What is that question (?) sign after a type declaration?

From: "....DotNet4Ever...." <hate.spam@xxxxxxxxxxx>
Date: Wed, 18 Jun 2008 16:06:43 +0200

I have been noticing declarations like this:

private DateTime? _creationdate;

and have been wondering what is the function or meaning of that question sign after the member type name. I searched the web but naturally with such a thing in a query yields a lot of noise and no suitable results.

Can somebody enlighten me?

Cheers,
Emilio

.

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