VSTHRD114: Do not return null from non-async Task method

src/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD112AvoidNullReturnInNonAsyncTaskMethodAnalyzerTests.cs

@@ -0,0 +1,137 @@
+namespace Microsoft.VisualStudio.Threading.Analyzers.Tests
+{
+    using System.Threading.Tasks;
+    using Xunit;
+    using Verify = CSharpCodeFixVerifier<VSTHRD112AvoidNullReturnInNonAsyncTaskMethodAnalyzer, CodeAnalysis.Testing.EmptyCodeFixProvider>;
+
+    public class VSTHRD112AvoidNullReturnInNonAsyncTaskMethodAnalyzerTests
+    {
+        [Fact]
+        public async Task TaskOfTReturnsNull_Diagnostic()
+        {
+            var test = @"
+using System.Threading.Tasks;
+
+class Test
+{
+    public Task<object> GetTaskObj()
+    {
+        return null;
+    }
+}
+";
+            await new Verify.Test
+            {
+                TestCode = test,
+                ExpectedDiagnostics = { Verify.Diagnostic().WithSpan(8, 9, 8, 21), },
+            }.RunAsync();
+        }
+
+        [Fact]
+        public async Task TaskReturnsNull_Diagnostic()
+        {
+            var test = @"
+using System.Threading.Tasks;
+
+class Test
+{
+    public Task GetTask()
+    {
+        return null;
+    }
+}
+";
+            await new Verify.Test
+            {
+                TestCode = test,
+                ExpectedDiagnostics = { Verify.Diagnostic().WithSpan(8, 9, 8, 21), },
+            }.RunAsync();
+        }
+
+        [Fact]
+        public async Task AsyncReturnsNull_NoDiagnostic()
+        {
+            var test = @"
+using System.Threading.Tasks;
+
+class Test
+{
+    public async Task<object> GetTaskObj()
+    {
+        return null;
+    }
+}
+";
+            await new Verify.Test
+            {
+                TestCode = test,
+            }.RunAsync();
+        }
+
+        [Fact]
+        public async Task VariableIsNullAndReturned_NoDiagnostic_FalseNegative()
+        {
+            var test = @"
+using System.Threading.Tasks;
+
+class Test
+{
+    public Task<object> GetTaskObj()
+    {
+        Task<object> o = null;
+        return o;
+    }
+}
+";
+            await new Verify.Test
+            {
+                TestCode = test,
+            }.RunAsync();
+        }
+
+        [Fact]
+        public async Task NullInTernary_NoDiagnostic_FalseNegative()
+        {
+            var test = @"
+using System.Threading.Tasks;
+
+class Test
+{
+    public Task<object> GetTaskObj(bool b)
+    {
+        return b ? default(Task<object>) : null;
+    }
+}
+";
+            await new Verify.Test
+            {
+                TestCode = test,
+            }.RunAsync();
+        }
+
+        [Fact]
+        public async Task MultipleFaultyReturns_MultipleDiagnostics()
+        {
+            var test = @"
+using System.Threading.Tasks;
+
+class Test
+{
+    public Task<object> GetTaskObj(string s)
+    {
+        if (string.IsNullOrEmpty(s))
+        {
+            {|VSTHRD112:return null;|}
+        }
+
+        {|VSTHRD112:return null;|}
+    }
+}
+";
+            await new Verify.Test
+            {
+                TestCode = test,
+            }.RunAsync();
+        }
+    }
+}

src/Microsoft.VisualStudio.Threading.Analyzers/Strings.resx

@@ -309,4 +309,10 @@ Use AsyncLazy&lt;T&gt; instead.</value>
     <value>Call ThrowIfCancellationRequested()</value>
     <comment>"ThrowIfCancellationRequested" is a method name and should not be translated.</comment>
   </data>
-</root>
+  <data name="VSTHRD112_MessageFormat" xml:space="preserve">
+    <value>Avoid returning null from a non-async awaitable method</value>
+  </data>
+  <data name="VSTHRD112_Title" xml:space="preserve">
+    <value>Avoid returning null from a non-async awaitable method</value>
+  </data>
+</root>

src/Microsoft.VisualStudio.Threading.Analyzers/Utils.cs

@@ -101,6 +101,36 @@ internal static Action<OperationAnalysisContext> DebuggableWrapper(Action<Operat
             };
         }
 
