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 · 2026-05-16T14:17:19Z

@copilot address review comments

Copilot · 2026-05-17T19:23:43Z

@copilot address review comments

Addressed in 687b203. The fixer now unwraps cast/parenthesized lambda arguments before rewriting, the analyzer skips exception filters, and the current PR file list no longer includes the unrelated TestFramework XLF file.

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

Copilot

Copilot's findings

Files reviewed: 33/33 changed files
Comments generated: 7

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


+            if (current is IAnonymousFunctionOperation or ILocalFunctionOperation ||
+                current.Kind == OperationKind.Lock ||
+                (current.Parent is ICatchClauseOperation catchClauseOperation && ReferenceEquals(catchClauseOperation.Filter, current)))


+        InvocationExpressionSyntax newInvocationExpression = ReplaceAssertMethodName(invocationExpression);
+        if (TryGetActionArgumentIndex(invocationExpression, semanticModel, cancellationToken, out int actionArgumentIndex) &&
+            TryReplaceLambda(newInvocationExpression.ArgumentList.Arguments[actionArgumentIndex], out ArgumentSyntax? newArgument))


+        if (newMethodDeclaration.ReturnType.IsVoid())
+        {
+            newMethodDeclaration = newMethodDeclaration.WithReturnType(GetTaskReturnType(originalMethodDeclaration, semanticModel, cancellationToken).WithTriviaFrom(newMethodDeclaration.ReturnType));


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


+        if (expression is not LambdaExpressionSyntax lambdaExpression ||
+            !TryGetBlockedTaskExpressionFromLambda(lambdaExpression, out ExpressionSyntax? asyncExpression))
+        {
+            newExpression = null;
+            return false;


+        if (invocationExpression.Ancestors().OfType<MethodDeclarationSyntax>().FirstOrDefault() is { } methodDeclaration)
+        {
+            MethodDeclarationSyntax newMethodDeclaration = methodDeclaration.ReplaceNode(invocationExpression, awaitExpression);
+            editor.ReplaceNode(methodDeclaration, AddAsyncModifierAndTaskReturnType(newMethodDeclaration, methodDeclaration, semanticModel, cancellationToken));


Evangelink · 2026-05-18T13:18:18Z

@copilot address review comments

…ions

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

Copilot · 2026-05-18T13:36:46Z

@copilot address review comments

Addressed in dc11ebe. MSTEST0064 now avoids unsafe/unfixable contexts and interpolated-string-handler overloads, and the fixer handles ValueTask returns, result member access, anonymous delegates, and expression-bodied void tests with regression coverage.

…explicit delegate creations (#8256 follow-up) (#8329) Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

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

Closed

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

View reviewed changes

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

Copilot AI requested review from Copilot and removed request for Copilot May 17, 2026 19:23

Copilot finished work on behalf of Evangelink May 17, 2026 19:24

Handle converted async assertion arguments

ec68da1

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

Copilot AI review requested due to automatic review settings May 18, 2026 08:53

Copilot started reviewing on behalf of Evangelink May 18, 2026 08:54 View session

Copilot AI reviewed May 18, 2026

View reviewed changes

Merge branch 'main' into copilot/add-analyzer-favorizing-async-assert…

800c895

…ions

Copilot started work on behalf of Evangelink May 18, 2026 13:18 View session

Address async assertion review edge cases

ad7dd57

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

Copilot AI requested review from Copilot and removed request for Copilot May 18, 2026 13:29

Incorporate validation feedback

394e045

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

Copilot AI requested review from Copilot and removed request for Copilot May 18, 2026 13:32

Polish async assertion fixes

2983e26

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

Copilot AI requested review from Copilot and removed request for Copilot May 18, 2026 13:34

Address final validation suggestions

dc11ebe

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

Copilot AI requested review from Copilot and removed request for Copilot May 18, 2026 13:36

Copilot finished work on behalf of Evangelink May 18, 2026 13:37

Evangelink merged commit 9eb2438 into main May 18, 2026
10 checks passed

Evangelink deleted the copilot/add-analyzer-favorizing-async-assertions branch May 18, 2026 15:02

Evangelink mentioned this pull request May 18, 2026

MSTEST0064: gate code fix against await-forbidden returns and unwrap explicit delegate creations (#8256 follow-up) #8329

Merged

Evangelink added this to the MSTest 4.3 / MTP 2.3 milestone Jun 16, 2026

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Add MSTEST0064 to prefer async assertions#8256

Add MSTEST0064 to prefer async assertions#8256
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