Example of the code, that runs in child AppDomain. Main goal is to remove shared static state between executions.

WorkInChildAppDomaon.cs

using System;
using System.Diagnostics;

public class Program
{
    public static class Runner
    {
        public static int Test1 = 0;
        public static void CodeInChildAppDomain()
        {
            Console.WriteLine("Test {0} + {1}", AppDomain.CurrentDomain.FriendlyName, ++Test1);
        }
    }

    static void Main()
    {        
        var s = new Stopwatch();
        s.Start();

        for (var i = 0; i < 1000; i++)
        {
            var appDomain = AppDomain.CreateDomain("SecondAppDomain" + i);
            appDomain.DoCallBack(() =>
            {
                Runner.CodeInChildAppDomain();
                Runner.CodeInChildAppDomain();
            });    
        }

        s.Stop();
        Console.WriteLine("Average: {0}ms", s.Elapsed.TotalMilliseconds / 1000);       
    }
}