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thomhurst merged 3 commits into from
Jul 28, 2026
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feat(mocks): accept an async factory in Returns() on async members #6503

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3ab6b32
feat(mocks): accept an async factory in Returns() on async members
thomhurst Jul 28, 2026
bb4843d
fix(mocks): deprioritise the async Returns overload to keep null lamb…
thomhurst Jul 28, 2026
a51c7ee
fix(mocks): gate the async Returns alias to net9.0+
thomhurst Jul 28, 2026
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35 changes: 35 additions & 0 deletions src/TUnit.Mocks.SourceGenerator/Builders/MockMembersBuilder.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -718,6 +718,21 @@ private static void GenerateTypedReturnsAsyncOverload(CodeWriter writer, List<Mo
writer.AppendLine($"EnsureSetup().ReturnsRaw(args => (object?)factory({castArgs}));");
writer.AppendLine("return this;");
}

// Returns alias, so an `async (a, b) => ...` lambda binds here rather than failing against
// the synchronous typed overload with CS4010. Deprioritised — and therefore net9.0+ only —
// for the same reasons as the parameterless alias; see EmitReturnsAsyncOverloads.
// See issue #6495.
writer.AppendLine();
writer.AppendLine("#if NET9_0_OR_GREATER");
writer.AppendLine("/// <summary>Configure a typed computed async return value using the actual method parameters. The returned task is handed back as-is, so an async factory stays pending until it completes.</summary>");

@greptile-apps greptile-apps Bot Jul 28, 2026

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P1 Async Returns remains unavailable on net8

When a .NET 8 consumer configures an async member with Returns(async (...) => ...), this conditional removes both new async-factory overload forms from the generated mock, so the lambda is still considered against the synchronous factory overload and compilation fails with CS4010.

writer.AppendLine(PriorityMinusOneAttribute);
using (writer.Block($"public {wrapperName} Returns({funcType} factory)"))
{
writer.AppendLine($"EnsureSetup().ReturnsRaw(args => (object?)factory({castArgs}));");
writer.AppendLine("return this;");
}
writer.AppendLine("#endif");
}

private static void GenerateTypedCallbackOverload(CodeWriter writer, List<MockParameterModel> nonOutParams,
Expand Down Expand Up @@ -1778,6 +1793,26 @@ private static void EmitReturnsAsyncOverloads(CodeWriter writer, string wrapperN
writer.AppendLine($"public {wrapperName} ReturnsAsync({taskType} task) {{ EnsureSetup().ReturnsRaw(task); return this; }}");
writer.AppendLine($"/// <summary>Return a pre-built {taskLabel} from a factory, invoked on each call.</summary>");
writer.AppendLine($"public {wrapperName} ReturnsAsync(global::System.Func<{taskType}> taskFactory) {{ EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }}");
// Same factory, spelled Returns — so an `async () => ...` lambda binds without the caller
// having to know about ReturnsAsync (the synchronous Func<T> overload rejects it with
// CS4010). The returned task is handed back as-is, so it stays pending. See issue #6495.
Comment thread
greptile-apps[bot] marked this conversation as resolved.
//
// Deprioritised against the synchronous Returns(Func<T>) sibling: when T is a reference
// type, a lambda whose body pins nothing — Returns(() => null), Returns(() => throw ...) —
// converts equally well to Func<T> and Func<Task<T>>, which would be CS0121. The priority
// breaks that tie back to the pre-existing synchronous meaning. A genuine async lambda is
Comment thread
greptile-apps[bot] marked this conversation as resolved.
// unaffected: it is not convertible to Func<T> at all, so it is the only candidate.
//
// That makes the alias inseparable from the attribute, which only reaches the consumer's
// compilation on net9.0+ — TUnit.Mocks polyfills it internally for its own build, so a
// net8.0 consumer would hit CS0246. Emitting the overload there without the priority would
// hand them the ambiguity instead, so the whole alias is net9.0+ (matching the framework
// polyfills below); net8.0 keeps ReturnsAsync, which already returns the task as-is.
writer.AppendLine("#if NET9_0_OR_GREATER");
writer.AppendLine($"/// <summary>Return a {taskLabel} from a factory, invoked on each call. The {taskLabel} is returned as-is, so an async factory stays pending until it completes.</summary>");
writer.AppendLine(PriorityMinusOneAttribute);
writer.AppendLine($"public {wrapperName} Returns(global::System.Func<{taskType}> taskFactory) {{ EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }}");
writer.AppendLine("#endif");
}

