Object Activator / Build Instance Factory Delegate

ActivatorEx.cs

using System;
using System.Reflection;
using System.Linq.Expressions;
using System.Collections;
using System.Linq;

public class Program
{
	public static void Main()
	{
		var factory = ActivatorEx.CreateInstanceFactoryDelegate<Hello>();
		factory("hello", 777);
		
		typeof(Hello).CreateFactory<Hello>().Invoke("hello",888);
	}
}

public class Hello
{
	public Hello(string greetMessage, int age)
	{
		Console.WriteLine(greetMessage + age.ToString());
	}
}

public static class ActivatorEx
{
	public delegate T ObjectActivator<T>(params object[] args);
	public static ObjectActivator<T> GetActivator<T>(ConstructorInfo ctor)
	{
		Type type = ctor.DeclaringType;
		ParameterInfo[] paramsInfo = ctor.GetParameters();
		//create a single param of type object[]
		ParameterExpression param = Expression.Parameter(typeof (object[]), "args");
		Expression[] argsExp = new Expression[paramsInfo.Length];
		//pick each arg from the params array 
		//and create a typed expression of them
		for (int i = 0; i < paramsInfo.Length; i++)
		{
			Expression index = Expression.Constant(i);
			Type paramType = paramsInfo[i].ParameterType;
			Expression paramAccessorExp = Expression.ArrayIndex(param, index);
			Expression paramCastExp = Expression.Convert(paramAccessorExp, paramType);
			argsExp[i] = paramCastExp;
		}

		//make a NewExpression that calls the
		//ctor with the args we just created
		NewExpression newExp = Expression.New(ctor, argsExp);
		//create a lambda with the New
		//Expression as body and our param object[] as arg
		LambdaExpression lambda = Expression.Lambda(typeof (ObjectActivator<T>), newExp, param);
		//compile it
		ObjectActivator<T> compiled = (ObjectActivator<T>)lambda.Compile();
		return compiled;
	}

	public static ObjectActivator<T> CreateInstanceFactoryDelegate<T>()
	{
		ConstructorInfo ctor = typeof (T).GetConstructors().First();
		ObjectActivator<T> createdActivator = GetActivator<T>(ctor);
		return createdActivator;
	}
	
	public static ObjectActivator<T> CreateFactory<T>(this Type t){
		return CreateInstanceFactoryDelegate<T>();
	}
	
	
}