Why is there a questionmark on the private variable definition?

Posted by Booser on Stack Overflow See other posts from Stack Overflow or by Booser
Published on 2010-02-24T13:08:55Z Indexed on 2010/04/07 2:53 UTC
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I am reading an article about the MVVP Pattern and how to implement it with WPF. In the source code there are multiple lines where I cannot figure out what the question marks in it stand for.

private DateTime? _value;

What does the ? mean in the definition? I tried to find it in the help from VS but failed.

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