </param>
        /// <param name="nameSyntax">A syntax node that represents a name. This syntax
        /// node need not and typically does not appear in the source code referred to by the
        /// SemanticModel instance.</param>
        /// <param name="bindingOption">Indicates whether to binding the name as a full expression,
        /// or as a type or namespace. If SpeculativeBindingOption.BindAsTypeOrNamespace is supplied, then
        /// expression should derive from TypeSyntax.</param>
        /// <remarks>The passed in name is interpreted as a stand-alone name, as if it
        /// appeared by itself somewhere within the scope that encloses "position".</remarks>
        internal IAliasSymbol GetSpeculativeAliasInfo(int position, SyntaxNode nameSyntax, SpeculativeBindingOption bindingOption)
        {
            return GetSpeculativeAliasInfoCore(position, nameSyntax, bindingOption);
        }

        /// <summary>
        /// Binds the name in the context of the specified location and sees if it resolves to an
        /// alias name. If it does, return the AliasSymbol corresponding to it. Otherwise, return null.
        /// </summary>
        /// <param name="position">A character position used to identify a declaration scope and
        /// accessibility. This character position must be within the FullSpan of the Root syntax
        /// node in this SemanticModel.
        /// </param>
        /// <param name="nameSyntax">A syntax node that represents a name. This syntax
        /// node need not and typically does not appear in the source code referred to by the
        /// SemanticModel instance.</param>
        /// <param name="bindingOption">Indicates whether to binding the name as a full expression,
        /// or as a type or namespace. If SpeculativeBindingOption.BindAsTypeOrNamespace is supplied, then
        /// expression should derive from TypeSyntax.</param>
        /// <remarks>The passed in name is interpreted as a stand-alone name, as if it
        /// appeared by itself somewhere within the scope that encloses "position".</remarks>
        protected abstract IAliasSymbol GetSpeculativeAliasInfoCore(int position, SyntaxNode nameSyntax, SpeculativeBindingOption bindingOption);

        /// <summary>
        /// Get all of the syntax errors within the syntax tree associated with this
        /// object. Does not get errors involving declarations or compiling method bodies or initializers.
        /// </summary>
        /// <param name="span">Optional span within the syntax tree for which to get diagnostics.
        /// If no argument is specified, then diagnostics for the entire tree are returned.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the diagnostics.</param>
        public abstract ImmutableArray<Diagnostic> GetSyntaxDiagnostics(TextSpan? span = null, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Get all of the declaration errors within the syntax tree associated with this
        /// object. Does not get errors involving incorrect syntax, compiling method bodies or initializers.
        /// </summary>
        /// <param name="span">Optional span within the syntax tree for which to get diagnostics.
        /// If no argument is specified, then diagnostics for the entire tree are returned.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the diagnostics.</param>
        /// <remarks>The declaration errors for a syntax tree are cached. The first time this method
        /// is called, all declarations are analyzed for diagnostics. Calling this a second time
        /// will return the cached diagnostics.
        /// </remarks>
        public abstract ImmutableArray<Diagnostic> GetDeclarationDiagnostics(TextSpan? span = null, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Get all of the method body and initializer errors within the syntax tree associated with this
        /// object. Does not get errors involving incorrect syntax or declarations.
        /// </summary>
        /// <param name="span">Optional span within the syntax tree for which to get diagnostics.
        /// If no argument is specified, then diagnostics for the entire tree are returned.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the diagnostics.</param>
        /// <remarks>The method body errors for a syntax tree are not cached. The first time this method
        /// is called, all method bodies are analyzed for diagnostics. Calling this a second time
        /// will repeat this work.
        /// </remarks>
        public abstract ImmutableArray<Diagnostic> GetMethodBodyDiagnostics(TextSpan? span = null, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Get all the errors within the syntax tree associated with this object. Includes errors
        /// involving compiling method bodies or initializers, in addition to the errors returned by
        /// GetDeclarationDiagnostics.
        /// </summary>
        /// <param name="span">Optional span within the syntax tree for which to get diagnostics.
        /// If no argument is specified, then diagnostics for the entire tree are returned.</param>
        /// <param name="cancellationToken">A cancellation token that can be used to cancel the
        /// process of obtaining the diagnostics.</param>
        /// <remarks>
        /// Because this method must semantically bind all method bodies and initializers to check
        /// for diagnostics, it may take a significant amount of time. Unlike
        /// GetDeclarationDiagnostics, diagnostics for method bodies and initializers are not
        /// cached, any semantic information used to obtain the diagnostics is discarded.
        /// </remarks>
        public abstract ImmutableArray<Diagnostic> GetDiagnostics(TextSpan? span = null, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Gets the symbol associated with a declaration syntax node.
        /// </summary>
        /// <param name="declaration">A syntax node that is a declaration. This can be any type
        /// derived from MemberDeclarationSyntax, TypeDeclarationSyntax, EnumDeclarationSyntax,
        /// NamespaceDeclarationSyntax, ParameterSyntax, TypeParameterSyntax, or the alias part of a
        /// UsingDirectiveSyntax</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <returns>The symbol declared by the node or null if the node is not a declaration.</returns>
        internal ISymbol GetDeclaredSymbolForNode(SyntaxNode declaration, CancellationToken cancellationToken = default(CancellationToken))
        {
            return GetDeclaredSymbolCore(declaration, cancellationToken);
        }