private static void EmitEnsureSetup(CodeWriter writer, string builderType, bool hasTypeArguments)
Expand Down
35 changes: 35 additions & 0 deletions ...s.SourceGenerator.Tests/Snapshots/Interface_FluentUI_Shape_Nullable_Warnings.verified.txt
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Expand Up @@ -202,6 +202,11 @@ public sealed class IDialogReference_GetReturnValueAsync_M0_MockCall<T> : global
public IDialogReference_GetReturnValueAsync_M0_MockCall<T> ReturnsAsync(global::System.Threading.Tasks.Task<T?> task) { EnsureSetup().ReturnsRaw(task); return this; }
/// <summary>Return a pre-built Task from a factory, invoked on each call.</summary>
public IDialogReference_GetReturnValueAsync_M0_MockCall<T> ReturnsAsync(global::System.Func<global::System.Threading.Tasks.Task<T?>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#if NET9_0_OR_GREATER
/// <summary>Return a Task from a factory, invoked on each call. The Task is returned as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IDialogReference_GetReturnValueAsync_M0_MockCall<T> Returns(global::System.Func<global::System.Threading.Tasks.Task<T?>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#endif

// ICallVerification
/// <inheritdoc />
Expand Down Expand Up @@ -587,6 +592,11 @@ public sealed class IDialogService_UpdateDialogAsync_M0_MockCall<TData> : global
public IDialogService_UpdateDialogAsync_M0_MockCall<TData> ReturnsAsync(global::System.Threading.Tasks.Task<global::IDialogReference?> task) { EnsureSetup().ReturnsRaw(task); return this; }
/// <summary>Return a pre-built Task from a factory, invoked on each call.</summary>
public IDialogService_UpdateDialogAsync_M0_MockCall<TData> ReturnsAsync(global::System.Func<global::System.Threading.Tasks.Task<global::IDialogReference?>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#if NET9_0_OR_GREATER
/// <summary>Return a Task from a factory, invoked on each call. The Task is returned as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IDialogService_UpdateDialogAsync_M0_MockCall<TData> Returns(global::System.Func<global::System.Threading.Tasks.Task<global::IDialogReference?>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#endif

/// <summary>Configure a typed computed return value using the actual method parameters.</summary>
public IDialogService_UpdateDialogAsync_M0_MockCall<TData> Returns(global::System.Func<string, global::DialogParameters<TData>, global::IDialogReference?> factory)
Expand All @@ -602,6 +612,16 @@ public sealed class IDialogService_UpdateDialogAsync_M0_MockCall<TData> : global
return this;
}

#if NET9_0_OR_GREATER
/// <summary>Configure a typed computed async return value using the actual method parameters. The returned task is handed back as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IDialogService_UpdateDialogAsync_M0_MockCall<TData> Returns(global::System.Func<string, global::DialogParameters<TData>, global::System.Threading.Tasks.Task<global::IDialogReference?>> factory)
{
EnsureSetup().ReturnsRaw(args => (object?)factory((string)args[0]!, (global::DialogParameters<TData>)args[1]!));
return this;
}
#endif

/// <summary>Execute a typed callback using the actual method parameters.</summary>
public IDialogService_UpdateDialogAsync_M0_MockCall<TData> Callback(global::System.Action<string, global::DialogParameters<TData>> callback)
{
Expand Down Expand Up @@ -694,6 +714,11 @@ public sealed class IDialogService_ShowDialogAsync_M1_MockCall<TDialog> : global
public IDialogService_ShowDialogAsync_M1_MockCall<TDialog> ReturnsAsync(global::System.Threading.Tasks.Task<global::IDialogReference> task) { EnsureSetup().ReturnsRaw(task); return this; }
/// <summary>Return a pre-built Task from a factory, invoked on each call.</summary>
public IDialogService_ShowDialogAsync_M1_MockCall<TDialog> ReturnsAsync(global::System.Func<global::System.Threading.Tasks.Task<global::IDialogReference>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#if NET9_0_OR_GREATER
/// <summary>Return a Task from a factory, invoked on each call. The Task is returned as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IDialogService_ShowDialogAsync_M1_MockCall<TDialog> Returns(global::System.Func<global::System.Threading.Tasks.Task<global::IDialogReference>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#endif

