dotnet/BenchmarkDotNet · error · ArgumentException

The optionalLocalThis should be null for static calls

Error message

The optionalLocalThis should be null for static calls

What it means

EmitCallCore (internal) throws ArgumentException when emitting a static call (methodToCall.IsStatic) but optionalLocalThis is non-null. A static call has no 'this', so providing a local for 'this' is contradictory and would produce invalid IL.

Source

Thrown at src/BenchmarkDotNet/Helpers/Reflection.Emit/IlGeneratorCallExtensions.cs:81

            MethodInfo methodToCall,
            IEnumerable<LocalBuilder> argLocals,
            bool forceDirectCall = false)
        {
            if (methodToCall.DeclaringType == null)
                throw new ArgumentException($"The {nameof(methodToCall)} should have non-null {nameof(methodToCall.DeclaringType)}.");

            /*
                IL_0000: call class [mscorlib]System.Threading.Tasks.Task [mscorlib]System.Threading.Tasks.Task::get_CompletedTask()
                -or-
                IL_0007: callvirt instance valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter`1<!0> class [mscorlib]System.Threading.Tasks.Task`1<int32>::GetAwaiter()
                --or-
                IL_000d: ldloca.s 0
                IL_000f: call instance !0 valuetype [mscorlib]System.Runtime.CompilerServices.TaskAwaiter`1<int32>::GetResult()
            */
            if (methodToCall.IsStatic)
            {
                if (optionalLocalThis != null)
                    throw new ArgumentException($"The {nameof(optionalLocalThis)} should be null for static calls", nameof(optionalLocalThis));

                ilBuilder.EmitLdLocals(argLocals);
                ilBuilder.Emit(OpCodes.Call, methodToCall);
            }
            else if (methodToCall.DeclaringType.IsValueType)
            {
                ArgumentNullException.ThrowIfNull(optionalLocalThis);

                ilBuilder.EmitLdloca(optionalLocalThis);
                ilBuilder.EmitLdLocals(argLocals);
                ilBuilder.Emit(OpCodes.Call, methodToCall);
            }
            else
            {
                if (optionalLocalThis != null)
                    ilBuilder.EmitLdloc(optionalLocalThis);

                ilBuilder.EmitLdLocals(argLocals);

View on GitHub (pinned to b515068b61)

Solutions

1. Retry with a simpler benchmark method (plain instance method) to avoid the misrouted emit path.
2. Upgrade BenchmarkDotNet; report a reproducible issue if it continues.

try { runner.Run(); }
catch (ArgumentException ex) when (ex.Message.Contains("should be null for static calls")) { /* simplify benchmark, report issue */ }

When it happens

Trigger: The emitter routes to the static-call branch while a LocalBuilder for 'this' was also supplied, i.e. an internal caller passed conflicting arguments.

Common situations: Library codegen bug misclassifying a call as static while still supplying a this-local; not user-facing configuration.

Related errors

• Method {methodToCall} should be static method.
• The methodToCall should have non-null DeclaringType.
• Method {methodToCall} should be instance method.
• Can not emit base call for {constructorBuilder}
• The constructorBuilder should have non-null DeclaringType.

AI-assisted analysis of dotnet/BenchmarkDotNet@b515068b61 (2026-08-13). Data as JSON: /api/errors/855573c4ebe92354. Report an issue: GitHub.