        /// <summary>
        /// Gets the symbol associated with a declaration syntax node.
        /// </summary>
        /// <param name="declaration">A syntax node that is a declaration. This can be any type
        /// derived from MemberDeclarationSyntax, TypeDeclarationSyntax, EnumDeclarationSyntax,
        /// NamespaceDeclarationSyntax, ParameterSyntax, TypeParameterSyntax, or the alias part of a
        /// UsingDirectiveSyntax</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <returns>The symbol declared by the node or null if the node is not a declaration.</returns>
        protected abstract ISymbol GetDeclaredSymbolCore(SyntaxNode declaration, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Gets the symbol associated with a declaration syntax node. Unlike <see cref="GetDeclaredSymbolForNode(SyntaxNode, CancellationToken)"/>,
        /// this method returns all symbols declared by a given declaration syntax node. Specifically, in the case of field declaration syntax nodes,
        /// which can declare multiple symbols, this method returns all declared symbols.
        /// </summary>
        /// <param name="declaration">A syntax node that is a declaration. This can be any type
        /// derived from MemberDeclarationSyntax, TypeDeclarationSyntax, EnumDeclarationSyntax,
        /// NamespaceDeclarationSyntax, ParameterSyntax, TypeParameterSyntax, or the alias part of a
        /// UsingDirectiveSyntax</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <returns>The symbols declared by the node.</returns>
        internal ImmutableArray<ISymbol> GetDeclaredSymbolsForNode(SyntaxNode declaration, CancellationToken cancellationToken = default(CancellationToken))
        {
            return GetDeclaredSymbolsCore(declaration, cancellationToken);
        }

        /// <summary>
        /// Gets the symbol associated with a declaration syntax node. Unlike <see cref="GetDeclaredSymbolForNode(SyntaxNode, CancellationToken)"/>,
        /// this method returns all symbols declared by a given declaration syntax node. Specifically, in the case of field declaration syntax nodes,
        /// which can declare multiple symbols, this method returns all declared symbols.
        /// </summary>
        /// <param name="declaration">A syntax node that is a declaration. This can be any type
        /// derived from MemberDeclarationSyntax, TypeDeclarationSyntax, EnumDeclarationSyntax,
        /// NamespaceDeclarationSyntax, ParameterSyntax, TypeParameterSyntax, or the alias part of a
        /// UsingDirectiveSyntax</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <returns>The symbols declared by the node.</returns>
        protected abstract ImmutableArray<ISymbol> GetDeclaredSymbolsCore(SyntaxNode declaration, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Gets the available named symbols in the context of the specified location and optional container. Only
        /// symbols that are accessible and visible from the given location are returned.
        /// </summary>
        /// <param name="position">The character position for determining the enclosing declaration scope and
        /// accessibility.</param>
        /// <param name="container">The container to search for symbols within. If null then the enclosing declaration
        /// scope around position is used.</param>
        /// <param name="name">The name of the symbol to find. If null is specified then symbols
        /// with any names are returned.</param>
        /// <param name="includeReducedExtensionMethods">Consider (reduced) extension methods.</param>
        /// <returns>A list of symbols that were found. If no symbols were found, an empty list is returned.</returns>
        /// <remarks>
        /// The "position" is used to determine what variables are visible and accessible. Even if "container" is
        /// specified, the "position" location is significant for determining which members of "containing" are
        /// accessible. 
        /// 
        /// Labels are not considered (see <see cref="LookupLabels"/>).
        /// 
        /// Non-reduced extension methods are considered regardless of the value of <paramref name="includeReducedExtensionMethods"/>.
        /// </remarks>
        public ImmutableArray<ISymbol> LookupSymbols(
            int position,
            INamespaceOrTypeSymbol container = null,
            string name = null,
            bool includeReducedExtensionMethods = false)
        {
            return LookupSymbolsCore(position, container, name, includeReducedExtensionMethods);
        }

