A General Fast Method Invoker

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Introduction

Sometimes, I run across the need to dynamically invoke the method of an object, where the actual method might not be known until run-time. Usually, Reflecting is nice, but frequently doing it can be too slow. This article describes an alternative method for dynamic method invocation.

Background

When I read the article Fast Dynamic Property Accessors, I was thinking about my project, it has a lots of reflecting methods in circle. But it's methods not properties. But the DynamicMethod reminded me, maybe I could use Emit to generate a DynamicMethod to bind a special method before it can be invoked. I hope it will improve performance.

Using the Code

First, I reflected out the method which will be invoked:

MethodInfo methodInfo = typeof(Person).GetMethod("Say");

Then, I get the MethodInvoker to invoke:

FastInvokeHandler fastInvoker = GetMethodInvoker(methodInfo);
fastInvoker(new Person(), new object[]{"hello"});

Instead of using reflection method, invoke in the past:

methodInfo.Invoke(new Person(), new object[]{"hello"});

Implementation

First, I need to define a delegate to adapt the dynamic method:

public delegate object FastInvokeHandler(object target, 
                                   object[] paramters);

It looks the same as the class MethodInfo's Invoke method. Yes, that means I can write the same code to use it like in the past.

This code generates the DynamicMethod:

public static FastInvokeHandler GetMethodInvoker(MethodInfo methodInfo)
{
    DynamicMethod dynamicMethod = new DynamicMethod(string.Empty, 
                     typeof(object), new Type[] { typeof(object), 
                     typeof(object[]) }, 
                     methodInfo.DeclaringType.Module);
    ILGenerator il = dynamicMethod.GetILGenerator();
    ParameterInfo[] ps = methodInfo.GetParameters();
    Type[] paramTypes = new Type[ps.Length];
    for (int i = 0; i < paramTypes.Length; i++)
    {
        paramTypes[i] = ps[i].ParameterType;
    }
    LocalBuilder[] locals = new LocalBuilder[paramTypes.Length];
    for (int i = 0; i < paramTypes.Length; i++)
    {
        locals[i] = il.DeclareLocal(paramTypes[i]);
    }
    for (int i = 0; i < paramTypes.Length; i++)
    {
        il.Emit(OpCodes.Ldarg_1);
        EmitFastInt(il, i);
        il.Emit(OpCodes.Ldelem_Ref);
        EmitCastToReference(il, paramTypes[i]);
        il.Emit(OpCodes.Stloc, locals[i]);
    }
    il.Emit(OpCodes.Ldarg_0);
    for (int i = 0; i < paramTypes.Length; i++)
    {
        il.Emit(OpCodes.Ldloc, locals[i]);
    }
    il.EmitCall(OpCodes.Call, methodInfo, null);
    if (methodInfo.ReturnType == typeof(void))
        il.Emit(OpCodes.Ldnull);
    else
        EmitBoxIfNeeded(il, methodInfo.ReturnType);
    il.Emit(OpCodes.Ret);
    FastInvokeHandler invoder = 
      (FastInvokeHandler)dynamicMethod.CreateDelegate(
      typeof(FastInvokeHandler));
    return invoder;
}

Conclusion

Well, I think this is a general way that can be used instead of most of the reflection methods to get about 50 times performance improvement. Any suggestions for improvements are welcome.

Extra advantage (reminded by MaxGuernsey): If an exception occurs in your code, FastInovker would throw the original one, but the Method.Invoke would throw a TargetInvocationException.

History

• 2006-7-05: Updated to add static method support. Thanks Manuel Abadia.
• 2006-6-30: Updated to add ref/out parameter support. Thanks Roger for his nice suggestion.

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