Question

An open delegate is a delegate to an instance method without the target. To call it you supply the target as its first parameter. They are a clever way to optimize code that otherwise would use reflection and have poor performance. For an intro to open delegates see this. The way you would use it in practice is to have expensive reflection code to build these open delegates, but then you would be able to call them very cheaply as a simple Delegate call.

I’m trying to write code that will transform an arbitrary PropertyInfo, into such an delegate for its setter. So far I came up with this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Reflection;

namespace Test
{
    class TestClass
    {
        static Action<T, object> MakeSetterDelegate<T>(PropertyInfo property)
        {
            MethodInfo setMethod = property.GetSetMethod();
            if (setMethod != null && setMethod.GetParameters().Length == 1) //skips over nasty index properties
            {
                //To be able to bind to the delegate we have to create a delegate 
                //type like: Action<T,actualType> rather than Action<T,object>.
                //We use reflection to do that
                Type setterGenericType = typeof(Action<,>);
                Type delegateType = setterGenericType.MakeGenericType(new Type[] { typeof(T), property.PropertyType });
                var untypedDelegate = Delegate.CreateDelegate(delegateType, setMethod);

                //we wrap the Action<T,actualType> delegate into an Action<T,object>
                Action<T, object> setter = (instance, value) =>
                {
                    untypedDelegate.DynamicInvoke(new object[] { instance, value });
                };
                return setter;
            }
            else
            {
                return null;
            }
        }

        int TestProp 
        {
            set
            {
                System.Diagnostics.Debug.WriteLine("Called set_TestProp");
            }
        }

        static void Test() 
        {
            PropertyInfo property = typeof(TestClass).GetProperty("TestProp");
            Action<TestClass, object> setter = MakeSetterDelegate<TestClass>(property);
            TestClass instance = new TestClass();
            setter(instance, 5);
        }
    }
}

Similar code would be written for the getter. It works, but the setter delegate uses a DynamicInvoke to convert from an Action<derivedType> to Action<object>, which I suspect is eating a good part of the optimization I’m after. So the questions are:

1. Is the DynamicInvoke a real concern?
2. Is there anyway around it?

Answer 1

DynamicInvoke will not make a performant setter. Reflection against a generic inner type is your better option here, as this will allow you to use typed delegates. Another option is DynamicMethod, but then you need to worry about a few IL details.

You might want to look at HyperDescriptor, which wraps up the IL work into a PropertyDescriptor implementation. Another option is the Expression API (if you are using .NET 3.5 or above):

static Action<T, object> MakeSetterDelegate<T>(PropertyInfo property)
{
    MethodInfo setMethod = property.GetSetMethod();
    if (setMethod != null && setMethod.GetParameters().Length == 1)
    {
        var target = Expression.Parameter(typeof(T));
        var value = Expression.Parameter(typeof(object));
        var body = Expression.Call(target, setMethod,
            Expression.Convert(value, property.PropertyType));
        return Expression.Lambda<Action<T, object>>(body, target, value)
            .Compile();
    }
    else
    {
        return null;
    }
}

Or alternatively with a generic type:

    abstract class Setter<T>
    {
        public abstract void Set(T obj, object value);
    }
    class Setter<TTarget, TValue> : Setter<TTarget>
    {
        private readonly Action<TTarget, TValue> del;
        public Setter(MethodInfo method)
        {
            del = (Action<TTarget, TValue>)
                Delegate.CreateDelegate(typeof(Action<TTarget, TValue>), method);
        }
        public override void Set(TTarget obj, object value)
        {
            del(obj, (TValue)value);
        }

    }
    static Action<T, object> MakeSetterDelegate<T>(PropertyInfo property)
    {
        MethodInfo setMethod = property.GetSetMethod();
        if (setMethod != null && setMethod.GetParameters().Length == 1)
        {
            Setter<T> untyped = (Setter<T>) Activator.CreateInstance(
                typeof(Setter<,>).MakeGenericType(typeof(T),
                property.PropertyType), setMethod);
            return untyped.Set;
        }
        else
        {
            return null;
        }
    }