        /// <summary>
        /// Backing implementation of <see cref="LookupSymbols"/>.
        /// </summary>
        protected abstract ImmutableArray<ISymbol> LookupSymbolsCore(
            int position,
            INamespaceOrTypeSymbol container,
            string name,
            bool includeReducedExtensionMethods);

        /// <summary>
        /// Gets the available base type members in the context of the specified location.  Akin to
        /// calling <see cref="LookupSymbols"/> with the container set to the immediate base type of
        /// the type in which <paramref name="position"/> occurs.  However, the accessibility rules
        /// are different: protected members of the base type will be visible.
        /// 
        /// Consider the following example:
        /// 
        ///   public class Base
        ///   {
        ///       protected void M() { }
        ///   }
        ///   
        ///   public class Derived : Base
        ///   {
        ///       void Test(Base b)
        ///       {
        ///           b.M(); // Error - cannot access protected member.
        ///           base.M();
        ///       }
        ///   }
        /// 
        /// Protected members of an instance of another type are only accessible if the instance is known
        /// to be "this" instance (as indicated by the "base" keyword).
        /// </summary>
        /// <param name="position">The character position for determining the enclosing declaration scope and
        /// accessibility.</param>
        /// <param name="name">The name of the symbol to find. If null is specified then symbols
        /// with any names are returned.</param>
        /// <returns>A list of symbols that were found. If no symbols were found, an empty list is returned.</returns>
        /// <remarks>
        /// The "position" is used to determine what variables are visible and accessible.
        /// 
        /// Non-reduced extension methods are considered, but reduced extension methods are not.
        /// </remarks>
        public ImmutableArray<ISymbol> LookupBaseMembers(
            int position,
            string name = null)
        {
            return LookupBaseMembersCore(position, name);
        }

        /// <summary>
        /// Backing implementation of <see cref="LookupBaseMembers"/>.
        /// </summary>
        protected abstract ImmutableArray<ISymbol> LookupBaseMembersCore(
            int position,
            string name);

        /// <summary>
        /// Gets the available named static member symbols in the context of the specified location and optional container.
        /// Only members that are accessible and visible from the given location are returned.
        /// 
        /// Non-reduced extension methods are considered, since they are static methods.
        /// </summary>
        /// <param name="position">The character position for determining the enclosing declaration scope and
        /// accessibility.</param>
        /// <param name="container">The container to search for symbols within. If null then the enclosing declaration
        /// scope around position is used.</param>
        /// <param name="name">The name of the symbol to find. If null is specified then symbols
        /// with any names are returned.</param>
        /// <returns>A list of symbols that were found. If no symbols were found, an empty list is returned.</returns>
        /// <remarks>
        /// The "position" is used to determine what variables are visible and accessible. Even if "container" is
        /// specified, the "position" location is significant for determining which members of "containing" are
        /// accessible. 
        /// 
        /// Essentially the same as filtering instance members out of the results of an analogous <see cref="LookupSymbols"/> call.
        /// </remarks>
        public ImmutableArray<ISymbol> LookupStaticMembers(
            int position,
            INamespaceOrTypeSymbol container = null,
            string name = null)
        {
            return LookupStaticMembersCore(position, container, name);
        }

        /// <summary>
        /// Backing implementation of <see cref="LookupStaticMembers"/>.
        /// </summary>
        protected abstract ImmutableArray<ISymbol> LookupStaticMembersCore(
            int position,
            INamespaceOrTypeSymbol container,
            string name);

        /// <summary>
        /// Gets the available named namespace and type symbols in the context of the specified location and optional container.
        /// Only members that are accessible and visible from the given location are returned.
        /// </summary>
        /// <param name="position">The character position for determining the enclosing declaration scope and
        /// accessibility.</param>
        /// <param name="container">The container to search for symbols within. If null then the enclosing declaration
        /// scope around position is used.</param>
        /// <param name="name">The name of the symbol to find. If null is specified then symbols
        /// with any names are returned.</param>
        /// <returns>A list of symbols that were found. If no symbols were found, an empty list is returned.</returns>
        /// <remarks>
        /// The "position" is used to determine what variables are visible and accessible. Even if "container" is
        /// specified, the "position" location is significant for determining which members of "containing" are
        /// accessible. 
        /// 
        /// Does not return INamespaceOrTypeSymbol, because there could be aliases.
        /// </remarks>
        public ImmutableArray<ISymbol> LookupNamespacesAndTypes(
            int position,
            INamespaceOrTypeSymbol container = null,
            string name = null)
        {
            return LookupNamespacesAndTypesCore(position, container, name);
        }

