dotnet/BenchmarkDotNet · error · InvalidBenchmarkDeclarationException

{methodType} method {methodInfo.Name} is generic.\nGeneric {

Error message

{methodType} method {methodInfo.Name} is generic.\nGeneric {methodType} methods are not supported.

What it means

Thrown by AssertMethodIsNotGeneric when a benchmark or lifecycle method is a generic method (IsGenericMethod == true). BenchmarkDotNet cannot generate host code for generic methods because the type arguments cannot be determined at code-generation time. Unlike generic types (which BenchmarkSwitcher can handle via type arguments), generic methods are unsupported.

Source

Thrown at src/BenchmarkDotNet/Running/BenchmarkConverter.cs:277

            => benchmarks.Where(benchmark => filters.All(filter => filter.Predicate(benchmark))).ToImmutableArray();

        private static void AssertMethodHasCorrectSignature(string methodType, MethodInfo methodInfo)
        {
            if (methodInfo.GetParameters().Any() && !methodInfo.HasAttribute<ArgumentsAttribute>() && !methodInfo.HasAttribute<ArgumentsSourceAttribute>())
                throw new InvalidBenchmarkDeclarationException($"{methodType} method {methodInfo.Name} has incorrect signature.\nMethod shouldn't have any arguments.");
        }

        private static void AssertMethodIsAccessible(string methodType, MethodInfo methodInfo)
        {
            if (!methodInfo.IsPublic)
                throw new InvalidBenchmarkDeclarationException($"{methodType} method {methodInfo.Name} has incorrect access modifiers.\nMethod must be public.");
            /* Moved the code that verifies if DeclaringType of a given MethodInfo (a method) is publicly accessible to CompilationValidator */
        }

        private static void AssertMethodIsNotGeneric(string methodType, MethodInfo methodInfo)
        {
            if (methodInfo.IsGenericMethod)
                throw new InvalidBenchmarkDeclarationException($"{methodType} method {methodInfo.Name} is generic.\nGeneric {methodType} methods are not supported.");
        }

        private static object?[] GetValidValues(object?[] values, Type parameterType)
            => values.Select(value => Map(value, parameterType)).ToArray();

        private static object? Map(object? providedValue, Type type)
        {
            if (providedValue == null)
                return null;

            if (providedValue.GetType().IsArray)
            {
                return ArrayParam<IParam>.FromObject(providedValue);
            }
            // Usually providedValue contains all needed type information,
            // but in case of F# enum types in attributes are erased.
            // We can to restore them from types of arguments and fields.
            // See also:

View on GitHub (pinned to b515068b61)

Solutions

Remove the type parameters and make the method non-generic, using a concrete type.
If testing multiple types, create separate non-generic benchmark methods for each type.

Example fix

// before
[Benchmark]
public void Run<T>(T[] data) { /* ... */ }

// after
[Benchmark]
public void Run(int[] data) { /* ... */ }

[Benchmark]
public void RunString(string[] data) { /* ... */ }

Defensive patterns

Strategy: validation

Validate before calling

var method = typeof(MyBench).GetMethod(nameof(MyBench.Run))!;
if (method.IsGenericMethod)
    throw new InvalidOperationException($"{method.Name} is generic and cannot be a benchmark.");

Prevention

Never declare generic benchmark methods; use separate concrete methods per type.
If testing multiple types, create distinct [Benchmark] methods or benchmark classes.
Add a validation check in a base test class that scans for generic benchmark methods.

When it happens

Trigger: Declaring [Benchmark] public void Run<T>(T value) or [GlobalSetup] public void Setup<T>() — any method with type parameters annotated with a benchmark attribute.

Common situations: Writing a generic benchmark utility method and annotating it with [Benchmark]. Refactoring a helper into a benchmark method while leaving its generic type parameter.

Related errors

AI-assisted analysis of dotnet/BenchmarkDotNet@b515068b61 (2026-08-13). Data as JSON: /api/errors/8a35dc99ff7557b6. Report an issue: GitHub.