Null Reference Exception?

Sup /prog/

Im trying to create a multi-client server in C#. I'm making it access an ArrayList of the same class. (I just started C# a week ago and are trying to gain experience)

Anyways, once it gets to the clientThread (I pass the index of the ArrayList in which is contained TcpClient). I get a nullreference exception.

Why?

>>13
The "book" you are in a search for is called "MSDN", freely available over the Internet or for a local installation.

If your previous experience was limited to men, or maybe even the stuff like Python documentation, you might find this idea strange at first.