Fix VSTHRD110 firing in Expression-valued scenarios

src/Microsoft.VisualStudio.Threading.Analyzers/AbstractVSTHRD110ObserveResultOfAsyncCallsAnalyzer.cs

@@ -43,6 +43,98 @@ public override void Initialize(AnalysisContext context)
         });
     }
 
+    /// <summary>
+    /// Determines whether an invocation is within a lambda expression that is being converted to an Expression tree.
+    /// </summary>
+    /// <param name="operation">The invocation operation to check.</param>
+    /// <returns>True if the invocation is within a lambda converted to an Expression; false otherwise.</returns>
+    private static bool IsWithinExpressionLambda(IInvocationOperation operation)
+    {
+        // Walk up the operation tree to find the containing lambda
+        IOperation? current = operation.Parent;
+        while (current is not null)
+        {
+            if (current is IAnonymousFunctionOperation lambda)
+            {
+                // Found a lambda, now check if it's being converted to an Expression<>
+                return IsLambdaConvertedToExpression(lambda);
+            }
+
+            current = current.Parent;
+        }
+
+        return false;
+    }
+
+    /// <summary>
+    /// Determines whether a lambda is being converted to an Expression tree type.
+    /// </summary>
+    /// <param name="lambda">The lambda operation to check.</param>
+    /// <returns>True if the lambda is being converted to an Expression; false otherwise.</returns>
+    private static bool IsLambdaConvertedToExpression(IAnonymousFunctionOperation lambda)
+    {
+        // Walk up from the lambda to find conversion or argument operations
+        IOperation? current = lambda.Parent;
+        while (current is not null)
+        {
+            // Check if the lambda's parent is a conversion operation
+            if (current is IConversionOperation conversion)
+            {
+                // Check if the target type is Expression<> or a related expression tree type
+                return IsExpressionTreeType(conversion.Type);
+            }
+
+            // Check if the lambda is being passed as an argument to a method expecting Expression<>
+            if (current is IArgumentOperation argument &&
+                argument.Parameter?.Type is INamedTypeSymbol parameterType)
+            {
+                return IsExpressionTreeType(parameterType);
+            }
+
+            // Allow certain operations to be skipped (like parentheses)
+            if (current is IParenthesizedOperation)
+            {
+                current = current.Parent;
+                continue;
+            }
+
+            // Stop walking up at other operation types to avoid false positives
+            break;
+        }
+
+        return false;
+    }
+
+    /// <summary>
+    /// Determines whether a type is an Expression tree type (Expression&lt;T&gt; or related types).
+    /// </summary>
+    /// <param name="type">The type to check.</param>
+    /// <returns>True if the type is an Expression tree type; false otherwise.</returns>
+    private static bool IsExpressionTreeType(ITypeSymbol? type)
+    {
+        if (type is not INamedTypeSymbol namedType)
+        {
+            return false;
+        }
+
+        // Check for System.Linq.Expressions.Expression<T>
+        if (namedType.Name == "Expression" &&
+            namedType.ContainingNamespace?.ToDisplayString() == "System.Linq.Expressions" &&
+            namedType.IsGenericType)
+        {
+            return true;
+        }
+
+        // Check for LambdaExpression and other expression types
+        if (namedType.ContainingNamespace?.ToDisplayString() == "System.Linq.Expressions" &&
+            (namedType.Name == "LambdaExpression" || namedType.Name.EndsWith("Expression")))
+        {
+            return true;
+        }
+
+        return false;
+    }
+
     private void AnalyzeInvocation(OperationAnalysisContext context, CommonInterest.AwaitableTypeTester awaitableTypes)
     {
         var operation = (IInvocationOperation)context.Operation;
@@ -57,6 +149,13 @@ private void AnalyzeInvocation(OperationAnalysisContext context, CommonInterest.
             return;
         }
 
