this.ContentPlaceHolder.Controls.Clear();
this.ContentPlaceHolder.Controls.Add
    (
    LoadControl("modules/TestControl.ascx")
    );

Einführung as-Operator

object o = new MyDemoClass();
MyDemoClass mdc = o as MyDemoClass;

object o = new MyDemoClass();
MyDemoClass mdc = (MyDemoClass)o;

object o = new MyDemoClass();
MyDemoClass demoClass = 
  (o is MyDemoClass) ? (MyDemoClass)o : null;

Performance

using System;
using System.Collections.Generic;
using System.Text;
using System.Diagnostics;

namespace AsTest
{
    class Program
    {
        static void Main(string[] args)
        {
            Stopwatch sw = new Stopwatch();

            object o = new MyDemoClass();

            sw.Start();

            for (int i = 0; i < 1000000000; i++) 
            {
                MyDemoClass mdc = o as MyDemoClass;
            }

            sw.Stop();
            Console.WriteLine("as: " + 
                sw.ElapsedMilliseconds);
            
            sw.Reset();
            sw.Start();

            for (int i = 0; i < 1000000000; i++)
            {
                MyDemoClass mdc = (MyDemoClass)o;
            }

            sw.Stop();
            Console.WriteLine("cast: " + 
                sw.ElapsedMilliseconds);

            Console.ReadKey();
        }
    }

    public class MyDemoClass 
    {
    }
}

as: 6135
cast: 5583

Fazit