Add MSTEST0064 to prefer async assertions by Copilot · Pull Request #8256 · microsoft/testfx

Copilot · 2026-05-15T12:26:45Z

New Feature

What does this feature do?

Adds an MSTest analyzer/code fix that flags synchronous exception assertions blocking on async work and recommends async assertion APIs instead.

Assert.ThrowsExactly<SomeException>(() => foo.BarAsync().GetAwaiter().GetResult());

becomes:

await Assert.ThrowsExactlyAsync<SomeException>(() => foo.BarAsync());

Implementation details

Analyzer
- Adds MSTEST0064 for Assert.Throws / Assert.ThrowsExactly calls that block Task/Task<T> via GetAwaiter().GetResult().
- Limits diagnostics to MSTest test methods.
Code fix
- Rewrites to Assert.ThrowsAsync / Assert.ThrowsExactlyAsync.
- Removes the blocking GetAwaiter().GetResult() chain from the asserted lambda.
- Updates void test methods to async Task when required.
Resources and coverage
- Adds analyzer/code-fix resources and localization updates.
- Adds focused C# analyzer/code-fix tests for Task, Task<T>, and non-diagnostic cases.

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Copilot

Pull request overview

Adds MSTEST0064 to detect MSTest synchronous exception assertions that block on async Task work and provide a C# code fix to use async assertion APIs.

Changes:

Adds PreferAsyncAssertionAnalyzer and registers MSTEST0064.
Adds PreferAsyncAssertionFixer to rewrite assertions and async test signatures.
Adds resources, localization entries, release tracking, and analyzer/code-fix tests.

Show a summary per file

File	Description
`src/Analyzers/MSTest.Analyzers/PreferAsyncAssertionAnalyzer.cs`	Adds MSTEST0064 analyzer logic.
`src/Analyzers/MSTest.Analyzers.CodeFixes/PreferAsyncAssertionFixer.cs`	Adds C# code fix for async assertions.
`src/Analyzers/MSTest.Analyzers/Helpers/DiagnosticIds.cs`	Registers MSTEST0064 ID.
`src/Analyzers/MSTest.Analyzers/AnalyzerReleases.Unshipped.md`	Adds release tracking entry.
`src/Analyzers/MSTest.Analyzers/Resources.resx`	Adds analyzer resource strings.
`src/Analyzers/MSTest.Analyzers.CodeFixes/CodeFixResources.resx`	Adds code-fix title resource.
`test/UnitTests/MSTest.Analyzers.UnitTests/PreferAsyncAssertionAnalyzerTests.cs`	Adds focused analyzer/code-fix tests.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.cs.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.de.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.es.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.fr.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.it.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.ja.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.ko.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.pl.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.pt-BR.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.ru.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.tr.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.zh-Hans.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers/xlf/Resources.zh-Hant.xlf`	Adds localized analyzer resource placeholders.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.cs.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.de.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.es.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.fr.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.it.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.ja.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.ko.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.pl.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.pt-BR.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.ru.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.tr.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.zh-Hans.xlf`	Adds localized code-fix resource placeholder.
`src/Analyzers/MSTest.Analyzers.CodeFixes/xlf/CodeFixResources.zh-Hant.xlf`	Adds localized code-fix resource placeholder.

Copilot's findings

Comments suppressed due to low confidence (5)

src/Analyzers/MSTest.Analyzers.CodeFixes/PreferAsyncAssertionFixer.cs:90

The code fix does not handle static-imported assertions such as using static ...Assert; Throws<Exception>(...). The analyzer still reports those invocations, but this method returns the original Throws name, so the fix wraps await around the synchronous assertion instead of calling ThrowsAsync, which can break compilation after the lambda is rewritten to return a Task.

        if (invocationExpression.Expression is not MemberAccessExpressionSyntax memberAccessExpression)
        {
            return invocationExpression;
        }

src/Analyzers/MSTest.Analyzers.CodeFixes/PreferAsyncAssertionFixer.cs:140

Changing a void test method to return unqualified Task requires System.Threading.Tasks to already be in scope. If the original test uses fully qualified task types or implicit usings are disabled, applying the fix introduces an unresolved Task; the fixer should add the using or use a qualified/simplified type.

