Use pattern matching

src/Compilers/Core/Portable/InternalUtilities/EnumerableExtensions.cs

@@ -26,8 +26,7 @@ public static IEnumerable<T> Do<T>(this IEnumerable<T> source, Action<T> action)
             }
 
             // perf optimization. try to not use enumerator if possible
-            var list = source as IList<T>;
-            if (list != null)
+            if (source is IList<T> list)
             {
                 for (int i = 0, count = list.Count; i < count; i++)
                 {
@@ -168,14 +167,12 @@ public static bool IsSingle<T>(this IEnumerable<T> list)
 
         public static bool IsEmpty<T>(this IEnumerable<T> source)
         {
-            var readOnlyCollection = source as IReadOnlyCollection<T>;
-            if (readOnlyCollection != null)
+            if (source is IReadOnlyCollection<T> readOnlyCollection)
             {
                 return readOnlyCollection.Count == 0;
             }
 
-            var genericCollection = source as ICollection<T>;
-            if (genericCollection != null)
+            if (source is ICollection<T> genericCollection)
             {
                 return genericCollection.Count == 0;
             }

src/VisualStudio/CSharp/Impl/Debugging/DataTipInfoGetter.cs

@@ -28,8 +28,7 @@ internal static async Task<DebugDataTipInfo> GetInfoAsync(Document document, int
 
                 var token = root.FindToken(position);
 
-                var expression = token.Parent as ExpressionSyntax;
-                if (expression == null)
+                if (!(token.Parent is ExpressionSyntax expression))
                 {
                     return token.IsKind(SyntaxKind.IdentifierToken)
                         ? new DebugDataTipInfo(token.Span, text: null)

src/VisualStudio/CSharp/Impl/ProjectSystemShim/CSharpProjectShim.ICSharpVenusProjectSite.cs

@@ -65,12 +65,11 @@ private static CSharpProjectShim GetProjectSite(ICSharpProjectRoot project)
         {
             // Get the host back for the project
             var projectSiteGuid = typeof(ICSharpProjectSite).GUID;
-            var projectSite = project.GetProjectSite(ref projectSiteGuid) as CSharpProjectShim;
 
             // We should have gotten a ProjectSite back. If we didn't, that means we're being given
             // a project site that we didn't get BindToProject called on first which is a no-no by
             // the project system.
-            if (projectSite == null)
+            if (!(project.GetProjectSite(ref projectSiteGuid) is CSharpProjectShim projectSite))
             {
                 throw new ArgumentException($"{project} was not properly sited with the language service.", nameof(project));
             }

src/Workspaces/CSharp/Portable/Formatting/FormattingHelpers.cs

@@ -157,8 +157,7 @@ public static bool IsSemicolonOfEmbeddedStatement(this SyntaxToken token)
                 return false;
             }
 
-            var statement = token.Parent as StatementSyntax;
-            if (statement == null ||
+            if (!(token.Parent is StatementSyntax statement) ||
                 statement.GetLastToken() != token)
             {
                 return false;
@@ -184,8 +183,7 @@ public static bool IsCloseBraceOfEmbeddedBlock(this SyntaxToken token)
                 return false;
             }
 
-            var block = token.Parent as BlockSyntax;
-            if (block == null ||
+            if (!(token.Parent is BlockSyntax block) ||
                 block.CloseBraceToken != token)
             {
                 return false;