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?

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Subject: C# K&K equivalent
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I'm looking for a book that covers the C# standard library, just like K&K does with C.