        if (newMethodDeclaration.ReturnType.IsVoid())
        {
            newMethodDeclaration = newMethodDeclaration.WithReturnType(SyntaxFactory.IdentifierName("Task").WithTriviaFrom(newMethodDeclaration.ReturnType));
        }

src/Analyzers/MSTest.Analyzers.CodeFixes/PreferAsyncAssertionFixer.cs:160

The analyzer accepts block-bodied lambdas whose single operation is a return value, but the fixer only rewrites block bodies with an expression statement. For Task<T> cases like () => { return BarAsync().GetAwaiter().GetResult(); }, the fix renames to ThrowsAsync and adds await while leaving a lambda that returns the synchronous result, causing the fixed code not to compile.

        if (lambdaExpression.Body is BlockSyntax blockSyntax &&
            blockSyntax.Statements.Count == 1 &&
            blockSyntax.Statements[0] is ExpressionStatementSyntax expressionStatement)

src/Analyzers/MSTest.Analyzers/PreferAsyncAssertionAnalyzer.cs:127

This treats every non-return child operation as meaningful, but VB lambdas can include implicit ILabeledOperation nodes in the block. Because those labels are not skipped, otherwise valid VB cases such as a single GetAwaiter().GetResult() statement can look like multiple operations and fail to produce the diagnostic despite the analyzer being registered for Visual Basic.

            IOperation? candidateOperation = childOperation switch
            {
                IExpressionStatementOperation expressionStatementOperation => expressionStatementOperation.Operation,
                IReturnOperation { ReturnedValue: { } returnedValue } => returnedValue,
                IReturnOperation { IsImplicit: true } => null,
                _ => childOperation,

src/Analyzers/MSTest.Analyzers.CodeFixes/PreferAsyncAssertionFixer.cs:111

The analyzer unwraps conversions/delegate creation when identifying the lambda, but the fixer only handles a direct lambda syntax node. If the diagnosed action is explicitly cast (for example (Action)(() => ...)), the fix still renames to the async assertion and adds await while leaving the blocking casted lambda unchanged, which can make the fixed call fail overload resolution.

        if (argument.Expression is not LambdaExpressionSyntax lambdaExpression ||
            !TryGetBlockedTaskExpressionFromLambda(lambdaExpression, out ExpressionSyntax? asyncExpression))

Files reviewed: 33/33 changed files
Comments generated: 2

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…favorizing-async-assertions

Copilot

Copilot's findings

Files reviewed: 33/33 changed files
Comments generated: 6

Evangelink · 2026-05-16T17:33:00Z

+
+        if (newMethodDeclaration.ReturnType.IsVoid())
+        {
+            newMethodDeclaration = newMethodDeclaration.WithReturnType(SyntaxFactory.IdentifierName("Task").WithTriviaFrom(newMethodDeclaration.ReturnType));


Addressed in b71536c (in this push): when the test method is void the fixer now uses GetSpeculativeTypeInfo to decide between unqualified Task and fully-qualified System.Threading.Tasks.Task (see GetTaskReturnType).

Evangelink · 2026-05-16T17:33:08Z

+        if (lambdaExpression.Body is BlockSyntax blockSyntax &&
+            blockSyntax.Statements.Count == 1 &&
+            blockSyntax.Statements[0] is ExpressionStatementSyntax expressionStatement)
+        {
+            return TryGetBlockedTaskExpression(expressionStatement.Expression, out asyncExpression);


Addressed in b71536c: TryGetBlockedTaskExpressionFromLambda now also handles a BlockSyntax whose single statement is a ReturnStatementSyntax with an expression.

Evangelink · 2026-05-16T17:33:16Z

+    private static InvocationExpressionSyntax ReplaceAssertMethodName(InvocationExpressionSyntax invocationExpression)
+    {
+        if (invocationExpression.Expression is not MemberAccessExpressionSyntax memberAccessExpression)
+        {
+            return invocationExpression;


Addressed in b71536c: ReplaceAssertMethodName now handles both MemberAccessExpressionSyntax and bare SimpleNameSyntax (using static) invocations; WhenStaticallyImportedAssertion_CodeFixUsesAsyncAssertion covers the case.