+        // Check if this invocation is within a lambda that's being converted to an Expression<>
+        if (IsWithinExpressionLambda(operation))
+        {
+            // This invocation is within a lambda converted to an expression tree, so it's not actually being invoked.
+            return;
+        }
+
         // Only consider invocations that are direct statements (or are statements through limited steps).
         // Otherwise, we assume their result is awaited, assigned, or otherwise consumed.
         IOperation? parentOperation = operation.Parent;

test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD110ObserveResultOfAsyncCallsAnalyzerTests.cs

@@ -542,4 +542,158 @@ void DoOperation()
 
         await CSVerify.VerifyAnalyzerAsync(test);
     }
+
+    [Fact]
+    public async Task ExpressionLambda_ProducesNoDiagnostic()
+    {
+        string test = """
+            using System;
+            using System.Linq.Expressions;
+            using System.Threading.Tasks;
+
+            interface ILogger
+            {
+                Task InfoAsync(string message);
+            }
+
+            class MockVerifier
+            {
+                public static void Verify<T>(Expression<Func<T, Task>> expression)
+                {
+                }
+            }
+
+            class Test
+            {
+                void TestMethod()
+                {
+                    var logger = new MockLogger();
+                    MockVerifier.Verify<ILogger>(x => x.InfoAsync("test"));
+                }
+            }
+
+            class MockLogger : ILogger
+            {
+                public Task InfoAsync(string message) => Task.CompletedTask;
+            }
+            """;
+
+        await CSVerify.VerifyAnalyzerAsync(test);
+    }
+
+    [Fact]
+    public async Task ExpressionFuncLambda_ProducesNoDiagnostic()
+    {
+        string test = """
+            using System;
+            using System.Linq.Expressions;
+            using System.Threading.Tasks;
+
+            class Test
+            {
+                void TestMethod()
+                {
+                    SomeMethod(x => x.InfoAsync("test"));
+                }
+
+                void SomeMethod(Expression<Func<ILogger, Task>> expression)
+                {
+                }
+
+                Task InfoAsync(string message) => Task.CompletedTask;
+            }
+
+            interface ILogger
+            {
+                Task InfoAsync(string message);
+            }
+            """;
+
+        await CSVerify.VerifyAnalyzerAsync(test);
+    }
+
+    [Fact]
+    public async Task MoqLikeScenario_ProducesNoDiagnostic()
+    {
+        string test = """
+            using System;
+            using System.Linq.Expressions;
+            using System.Threading.Tasks;
+
+            interface ILogger
+            {
+                Task InfoAsync(string message);
+            }
+
+            class Mock<T>
+            {
+                public void Verify(Expression<Func<T, Task>> expression, Times times, string message)
+                {
+                }
+            }
+
+            enum Times
+            {
+                Never
+            }
+
+            class Test
+            {
+                void TestMethod()
+                {
+                    var mock = new Mock<ILogger>();
+                    mock.Verify(x => x.InfoAsync("test"), Times.Never, "No Log should have been written");
+                }
+            }
+            """;
+
+        await CSVerify.VerifyAnalyzerAsync(test);
+    }
+
+    [Fact]
+    public async Task DirectTaskCall_StillProducesDiagnostic()
+    {
+        string test = """
+            using System.Threading.Tasks;
+
+            class Test
+            {
+                void TestMethod()
+                {
+                    // This should still trigger VSTHRD110 - direct call not in expression
+                    [|TaskReturningMethod()|];
+                }
+
+                Task TaskReturningMethod() => Task.CompletedTask;
+            }
+            """;
+
+        await CSVerify.VerifyAnalyzerAsync(test);
+    }
+
+    [Fact]
+    public async Task ExpressionAssignment_ProducesNoDiagnostic()
+    {
+        string test = """
+            using System;
+            using System.Linq.Expressions;
+            using System.Threading.Tasks;
+
+            interface ILogger
+            {
+                Task InfoAsync(string message);
+            }
+
+            class Test
+            {
+                void TestMethod()
+                {
+                    // Assignment to Expression<> variable should not trigger VSTHRD110
+                    Expression<Func<ILogger, Task>> expr = x => x.InfoAsync("test");
+                }
+            }
+            """;
+
+        await CSVerify.VerifyAnalyzerAsync(test);
+    }
 }