+        internal static Action<OperationBlockStartAnalysisContext> DebuggableWrapper(Action<OperationBlockStartAnalysisContext> handler)
+        {
+            return ctxt =>
+            {
+                try
+                {
+                    handler(ctxt);
+                }
+                catch (OperationCanceledException)
+                {
+                    throw;
+                }
+                catch (Exception ex) when (LaunchDebuggerExceptionFilter())
+                {
+                    var messageBuilder = new StringBuilder();
+                    messageBuilder.AppendLine("Analyzer failure while processing syntax at");
+
+                    foreach (var operation in ctxt.OperationBlocks)
+                    {
+                        var lineSpan = operation.Syntax.GetLocation()?.GetLineSpan();
+                        messageBuilder.AppendLine($"- {operation.Syntax.SyntaxTree.FilePath}({lineSpan?.StartLinePosition.Line + 1},{lineSpan?.StartLinePosition.Character + 1}). Syntax: {operation.Syntax}");
+                    }
+
+                    messageBuilder.AppendLine($"{ex.GetType()} {ex.Message}");
+
+                    throw new Exception(messageBuilder.ToString(), ex);
+                }
+            };
+        }
+
         internal static ExpressionSyntax IsolateMethodName(InvocationExpressionSyntax invocation)
         {
             if (invocation == null)

src/Microsoft.VisualStudio.Threading.Analyzers/VSTHRD112AvoidNullReturnInNonAsyncTaskMethodAnalyzer.cs

@@ -0,0 +1,60 @@
+namespace Microsoft.VisualStudio.Threading.Analyzers
+{
+    using System.Collections.Immutable;
+    using System.Threading.Tasks;
+    using Microsoft.CodeAnalysis;
+    using Microsoft.CodeAnalysis.Diagnostics;
+    using Microsoft.CodeAnalysis.Operations;
+
+    /// <summary>
+    /// Finds await expressions on <see cref="Task"/> that do not use <see cref="Task.ConfigureAwait(bool)"/>.
+    /// Also works on <see cref="ValueTask"/>.
+    /// </summary>
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    public class VSTHRD112AvoidNullReturnInNonAsyncTaskMethodAnalyzer : DiagnosticAnalyzer
+    {
+        public const string Id = "VSTHRD112";
+
+        internal static readonly DiagnosticDescriptor Descriptor = new DiagnosticDescriptor(
+            id: Id,
+            title: new LocalizableResourceString(nameof(Strings.VSTHRD112_Title), Strings.ResourceManager, typeof(Strings)),
+            messageFormat: new LocalizableResourceString(nameof(Strings.VSTHRD112_MessageFormat), Strings.ResourceManager, typeof(Strings)),
+            helpLinkUri: Utils.GetHelpLink(Id),
+            category: "Usage",
+            defaultSeverity: DiagnosticSeverity.Warning,
+            isEnabledByDefault: true);
+
+        /// <inheritdoc />
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(Descriptor);
+
+        /// <inheritdoc />
+        public override void Initialize(AnalysisContext context)
+        {
+            context.EnableConcurrentExecution();
+            context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.Analyze);
+
+            context.RegisterOperationBlockStartAction(Utils.DebuggableWrapper(context => AnalyzeOperationBlockStart(context)));
+        }
+
+        private static void AnalyzeOperationBlockStart(OperationBlockStartAnalysisContext context)
+        {
+            if (context.OwningSymbol is IMethodSymbol method &&
+                !method.IsAsync &&
+                Utils.IsTask(method.ReturnType))
+            {
+                context.RegisterOperationAction(Utils.DebuggableWrapper(context => AnalyzerReturnOperation(context)), OperationKind.Return);
+            }
+        }
+
+        private static void AnalyzerReturnOperation(OperationAnalysisContext context)
+        {
+            var returnOperation = (IReturnOperation)context.Operation;
+
+            if (returnOperation.ReturnedValue.ConstantValue.HasValue &&
+                returnOperation.ReturnedValue.ConstantValue.Value == null)
+            {
+                context.ReportDiagnostic(Diagnostic.Create(Descriptor, returnOperation.Syntax.GetLocation()));
+            }
+        }
+    }
+}

src/Microsoft.VisualStudio.Threading.Analyzers/xlf/Strings.cs.xlf

@@ -249,6 +249,16 @@ Použijte místo toho AsyncLazy&lt;T&gt;.</target>
         <target state="translated">Používat ConfigureAwait(bool)</target>
         <note from="MultilingualBuild" annotates="source" priority="2">"ConfigureAwait(bool)" is a reference and should NOT be translated.</note>
       </trans-unit>
+      <trans-unit id="VSTHRD112_MessageFormat">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="VSTHRD112_Title">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
       <trans-unit id="VSTHRD200_AddAsync_MessageFormat" translate="yes" xml:space="preserve">
         <source>Use "Async" suffix in names of methods that return an awaitable type.</source>
         <target state="translated">V názvech metod, které vrací typ awaitable, používejte příponu „Async“.</target>

src/Microsoft.VisualStudio.Threading.Analyzers/xlf/Strings.de.xlf

@@ -249,6 +249,16 @@ Verwenden Sie stattdessen "AsyncLazy&lt;T&gt;".</target>
         <target state="translated">ConfigureAwait(bool) verwenden</target>
         <note from="MultilingualBuild" annotates="source" priority="2">"ConfigureAwait(bool)" is a reference and should NOT be translated.</note>
       </trans-unit>
+      <trans-unit id="VSTHRD112_MessageFormat">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="VSTHRD112_Title">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
       <trans-unit id="VSTHRD200_AddAsync_MessageFormat" translate="yes" xml:space="preserve">
         <source>Use "Async" suffix in names of methods that return an awaitable type.</source>
         <target state="translated">Verwenden Sie das Suffix "Async" in Namen von Methoden, die einen Typ zurückgeben, der "await" unterstützt.</target>