Evangelink · 2026-05-16T17:33:24Z

+        if (!newMethodDeclaration.Modifiers.Any(modifier => modifier.IsKind(SyntaxKind.AsyncKeyword)))
+        {
+            newMethodDeclaration = newMethodDeclaration.WithModifiers(newMethodDeclaration.Modifiers.Add(SyntaxFactory.Token(SyntaxKind.AsyncKeyword)));
+        }


Addressed in b71536c: when the containing method is already non-async Task-returning, TaskReturnStatementRewriter rewrites any return X; to await X; return; so the body remains valid after we add the async modifier. WhenNonAsyncTaskReturningTestMethodHasReturnExpression_CodeFixConvertsReturnToAwait covers the case.

Evangelink · 2026-05-16T17:33:32Z

+        AwaitExpressionSyntax awaitExpression = SyntaxFactory.AwaitExpression(newInvocationExpression.WithoutLeadingTrivia())
+            .WithLeadingTrivia(invocationExpression.GetLeadingTrivia())
+            .WithAdditionalAnnotations(Formatter.Annotation);


Addressed in b71536c at the analyzer level: IsInsideUnsupportedAwaitContext now also stops on OperationKind.Lock so the diagnostic is no longer raised inside a lock block. WhenAssertionIsInsideLockStatement_NoDiagnostic covers the case.

Evangelink · 2026-05-16T17:33:42Z

+            {
+                IExpressionStatementOperation expressionStatementOperation => expressionStatementOperation.Operation,
+                IReturnOperation { ReturnedValue: { } returnedValue } => returnedValue,
+                IReturnOperation { IsImplicit: true } => null,


Addressed in 190da15: the initial attempt to address this in b71536c by unconditionally skipping implicit operations was too aggressive — it silently dropped every C# expression-bodied-lambda diagnostic, because the synthetic IReturnOperation that wraps an expression-body's user expression is itself implicit. The helper now only filters out the specific VB Function synthetic implicit return (detected as an implicit ILocalReferenceOperation after walking down conversions on ReturnedValue) and still accepts C# implicit returns whose value is a real user expression. WhenVisualBasicFunctionLambdaBlocksOnGenericTask_Diagnostic covers the VB case; the previously-failing C# tests are green again.

Evangelink · 2026-05-16T14:17:19Z

@copilot address review comments

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Copilot · 2026-05-16T14:28:10Z

@copilot address review comments

Addressed in b71536c.

…urns The previous skip-all-implicit guard for VB was over-broad: in C# the implicit IReturnOperation generated for an expression-bodied lambda wraps the user's expression as its ReturnedValue, so skipping it silently dropped every expression-bodied-lambda diagnostic (e.g. `() => BarAsync().GetAwaiter().GetResult()`). Only the VB Function lambda's synthetic implicit return-of-function-name-local needs to be filtered out, which we now detect by looking at the unwrapped ReturnedValue for an implicit ILocalReferenceOperation. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

Copilot's findings

Comments suppressed due to low confidence (2)

src/Analyzers/MSTest.Analyzers.CodeFixes/PreferAsyncAssertionFixer.cs:175

This only rewrites returns for methods whose original return type is Task. MSTest also treats ValueTask test methods as valid, so applying this fix to a non-async ValueTask test method leaves return ValueTask...; statements in an async ValueTask method, which does not compile.

        else if (!isAsync && IsTaskReturnType(originalMethodDeclaration, semanticModel, cancellationToken) && newMethodDeclaration.Body is { } body)
        {
            newMethodDeclaration = newMethodDeclaration.WithBody((BlockSyntax)new TaskReturnStatementRewriter().Visit(body)!);

src/Analyzers/MSTest.Analyzers.CodeFixes/PreferAsyncAssertionFixer.cs:285

The rewriter converts every return expression in the method body into an await followed by return. If a different return statement is inside a lock block, the fix emits await inside lock, which C# forbids even though the diagnostic assertion itself is outside the lock.

                if (statement is ReturnStatementSyntax { Expression: { } returnExpression } returnStatement)
                {
                    rewrittenStatements ??= AddUnchangedStatements(node.Statements, i);
                    rewrittenStatements.AddRange(CreateAwaitAndReturnStatements(returnStatement, returnExpression));

