dont-use-activator-createinstance-or-constructorinfo-invoke-use-compiled-lambda-expressions

ObjectActivator.cs

namespace SView.Global
{
  using System;
  using System.Linq.Expressions;
  using System.Reflection;

  /// <summary>
  /// https://vagifabilov.wordpress.com/2010/04/02/dont-use-activator-createinstance-or-constructorinfo-invoke-use-compiled-lambda-expressions/
  /// </summary>
  public class Activator
  {
    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
      var compiled = (ObjectActivator<T>)lambda.Compile();
      return compiled;
    }
  }
}