src/Microsoft.VisualStudio.Threading.Analyzers/xlf/Strings.es.xlf

@@ -249,6 +249,16 @@ Use AsyncLazy&lt;T&gt; en su lugar.</target>
         <target state="translated">Usar ConfigureAwait(bool)</target>
         <note from="MultilingualBuild" annotates="source" priority="2">"ConfigureAwait(bool)" is a reference and should NOT be translated.</note>
       </trans-unit>
+      <trans-unit id="VSTHRD112_MessageFormat">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="VSTHRD112_Title">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
       <trans-unit id="VSTHRD200_AddAsync_MessageFormat" translate="yes" xml:space="preserve">
         <source>Use "Async" suffix in names of methods that return an awaitable type.</source>
         <target state="translated">Use el sufijo "Async" en nombres de métodos que devuelven un tipo que admite await.</target>

src/Microsoft.VisualStudio.Threading.Analyzers/xlf/Strings.fr.xlf

@@ -249,6 +249,16 @@ Utilisez AsyncLazy&lt;T&gt; à la place.</target>
         <target state="translated">Utiliser ConfigureAwait(bool)</target>
         <note from="MultilingualBuild" annotates="source" priority="2">"ConfigureAwait(bool)" is a reference and should NOT be translated.</note>
       </trans-unit>
+      <trans-unit id="VSTHRD112_MessageFormat">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="VSTHRD112_Title">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
       <trans-unit id="VSTHRD200_AddAsync_MessageFormat" translate="yes" xml:space="preserve">
         <source>Use "Async" suffix in names of methods that return an awaitable type.</source>
         <target state="translated">Utilisez le suffixe "Async" dans les noms des méthodes qui retournent un type awaitable.</target>

src/Microsoft.VisualStudio.Threading.Analyzers/xlf/Strings.it.xlf

@@ -249,6 +249,16 @@ In alternativa, usare AsyncLazy&lt;T&gt;.</target>
         <target state="translated">Usa ConfigureAwait(bool)</target>
         <note from="MultilingualBuild" annotates="source" priority="2">"ConfigureAwait(bool)" is a reference and should NOT be translated.</note>
       </trans-unit>
+      <trans-unit id="VSTHRD112_MessageFormat">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="VSTHRD112_Title">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
       <trans-unit id="VSTHRD200_AddAsync_MessageFormat" translate="yes" xml:space="preserve">
         <source>Use "Async" suffix in names of methods that return an awaitable type.</source>
         <target state="translated">Usare il suffisso "Async" in nomi di metodi che restituiscono un tipo awaitable.</target>

src/Microsoft.VisualStudio.Threading.Analyzers/xlf/Strings.ja.xlf

@@ -249,6 +249,16 @@ Use AsyncLazy&lt;T&gt; instead.</source>
         <target state="translated">ConfigureAwait(bool) を使用する</target>
         <note from="MultilingualBuild" annotates="source" priority="2">"ConfigureAwait(bool)" is a reference and should NOT be translated.</note>
       </trans-unit>
+      <trans-unit id="VSTHRD112_MessageFormat">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="VSTHRD112_Title">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
       <trans-unit id="VSTHRD200_AddAsync_MessageFormat" translate="yes" xml:space="preserve">
         <source>Use "Async" suffix in names of methods that return an awaitable type.</source>
         <target state="translated">待機可能な型を戻すメソッドの名前に "Async" サフィックスを使用します。</target>

src/Microsoft.VisualStudio.Threading.Analyzers/xlf/Strings.ko.xlf

@@ -249,6 +249,16 @@ Use AsyncLazy&lt;T&gt; instead.</source>
         <target state="translated">ConfigureAwait(bool)를 사용합니다.</target>
         <note from="MultilingualBuild" annotates="source" priority="2">"ConfigureAwait(bool)" is a reference and should NOT be translated.</note>
       </trans-unit>
+      <trans-unit id="VSTHRD112_MessageFormat">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="VSTHRD112_Title">
+        <source>Avoid returning null from a non-async awaitable method</source>
+        <target state="new">Avoid returning null from a non-async awaitable method</target>
+        <note />
+      </trans-unit>
       <trans-unit id="VSTHRD200_AddAsync_MessageFormat" translate="yes" xml:space="preserve">
         <source>Use "Async" suffix in names of methods that return an awaitable type.</source>
         <target state="translated">대기할 수 있는 형식을 반환하는 메서드의 이름에 "Async" 접미사를 사용합니다.</target>