Files reviewed: 46/46 changed files
Comments generated: 3

+        if (argument.Expression is not LambdaExpressionSyntax lambdaExpression ||
+            !TryGetBlockedTaskExpressionFromLambda(lambdaExpression, out ExpressionSyntax? asyncExpression))
+        {
+            newArgument = null;
+            return false;
+        }
+
+        LambdaExpressionSyntax newLambdaExpression = lambdaExpression switch
+        {
+            SimpleLambdaExpressionSyntax simpleLambda => simpleLambda.WithBody(asyncExpression.WithTriviaFrom(lambdaExpression.Body)),
+            ParenthesizedLambdaExpressionSyntax parenthesizedLambda => parenthesizedLambda.WithBody(asyncExpression.WithTriviaFrom(lambdaExpression.Body)),
+            _ => lambdaExpression,
+        };
+
+        newArgument = argument.WithExpression(newLambdaExpression);
+        return true;
+    }
+


+        for (IOperation? current = operation.Parent; current is not null; current = current.Parent)
+        {
+            if (current is IAnonymousFunctionOperation or ILocalFunctionOperation ||
+                current.Kind == OperationKind.Lock)


+      <trans-unit id="AreEquivalentFailedSummary">
+        <source>Expected values to be structurally equivalent.</source>
+        <target state="new">Expected values to be structurally equivalent.</target>
+        <note />
+      </trans-unit>


Initial plan

4bd6683

Copilot AI assigned Copilot and Evangelink May 15, 2026

Copilot AI requested review from Copilot and removed request for Copilot May 15, 2026 12:26

Copilot started work on behalf of Evangelink May 15, 2026 12:26 View session

Copilot AI linked an issue May 15, 2026 that may be closed by this pull request

Add analyzer favorizing Async assertions #7468

Open

Add async assertion analyzer

0da8df4

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot May 15, 2026 12:40

Address async assertion review feedback

dd56bd5

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot May 15, 2026 12:42

Share async blocking method name constants

1dd5492

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot May 15, 2026 12:43

Copilot AI changed the title ~~[WIP] Add analyzer favorizing async assertions~~ Add MSTEST0064 to prefer async assertions May 15, 2026

Copilot finished work on behalf of Evangelink May 15, 2026 12:44

Copilot AI requested a review from Evangelink May 15, 2026 12:44

Evangelink marked this pull request as ready for review May 15, 2026 13:30

Copilot AI review requested due to automatic review settings May 15, 2026 13:30

Copilot started reviewing on behalf of Evangelink May 15, 2026 13:34 View session

Copilot AI reviewed May 15, 2026

View reviewed changes

Comment thread src/Analyzers/MSTest.Analyzers.CodeFixes/PreferAsyncAssertionFixer.cs Outdated

Comment thread src/Analyzers/MSTest.Analyzers/PreferAsyncAssertionAnalyzer.cs

Evangelink and others added 2 commits May 15, 2026 20:46

Fix PreferAsyncAssertion review issues

96e9fc5

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Merge remote-tracking branch 'origin/main' into copilot/add-analyzer-…

8debda3

…favorizing-async-assertions

Copilot AI review requested due to automatic review settings May 16, 2026 11:58

Copilot started reviewing on behalf of Evangelink May 16, 2026 11:59 View session

Copilot AI reviewed May 16, 2026

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Copilot started work on behalf of Evangelink May 16, 2026 14:17 View session

Copilot AI and others added 2 commits May 16, 2026 14:22

Plan review feedback fixes

4c1fe75

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Address async assertion review feedback

b71536c

Co-authored-by: Evangelink <11340282+Evangelink@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot May 16, 2026 14:27

Copilot finished work on behalf of Evangelink May 16, 2026 14:28

Evangelink mentioned this pull request May 16, 2026

Fix TelemetryTests flake from parallel dotnet test races #8292

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Copilot AI review requested due to automatic review settings May 16, 2026 17:12

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Add MSTEST0064 to prefer async assertions#8256

Add MSTEST0064 to prefer async assertions#8256
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Copilot AI commented May 15, 2026 •

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