gistfile1.cs

public class Foo
	{
		private readonly StreamWriter streamWriter;
		private readonly string fileName;
		private readonly string directory;

		/// <summary>
		/// my heavy constructor
		/// </summary>
		/// <param name="streamWriter"></param>
		/// <param name="fileName"></param>
		/// <param name="directory"></param>
		public Foo(StreamWriter streamWriter, string fileName, string directory )
		{
			this.streamWriter = streamWriter;
			this.fileName = fileName;
			this.directory = directory;
		}

		public virtual string DoSomething()
		{
			return "abcd";
		}

	}

	[TestFixture]
	public class TestFixture
	{
		[Test]
		public void test()
		{
			var epa = (Foo)FormatterServices.GetUninitializedObject(typeof (Foo));
			Assert.AreEqual("abcd", epa.DoSomething()); 
		}
	}