Fixed #19260

src/EditorFeatures/CSharpTest/Completion/CompletionProviders/DeclarationNameCompletionProviderTests.cs

@@ -196,6 +196,68 @@ public class C
             await VerifyItemExistsAsync(markup, "cancellationToken", glyph: (int)Glyph.Parameter);
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
+        public async void EscapeKeywords1()
+        {
+            var markup = @"
+using System.Text;
+public class C
+{
+    void Foo(StringBuilder $$) {}
+}
+";
+            await VerifyItemExistsAsync(markup, "stringBuilder", glyph: (int) Glyph.Parameter);
+            await VerifyItemExistsAsync(markup, "@string", glyph: (int) Glyph.Parameter);
+            await VerifyItemExistsAsync(markup, "builder", glyph: (int) Glyph.Parameter);
+        }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
+        public async void EscapeKeywords2()
+        {
+            var markup = @"
+class For { }
+public class C
+{
+    void Foo(For $$) {}
+}
+";
+            await VerifyItemExistsAsync(markup, "@for", glyph: (int) Glyph.Parameter);
+        }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
+        public async void EscapeKeywords3()
+        {
+            var markup = @"
+class For { }
+public class C
+{
+    void foo()
+    {
+        For $$
+    }
+}
+";
+            await VerifyItemExistsAsync(markup, "@for");
+        }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
+        public async void EscapeKeywords4()
+        {
+            var markup = @"
+using System.Text;
+public class C
+{
+    void foo()
+    {
+        StringBuilder $$
+    }
+}
+";
+            await VerifyItemExistsAsync(markup, "stringBuilder");
+            await VerifyItemExistsAsync(markup, "@string");
+            await VerifyItemExistsAsync(markup, "builder");
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.Completion)]
         public async void NoSuggestionsForInt()
         {

src/Features/CSharp/Portable/Completion/CompletionProviders/DeclarationNameCompletionProvider.cs

@@ -177,7 +177,7 @@ private ITypeSymbol UnwrapType(ITypeSymbol type, Compilation compilation)
                     {
                         foreach (var baseName in baseNames)
                         {
-                            var name = rule.NamingStyle.CreateName(baseName);
+                            var name = EscapeIdentifier(rule.NamingStyle.CreateName(baseName));
                             if (name.Length > 1 && !result.ContainsKey(name)) // Don't add multiple items for the same name
                             {
                                 result.Add(name, symbolKind);
@@ -190,6 +190,14 @@ private ITypeSymbol UnwrapType(ITypeSymbol type, Compilation compilation)
             return result.Select(kvp => (kvp.Key, kvp.Value)).ToImmutableArray();
         }
 
+        private static string EscapeIdentifier(string identifier)
+        {
+            var kind = SyntaxFacts.GetKeywordKind(identifier);
+            return kind == SyntaxKind.None
+                ? identifier
+                : $"@{identifier}";
+        }
+
         CompletionItem CreateCompletionItem(string name, Glyph glyph, string sortText)
         {
             return CommonCompletionItem.Create(