/// <summary>Configure a typed computed return value using the actual method parameters.</summary>
public IDialogService_ShowDialogAsync_M1_MockCall<TDialog> Returns(global::System.Func<object, global::DialogParameters, global::IDialogReference> factory)
Expand All @@ -709,6 +734,16 @@ public sealed class IDialogService_ShowDialogAsync_M1_MockCall<TDialog> : global
return this;
}

#if NET9_0_OR_GREATER
/// <summary>Configure a typed computed async return value using the actual method parameters. The returned task is handed back as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IDialogService_ShowDialogAsync_M1_MockCall<TDialog> Returns(global::System.Func<object, global::DialogParameters, global::System.Threading.Tasks.Task<global::IDialogReference>> factory)
{
EnsureSetup().ReturnsRaw(args => (object?)factory((object)args[0]!, (global::DialogParameters)args[1]!));
return this;
}
#endif

/// <summary>Execute a typed callback using the actual method parameters.</summary>
public IDialogService_ShowDialogAsync_M1_MockCall<TDialog> Callback(global::System.Action<object, global::DialogParameters> callback)
{
Expand Down
40 changes: 40 additions & 0 deletions tests/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Async_Methods.verified.txt
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Expand Up @@ -299,6 +299,11 @@ public sealed class IAsyncService_GetValueAsync_M0_MockCall : global::TUnit.Mock
public IAsyncService_GetValueAsync_M0_MockCall ReturnsAsync(global::System.Threading.Tasks.Task<string> task) { EnsureSetup().ReturnsRaw(task); return this; }
/// <summary>Return a pre-built Task from a factory, invoked on each call.</summary>
public IAsyncService_GetValueAsync_M0_MockCall ReturnsAsync(global::System.Func<global::System.Threading.Tasks.Task<string>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#if NET9_0_OR_GREATER
/// <summary>Return a Task from a factory, invoked on each call. The Task is returned as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IAsyncService_GetValueAsync_M0_MockCall Returns(global::System.Func<global::System.Threading.Tasks.Task<string>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#endif

/// <summary>Configure a typed computed return value using the actual method parameters.</summary>
public IAsyncService_GetValueAsync_M0_MockCall Returns(global::System.Func<string, string> factory)
Expand All @@ -314,6 +319,16 @@ public sealed class IAsyncService_GetValueAsync_M0_MockCall : global::TUnit.Mock
return this;
}

#if NET9_0_OR_GREATER
/// <summary>Configure a typed computed async return value using the actual method parameters. The returned task is handed back as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IAsyncService_GetValueAsync_M0_MockCall Returns(global::System.Func<string, global::System.Threading.Tasks.Task<string>> factory)
{
EnsureSetup().ReturnsRaw(args => (object?)factory((string)args[0]!));
return this;
}
#endif

/// <summary>Execute a typed callback using the actual method parameters.</summary>
public IAsyncService_GetValueAsync_M0_MockCall Callback(global::System.Action<string> callback)
{
Expand Down Expand Up @@ -398,6 +413,11 @@ public sealed class IAsyncService_DoWorkAsync_M1_MockCall : global::TUnit.Mocks.
public IAsyncService_DoWorkAsync_M1_MockCall ReturnsAsync(global::System.Threading.Tasks.Task task) { EnsureSetup().ReturnsRaw(task); return this; }
/// <summary>Return a pre-built Task from a factory, invoked on each call.</summary>
public IAsyncService_DoWorkAsync_M1_MockCall ReturnsAsync(global::System.Func<global::System.Threading.Tasks.Task> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#if NET9_0_OR_GREATER
/// <summary>Return a Task from a factory, invoked on each call. The Task is returned as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IAsyncService_DoWorkAsync_M1_MockCall Returns(global::System.Func<global::System.Threading.Tasks.Task> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#endif

// ICallVerification
/// <inheritdoc />
Expand Down Expand Up @@ -474,6 +494,11 @@ public sealed class IAsyncService_ComputeAsync_M2_MockCall : global::TUnit.Mocks
public IAsyncService_ComputeAsync_M2_MockCall ReturnsAsync(global::System.Threading.Tasks.ValueTask<int> task) { EnsureSetup().ReturnsRaw(task); return this; }
/// <summary>Return a pre-built ValueTask from a factory, invoked on each call.</summary>
public IAsyncService_ComputeAsync_M2_MockCall ReturnsAsync(global::System.Func<global::System.Threading.Tasks.ValueTask<int>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#if NET9_0_OR_GREATER
/// <summary>Return a ValueTask from a factory, invoked on each call. The ValueTask is returned as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IAsyncService_ComputeAsync_M2_MockCall Returns(global::System.Func<global::System.Threading.Tasks.ValueTask<int>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#endif