        /// <summary>
        /// Backing implementation of <see cref="LookupNamespacesAndTypes"/>.
        /// </summary>
        protected abstract ImmutableArray<ISymbol> LookupNamespacesAndTypesCore(
            int position,
            INamespaceOrTypeSymbol container,
            string name);

        /// <summary>
        /// Gets the available named label symbols in the context of the specified location and optional container.
        /// Only members that are accessible and visible from the given location are returned.
        /// </summary>
        /// <param name="position">The character position for determining the enclosing declaration scope and
        /// accessibility.</param>
        /// <param name="name">The name of the symbol to find. If null is specified then symbols
        /// with any names are returned.</param>
        /// <returns>A list of symbols that were found. If no symbols were found, an empty list is returned.</returns>
        /// <remarks>
        /// The "position" is used to determine what variables are visible and accessible. Even if "container" is
        /// specified, the "position" location is significant for determining which members of "containing" are
        /// accessible. 
        /// </remarks>
        public ImmutableArray<ISymbol> LookupLabels(
            int position,
            string name = null)
        {
            return LookupLabelsCore(position, name);
        }

        /// <summary>
        /// Backing implementation of <see cref="LookupLabels"/>.
        /// </summary>
        protected abstract ImmutableArray<ISymbol> LookupLabelsCore(
            int position,
            string name);

        /// <summary>
        /// Analyze control-flow within a part of a method body. 
        /// </summary>
        /// <param name="firstStatement">The first node to be included within the analysis.</param>
        /// <param name="lastStatement">The last node to be included within the analysis.</param>
        /// <returns>An object that can be used to obtain the result of the control flow analysis.</returns>
        /// <exception cref="System.ArgumentException">The span is not with a method
        /// body.</exception>
        /// <remarks>
        /// The first and last nodes must be fully inside the same method body.
        /// </remarks>
        internal ControlFlowAnalysis AnalyzeControlFlow(SyntaxNode firstStatement, SyntaxNode lastStatement)
        {
            return AnalyzeControlFlowCore(firstStatement, lastStatement);
        }

        /// <summary>
        /// Analyze control-flow within a part of a method body. 
        /// </summary>
        /// <param name="firstStatement">The first node to be included within the analysis.</param>
        /// <param name="lastStatement">The last node to be included within the analysis.</param>
        /// <returns>An object that can be used to obtain the result of the control flow analysis.</returns>
        /// <exception cref="System.ArgumentException">The span is not with a method
        /// body.</exception>
        /// <remarks>
        /// The first and last nodes must be fully inside the same method body.
        /// </remarks>
        protected abstract ControlFlowAnalysis AnalyzeControlFlowCore(SyntaxNode firstStatement, SyntaxNode lastStatement);

        /// <summary>
        /// Analyze control-flow within a part of a method body. 
        /// </summary>
        /// <param name="statement">The statement to be analyzed.</param>
        /// <returns>An object that can be used to obtain the result of the control flow analysis.</returns>
        /// <exception cref="System.ArgumentException">The span is not with a method
        /// body.</exception>
        /// <remarks>
        /// The statement must be fully inside the same method body.
        /// </remarks>
        internal ControlFlowAnalysis AnalyzeControlFlow(SyntaxNode statement)
        {
            return AnalyzeControlFlowCore(statement);
        }

        /// <summary>
        /// Analyze control-flow within a part of a method body. 
        /// </summary>
        /// <param name="statement">The statement to be analyzed.</param>
        /// <returns>An object that can be used to obtain the result of the control flow analysis.</returns>
        /// <exception cref="System.ArgumentException">The span is not with a method
        /// body.</exception>
        /// <remarks>
        /// The statement must be fully inside the same method body.
        /// </remarks>
        protected abstract ControlFlowAnalysis AnalyzeControlFlowCore(SyntaxNode statement);

        /// <summary>
        /// Analyze data-flow within a part of a method body. 
        /// </summary>
        /// <param name="firstStatement">The first node to be included within the analysis.</param>
        /// <param name="lastStatement">The last node to be included within the analysis.</param>
        /// <returns>An object that can be used to obtain the result of the data flow analysis.</returns>
        /// <exception cref="System.ArgumentException">The span is not with a method
        /// body.</exception>
        /// <remarks>
        /// The first and last nodes must be fully inside the same method body.
        /// </remarks>
        internal DataFlowAnalysis AnalyzeDataFlow(SyntaxNode firstStatement, SyntaxNode lastStatement)
        {
            return AnalyzeDataFlowCore(firstStatement, lastStatement);
        }

