dotnet/BenchmarkDotNet · error · NotSupportedException

Default BenchmarkActionFactory does not support returning aw

Error message

Default BenchmarkActionFactory does not support returning awaitable types except (Value)Task(<T>).

What it means

The default InProcess NoEmit BenchmarkActionFactory only knows how to await (Value)Task(<T>); benchmarks returning other awaitable types throw NotSupportedException.

Source

Thrown at src/BenchmarkDotNet/Toolchains/InProcess/NoEmit/BenchmarkActionFactory.cs:78

                        ? typeof(BenchmarkActionBlockingTask<>).MakeGenericType(argType)
                        : typeof(BenchmarkActionTask<>).MakeGenericType(argType),
                    resultInstance,
                    targetMethod,
                    unrollFactor);

            if (typeof(ValueTask<>) == genericType)
                return Create(
                    consumeTasksSynchronously
                        ? typeof(BenchmarkActionBlockingValueTask<>).MakeGenericType(argType)
                        : typeof(BenchmarkActionValueTask<>).MakeGenericType(argType),
                    resultInstance,
                    targetMethod,
                    unrollFactor);
        }

        if (resultType.IsAwaitable(out _))
        {
            throw new NotSupportedException($"Default {nameof(BenchmarkActionFactory)} does not support returning awaitable types except (Value)Task(<T>).");
        }

        if (resultType.IsAsyncEnumerable(out var asyncEnumerableInfo))
        {
            var asyncEnumerableInterface = typeof(IAsyncEnumerable<>).MakeGenericType(asyncEnumerableInfo.ItemType);
            if (asyncEnumerableInterface.IsAssignableFrom(resultType))
            {
                return Create(
                    typeof(BenchmarkActionAsyncEnumerable<>).MakeGenericType(asyncEnumerableInfo.ItemType),
                    resultInstance,
                    targetMethod,
                    unrollFactor);
            }
            if (resultType.IsGenericType && resultType.GetGenericTypeDefinition() == typeof(ConfiguredCancelableAsyncEnumerable<>))
            {
                return Create(
                    typeof(BenchmarkActionConfiguredCancelableAsyncEnumerable<>).MakeGenericType(asyncEnumerableInfo.ItemType),
                    resultInstance,

View on GitHub (pinned to b515068b61)

Solutions

1. Return Task, Task<T>, ValueTask, or ValueTask<T> from the async benchmark method.
2. For other awaitable types, run the benchmark with the default out-of-process toolchain or provide a custom BenchmarkActionFactory.

When it happens

Trigger: Thrown at src/BenchmarkDotNet/Toolchains/InProcess/NoEmit/BenchmarkActionFactory.cs:78 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of dotnet/BenchmarkDotNet@b515068b61 (2026-08-13). Data as JSON: /api/errors/5dc4287e30b28d58. Report an issue: GitHub.