/// <summary>Configure a typed computed return value using the actual method parameters.</summary>
public IAsyncService_ComputeAsync_M2_MockCall Returns(global::System.Func<int, int> factory)
Expand All @@ -489,6 +514,16 @@ public sealed class IAsyncService_ComputeAsync_M2_MockCall : global::TUnit.Mocks
return this;
}

#if NET9_0_OR_GREATER
/// <summary>Configure a typed computed async return value using the actual method parameters. The returned task is handed back as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IAsyncService_ComputeAsync_M2_MockCall Returns(global::System.Func<int, global::System.Threading.Tasks.ValueTask<int>> factory)
{
EnsureSetup().ReturnsRaw(args => (object?)factory((int)args[0]!));
return this;
}
#endif

/// <summary>Execute a typed callback using the actual method parameters.</summary>
public IAsyncService_ComputeAsync_M2_MockCall Callback(global::System.Action<int> callback)
{
Expand Down Expand Up @@ -575,6 +610,11 @@ public sealed class IAsyncService_InitializeAsync_M3_MockCall : global::TUnit.Mo
public IAsyncService_InitializeAsync_M3_MockCall ReturnsAsync(global::System.Threading.Tasks.ValueTask task) { EnsureSetup().ReturnsRaw(task); return this; }
/// <summary>Return a pre-built ValueTask from a factory, invoked on each call.</summary>
public IAsyncService_InitializeAsync_M3_MockCall ReturnsAsync(global::System.Func<global::System.Threading.Tasks.ValueTask> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#if NET9_0_OR_GREATER
/// <summary>Return a ValueTask from a factory, invoked on each call. The ValueTask is returned as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IAsyncService_InitializeAsync_M3_MockCall Returns(global::System.Func<global::System.Threading.Tasks.ValueTask> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#endif

/// <summary>Execute a typed callback using the actual method parameters.</summary>
public IAsyncService_InitializeAsync_M3_MockCall Callback(global::System.Action<global::System.Threading.CancellationToken> callback)
Expand Down
15 changes: 15 additions & 0 deletions tests/TUnit.Mocks.SourceGenerator.Tests/Snapshots/Interface_With_Mixed_Members.verified.txt
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Expand Up @@ -313,6 +313,11 @@ public sealed class IService_GetAsync_M3_MockCall : global::TUnit.Mocks.Verifica
public IService_GetAsync_M3_MockCall ReturnsAsync(global::System.Threading.Tasks.Task<string> task) { EnsureSetup().ReturnsRaw(task); return this; }
/// <summary>Return a pre-built Task from a factory, invoked on each call.</summary>
public IService_GetAsync_M3_MockCall ReturnsAsync(global::System.Func<global::System.Threading.Tasks.Task<string>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#if NET9_0_OR_GREATER
/// <summary>Return a Task from a factory, invoked on each call. The Task is returned as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IService_GetAsync_M3_MockCall Returns(global::System.Func<global::System.Threading.Tasks.Task<string>> taskFactory) { EnsureSetup().ReturnsRaw(() => (object?)taskFactory()); return this; }
#endif

/// <summary>Configure a typed computed return value using the actual method parameters.</summary>
public IService_GetAsync_M3_MockCall Returns(global::System.Func<int, string> factory)
Expand All @@ -328,6 +333,16 @@ public sealed class IService_GetAsync_M3_MockCall : global::TUnit.Mocks.Verifica
return this;
}

#if NET9_0_OR_GREATER
/// <summary>Configure a typed computed async return value using the actual method parameters. The returned task is handed back as-is, so an async factory stays pending until it completes.</summary>
[global::System.Runtime.CompilerServices.OverloadResolutionPriority(-1)]
public IService_GetAsync_M3_MockCall Returns(global::System.Func<int, global::System.Threading.Tasks.Task<string>> factory)
{
EnsureSetup().ReturnsRaw(args => (object?)factory((int)args[0]!));
return this;
}
#endif

/// <summary>Execute a typed callback using the actual method parameters.</summary>
public IService_GetAsync_M3_MockCall Callback(global::System.Action<int> callback)
{
Expand Down
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