        /// <summary>
        /// Analyze data-flow within a part of a method body. 
        /// </summary>
        /// <param name="firstStatement">The first node to be included within the analysis.</param>
        /// <param name="lastStatement">The last node to be included within the analysis.</param>
        /// <returns>An object that can be used to obtain the result of the data flow analysis.</returns>
        /// <exception cref="System.ArgumentException">The span is not with a method
        /// body.</exception>
        /// <remarks>
        /// The first and last nodes must be fully inside the same method body.
        /// </remarks>
        protected abstract DataFlowAnalysis AnalyzeDataFlowCore(SyntaxNode firstStatement, SyntaxNode lastStatement);

        /// <summary>
        /// Analyze data-flow within a part of a method body. 
        /// </summary>
        /// <param name="statementOrExpression">The statement or expression to be analyzed.</param>
        /// <returns>An object that can be used to obtain the result of the data flow analysis.</returns>
        /// <exception cref="System.ArgumentException">The statement or expression is not with a method
        /// body or field or property initializer.</exception>
        /// <remarks>
        /// The statement or expression must be fully inside a method body.
        /// </remarks>
        internal DataFlowAnalysis AnalyzeDataFlow(SyntaxNode statementOrExpression)
        {
            return AnalyzeDataFlowCore(statementOrExpression);
        }

        /// <summary>
        /// Analyze data-flow within a part of a method body. 
        /// </summary>
        /// <param name="statementOrExpression">The statement or expression to be analyzed.</param>
        /// <returns>An object that can be used to obtain the result of the data flow analysis.</returns>
        /// <exception cref="System.ArgumentException">The statement or expression is not with a method
        /// body or field or property initializer.</exception>
        /// <remarks>
        /// The statement or expression must be fully inside a method body.
        /// </remarks>
        protected abstract DataFlowAnalysis AnalyzeDataFlowCore(SyntaxNode statementOrExpression);

        /// <summary>
        /// If the node provided has a constant value an Optional value will be returned with
        /// HasValue set to true and with Value set to the constant.  If the node does not have an
        /// constant value, an Optional will be returned with HasValue set to false.
        /// </summary>
        public Optional<object> GetConstantValue(SyntaxNode node, CancellationToken cancellationToken = default(CancellationToken))
        {
            return GetConstantValueCore(node, cancellationToken);
        }

        /// <summary>
        /// If the node provided has a constant value an Optional value will be returned with
        /// HasValue set to true and with Value set to the constant.  If the node does not have an
        /// constant value, an Optional will be returned with HasValue set to false.
        /// </summary>
        protected abstract Optional<object> GetConstantValueCore(SyntaxNode node, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// When getting information for a symbol that resolves to a method group or property group,
        /// from which a method is then chosen; the chosen method or property is present in Symbol;
        /// all methods in the group that was consulted are placed in this property.
        /// </summary>
        internal ImmutableArray<ISymbol> GetMemberGroup(SyntaxNode node, CancellationToken cancellationToken = default(CancellationToken))
        {
            return GetMemberGroupCore(node, cancellationToken);
        }

        /// <summary>
        /// When getting information for a symbol that resolves to a method group or property group,
        /// from which a method is then chosen; the chosen method or property is present in Symbol;
        /// all methods in the group that was consulted are placed in this property.
        /// </summary>
        protected abstract ImmutableArray<ISymbol> GetMemberGroupCore(SyntaxNode node, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Given a position in the SyntaxTree for this SemanticModel returns the innermost Symbol
        /// that the position is considered inside of.
        /// </summary>
        public ISymbol GetEnclosingSymbol(int position, CancellationToken cancellationToken = default(CancellationToken))
        {
            return GetEnclosingSymbolCore(position, cancellationToken);
        }

        /// <summary>
        /// Given a position in the SyntaxTree for this SemanticModel returns the innermost Symbol
        /// that the position is considered inside of.
        /// </summary>
        protected abstract ISymbol GetEnclosingSymbolCore(int position, CancellationToken cancellationToken = default(CancellationToken));

