Merge pull request #17850 from CyrusNajmabadi/noAsync

Do not offer to convert method to async if we can't find the type for 'Task' and 'Task<T>'.

src/Features/Core/Portable/MakeMethodAsynchronous/AbstractMakeMethodAsynchronousCodeFixProvider.cs

@@ -34,6 +34,19 @@ public override async Task RegisterCodeFixesAsync(CodeFixContext context)
             }
 
             var semanticModel = await context.Document.GetSemanticModelAsync(cancellationToken).ConfigureAwait(false);
+            var compilation = semanticModel.Compilation;
+
+            // Find the symbols for Task, Task<T> and ValueTask<T>.  Note that the first
+            // two are mandatory (since we need them to generate the return types for our
+            // method if we convert it.  The last is optional.  It is only needed to know
+            // if our member is already Task-Like, and that functionality recognizes
+            // ValueTask if it is available, but does not care if it is not.
+            var (taskType, taskOfTType, valueTaskOfTTypeOpt) = GetTaskTypes(compilation);
+            if (taskType == null || taskOfTType == null)
+            {
+                return;
+            }
+
             var symbol = semanticModel.GetDeclaredSymbol(node, cancellationToken) as IMethodSymbol;
 
             // If it's a void returning method, offer to keep the void return type, or convert to 
@@ -60,6 +73,15 @@ public override async Task RegisterCodeFixesAsync(CodeFixContext context)
             }
         }
 
+        private (INamedTypeSymbol taskType, INamedTypeSymbol taskOfTType, INamedTypeSymbol valueTaskOfTTypeOpt) GetTaskTypes(Compilation compilation)
+        {
+            var taskType = compilation.GetTypeByMetadataName("System.Threading.Tasks.Task");
+            var taskOfTType = compilation.GetTypeByMetadataName("System.Threading.Tasks.Task`1");
+            var valueTaskOfTType = compilation.GetTypeByMetadataName("System.Threading.Tasks.ValueTask`1");
+
+            return (taskType, taskOfTType, valueTaskOfTType);
+        }
+
         protected abstract string GetMakeAsyncTaskFunctionResource();
 
         protected abstract string GetMakeAsyncVoidFunctionResource();
@@ -128,9 +150,7 @@ private SyntaxNode GetContainingFunction(Diagnostic diagnostic, CancellationToke
             SyntaxNode node, CancellationToken cancellationToken)
         {
             var compilation = await document.Project.GetCompilationAsync(cancellationToken).ConfigureAwait(false);
-            var taskType = compilation.GetTypeByMetadataName("System.Threading.Tasks.Task");
-            var taskOfTType = compilation.GetTypeByMetadataName("System.Threading.Tasks.Task`1");
-            var valueTaskOfTType = compilation.GetTypeByMetadataName("System.Threading.Tasks.ValueTask`1");
+            var (taskType, taskOfTType, valueTaskOfTType) = GetTaskTypes(compilation);
 
             var newNode = AddAsyncTokenAndFixReturnType(keepVoid, methodSymbolOpt, node, taskType, taskOfTType, valueTaskOfTType)
                 .WithAdditionalAnnotations(Formatter.Annotation);