dotnet/BenchmarkDotNet · error · InvalidBenchmarkDeclarationException

{memberInfo.Name} of type {type.Name} does not implement IEn

Error message

{memberInfo.Name} of type {type.Name} does not implement IEnumerable, unable to read values for [ParamsSource]

What it means

Thrown by ToArray when the member referenced by [ParamsSource] returns a value that does not implement IEnumerable. BenchmarkDotNet requires the source to produce a collection of values to enumerate for parameter generation; a non-enumerable return type (e.g., a single int, a custom object) cannot be iterated.

Source

Thrown at src/BenchmarkDotNet/Running/BenchmarkConverter.cs:328

                    paramsSourceMethod.Invoke(paramsSourceMethod.IsStatic ? null : Activator.CreateInstance(sourceType), null)!,
                    paramsSourceMethod,
                    sourceType));

            var paramsSourceProperty = sourceType.GetAllProperties().FirstOrDefault(property => property.Name == sourceName && property.GetMethod?.IsPublic == true);

            if (paramsSourceProperty == null)
                throw new InvalidBenchmarkDeclarationException($"{sourceType.Name} has no public, accessible method/property called {sourceName}, unable to read values for [ParamsSource]");

            return (paramsSourceProperty, ToArray(
                paramsSourceProperty.GetValue(paramsSourceProperty.GetMethod!.IsStatic ? null : Activator.CreateInstance(sourceType)!)!,
                paramsSourceProperty,
                sourceType));
        }

        private static object[] ToArray(object sourceValue, MemberInfo memberInfo, Type type)
        {
            if (!(sourceValue is IEnumerable collection))
                throw new InvalidBenchmarkDeclarationException($"{memberInfo.Name} of type {type.Name} does not implement IEnumerable, unable to read values for [ParamsSource]");

            return collection.Cast<object>().ToArray();
        }

        private static object?[] GetAllValidValues(Type parameterType)
        {
            if (parameterType == typeof(bool))
                return [false, true];

            if (parameterType.GetTypeInfo().IsEnum)
            {
                if (parameterType.GetTypeInfo().IsDefined(typeof(FlagsAttribute)))
                    return [Activator.CreateInstance(parameterType)!];

                return Enum.GetValues(parameterType).Cast<object>().ToArray();
            }

            var nullableUnderlyingType = Nullable.GetUnderlyingType(parameterType);

View on GitHub (pinned to b515068b61)

Solutions

1. Change the source member to return IEnumerable<T> or IEnumerable<object>, e.g. a list or array.
2. Wrap a single value in a collection if only one value is needed: new[] { 42 }.
3. Ensure the return type implements IEnumerable (arrays, List<T>, HashSet<T>, etc.).

Example fix

// before
public int Source => 42;
[Params]
[ParamsSource(nameof(Source))]
public int Number;

// after
public IEnumerable<int> Source => new[] { 42 };
[Params]
[ParamsSource(nameof(Source))]
public int Number;

var member = typeof(MyBench).GetProperty(nameof(MyBench.Source))?.PropertyType
    ?? typeof(MyBench).GetMethod(nameof(MyBench.Source))?.ReturnType;
if (member is null || !typeof(IEnumerable).IsAssignableFrom(member))
    throw new InvalidOperationException("ParamsSource member must return an IEnumerable.");

static bool IsEnumerableSource(Type? sourceReturnType) =>
    sourceReturnType is not null && typeof(IEnumerable).IsAssignableFrom(sourceReturnType);

When it happens

Trigger: Declaring [ParamsSource(nameof(Source))] where Source is a public property/method returning a non-IEnumerable type, e.g. public int Source => 42 or public MyCustomObject Source => new MyCustomObject().

Common situations: Misunderstanding that ParamsSource must return a collection. Accidentally returning a single value. Declaring a property that returns a custom type without implementing IEnumerable.

Related errors

• {sourceType.Name} has no public, accessible method/property
• Benchmark {benchmark.Name} has invalid type of arguments pro
• {type.Name} is generic type definition, use BenchmarkSwitche
• {methodType} method {methodInfo.Name} has incorrect signatur
• {methodType} method {methodInfo.Name} has incorrect access m

AI-assisted analysis of dotnet/BenchmarkDotNet@b515068b61 (2026-08-13). Data as JSON: /api/errors/edb57cb82cb718e0. Report an issue: GitHub.