        /// <summary>
        /// Determines if the symbol is accessible from the specified location. 
        /// </summary>
        /// <param name="position">A character position used to identify a declaration scope and
        /// accessibility. This character position must be within the FullSpan of the Root syntax
        /// node in this SemanticModel.
        /// </param>
        /// <param name="symbol">The symbol that we are checking to see if it accessible.</param>
        /// <returns>
        /// True if "symbol is accessible, false otherwise.</returns>
        /// <remarks>
        /// This method only checks accessibility from the point of view of the accessibility
        /// modifiers on symbol and its containing types. Even if true is returned, the given symbol
        /// may not be able to be referenced for other reasons, such as name hiding.
        /// </remarks>
        public bool IsAccessible(int position, ISymbol symbol)
        {
            return IsAccessibleCore(position, symbol);
        }

        /// <summary>
        /// Determines if the symbol is accessible from the specified location. 
        /// </summary>
        /// <param name="position">A character position used to identify a declaration scope and
        /// accessibility. This character position must be within the FullSpan of the Root syntax
        /// node in this SemanticModel.
        /// </param>
        /// <param name="symbol">The symbol that we are checking to see if it accessible.</param>
        /// <returns>
        /// True if "symbol is accessible, false otherwise.</returns>
        /// <remarks>
        /// This method only checks accessibility from the point of view of the accessibility
        /// modifiers on symbol and its containing types. Even if true is returned, the given symbol
        /// may not be able to be referenced for other reasons, such as name hiding.
        /// </remarks>
        protected abstract bool IsAccessibleCore(int position, ISymbol symbol);

        /// <summary>
        /// Field-like events can be used as fields in types that can access private
        /// members of the declaring type of the event.
        /// </summary>
        /// <remarks>
        /// Always false for VB events.
        /// </remarks>
        public bool IsEventUsableAsField(int position, IEventSymbol eventSymbol)
        {
            return IsEventUsableAsFieldCore(position, eventSymbol);
        }

        /// <summary>
        /// Field-like events can be used as fields in types that can access private
        /// members of the declaring type of the event.
        /// </summary>
        /// <remarks>
        /// Always false for VB events.
        /// </remarks>
        protected abstract bool IsEventUsableAsFieldCore(int position, IEventSymbol eventSymbol);

        /// <summary>
        /// If <paramref name="nameSyntax"/> is an identifier name syntax node, return the <see cref="PreprocessingSymbolInfo"/> corresponding
        /// to it.
        /// </summary>
        /// <param name="nameSyntax">The nameSyntax node to get semantic information for.</param>
        public PreprocessingSymbolInfo GetPreprocessingSymbolInfo(SyntaxNode nameSyntax)
        {
            return GetPreprocessingSymbolInfoCore(nameSyntax);
        }

        /// <summary>
        /// If <paramref name="nameSyntax"/> is an identifier name syntax node, return the <see cref="PreprocessingSymbolInfo"/> corresponding
        /// to it.
        /// </summary>
        /// <param name="nameSyntax">The nameSyntax node to get semantic information for.</param>
        protected abstract PreprocessingSymbolInfo GetPreprocessingSymbolInfoCore(SyntaxNode nameSyntax);

        /// <summary>
        /// Gets the <see cref="DeclarationInfo"/> for all the declarations whose span overlaps with the given <paramref name="span"/>.
        /// </summary>
        /// <param name="span">Span to get declarations.</param>
        /// <param name="getSymbol">Flag indicating whether <see cref="DeclarationInfo.DeclaredSymbol"/> should be computed for the returned declaration infos.
        /// If false, then <see cref="DeclarationInfo.DeclaredSymbol"/> is always null.</param>
        /// <param name="builder">Builder to add declarations.</param>
        /// <param name="cancellationToken">Cancellation token.</param>
        internal abstract void ComputeDeclarationsInSpan(TextSpan span, bool getSymbol, List<DeclarationInfo> builder, CancellationToken cancellationToken);

        /// <summary>
        /// Takes a node and returns a set of declarations that overlap the node's span.
        /// </summary>
        internal abstract void ComputeDeclarationsInNode(SyntaxNode node, bool getSymbol, List<DeclarationInfo> builder, CancellationToken cancellationToken, int? levelsToCompute = null);

        /// <summary>
        /// Takes a Symbol and syntax for one of its declaring syntax reference and returns the topmost syntax node to be used by syntax analyzer.
        /// </summary>
        protected internal virtual SyntaxNode GetTopmostNodeForDiagnosticAnalysis(ISymbol symbol, SyntaxNode declaringSyntax)
        {
            return declaringSyntax;
        }
    }
}