AppDomain testDomain = AppDomain.CreateDomain("testDomain");
Assembly testUtils;
using (FileStream fs = File.Open("TestUtility.dll", FileMode.Open,FileAccess.Read,FileShare.Read))
{
	using (MemoryStream ms = new MemoryStream())
	{
		byte buffer = new byte;
		int read;
		while ((read = fs.Read(buffer, 0, 1024)) > 0) { ms.Write(buffer, 0, read); }
		testUtils = testDomain.Load(ms.ToArray());
	}
}
if (testUtils == null) throw new Exception("testUtils is null!");
MyClass myClassInstance = testUtils.CreateInstance("TestUtils.MyClass") as MyClass;
// use myClassInstance as you wish! The "TestUtility.dll" that contains its impl is free and can be deleted.