Address review comments

src/Compilers/CSharp/Portable/Operations/CSharpArgument.cs

@@ -17,7 +17,7 @@ internal abstract class BaseCSharpArgument : BaseArgument
         public override CommonConversion OutConversion => new CommonConversion(exists: true, isIdentity: true, isNumeric: false, isReference: false, methodSymbol: null);
     }
 
-    internal sealed partial class CSharpArgument : BaseCSharpArgument
+    internal sealed class CSharpArgument : BaseCSharpArgument
     {
         public CSharpArgument(ArgumentKind argumentKind, IParameterSymbol parameter, IOperation value, SemanticModel semanticModel, SyntaxNode syntax, ITypeSymbol type, Optional<object> constantValue, bool isImplicit) :
             base(argumentKind, parameter, semanticModel, syntax, type, constantValue, isImplicit)
@@ -28,7 +28,7 @@ internal sealed partial class CSharpArgument : BaseCSharpArgument
         protected override IOperation ValueImpl { get; }
     }
 
-    internal sealed partial class LazyCSharpArgument : BaseCSharpArgument
+    internal sealed class LazyCSharpArgument : BaseCSharpArgument
     {
         private readonly Lazy<IOperation> _lazyValue;
 

src/Compilers/CSharp/Test/Semantic/IOperation/IOperationTests_IArgument.cs

@@ -2224,7 +2224,7 @@ struct G<T>
 
         [CompilerTrait(CompilerFeature.IOperation)]
         [Fact]
-        public void GettingInOutConverionFromCSharpArgumentShouldThrowException()
+        public void GettingInOutConversionFromCSharpArgumentShouldThrowException()
         {
             string source = @"
 class P
@@ -2245,6 +2245,7 @@ static void M2(int x)
             var invocation = (IInvocationExpression)operation;
             var argument = invocation.ArgumentsInEvaluationOrder[0];
 
+            // We are calling VB extension methods on IArgument in C# code, therefore exception is expteced here.
             Assert.Throws<ArgumentException>(() => argument.GetInConversion());
             Assert.Throws<ArgumentException>(() => argument.GetOutConversion());
         }

src/Compilers/Test/Utilities/VisualBasic/BasicTestBase.vb

@@ -861,12 +861,7 @@ Public MustInherit Class BasicTestBaseBase
         
         Dim semanticModel = compilation.GetSemanticModel(node.SyntaxTree)
         Dim operation = semanticModel.GetOperationInternal(node)
-        If operation IsNot Nothing Then
-            Return (operation, node)
-        Else
-            Return (Nothing, Nothing)
-        End If
-
+        Return (operation, node)
     End Function
 
 #End Region

src/Compilers/VisualBasic/Portable/Operations/VisualBasicOperationFactory_Methods.vb

@@ -105,47 +105,37 @@ Namespace Microsoft.CodeAnalysis.Semantics
                         isImplicit:=isImplicit)
                 Case Else
                     Dim lastParameterIndex = parameters.Length - 1
+                    Dim kind As ArgumentKind
+                    Dim parameter As ParameterSymbol
+
                     If index = lastParameterIndex AndAlso ParameterIsParamArray(parameters(lastParameterIndex)) Then
                         ' TODO: figure out if this is true:
                         '       a compiler generated argument for a ParamArray parameter is created iff
                         '       a list of arguments (including 0 argument) is provided for ParamArray parameter in source
                         '       https://github.com/dotnet/roslyn/issues/18550
-                        Dim kind = If(argument.WasCompilerGenerated AndAlso argument.Kind = BoundKind.ArrayCreation, ArgumentKind.ParamArray, ArgumentKind.Explicit)
-                        Dim parameter = parameters(lastParameterIndex)
-                        Dim value = Create(argument)
-
-                        Return New VisualBasicArgument(
-                            kind,
-                            parameter,
-                            value,
-                            inConversion:=New Conversion(Conversions.Identity),
-                            outConversion:=New Conversion(Conversions.Identity),
-                            semanticModel:=_semanticModel,
-                            syntax:=value.Syntax,
-                            type:=Nothing,
-                            constantValue:=Nothing,
-                            isImplicit:=isImplicit)
+                        kind = If(argument.WasCompilerGenerated AndAlso argument.Kind = BoundKind.ArrayCreation, ArgumentKind.ParamArray, ArgumentKind.Explicit)
+                        parameter = parameters(lastParameterIndex)
                     Else
                         ' TODO: figure our if this is true:
                         '       a compiler generated argument for an Optional parameter is created iff
                         '       the argument is omitted from the source
                         '       https://github.com/dotnet/roslyn/issues/18550
-                        Dim kind = If(argument.WasCompilerGenerated, ArgumentKind.DefaultValue, ArgumentKind.Explicit)
-                        Dim parameter = parameters(index)
-                        Dim value = Create(argument)
-
-                        Return New VisualBasicArgument(
-                            kind,
-                            parameter,
-                            value,
-                            inConversion:=New Conversion(Conversions.Identity),
-                            outConversion:=New Conversion(Conversions.Identity),
-                            semanticModel:=_semanticModel,
-                            syntax:=value.Syntax,
-                            type:=Nothing,
-                            constantValue:=Nothing,
-                            isImplicit:=isImplicit)
+                        kind = If(argument.WasCompilerGenerated, ArgumentKind.DefaultValue, ArgumentKind.Explicit)
+                        parameter = parameters(index)
                     End If
+                    
+                    Dim value = Create(argument)
+                    Return New VisualBasicArgument(
+                        kind,
+                        parameter,
+                        Create(argument),
+                        inConversion:=New Conversion(Conversions.Identity),
+                        outConversion:=New Conversion(Conversions.Identity),
+                        semanticModel:=_semanticModel,
+                        syntax:=value.Syntax,
+                        type:=Nothing,
+                        constantValue:=Nothing,
+                        isImplicit:=isImplicit)
             End Select
         End Function