Can i define the TSource type in the method of Single()

Hi,

I have an issue of the Single<TSource>()

Why I can't set the TSource of the a concrete type while the collection is the astract type. The following is the example of my question.

public abstract class TestBase

{

}

public class Test : TestBase

{

    public string Code {get; set;}

}

public static void Main(string[] args)

{

    List<TestBase> tests = new List<TestBase>();

    tests.Add(new Test() { Code = "A"});

    Test theTest = tests.Single<Test>();

}

Thanks

Jobria