<p>I am reading a .NET book, and in one of the code examples there is a class definition with this field:</p>

<pre><code>private DateTime? startdate
</code></pre>

<p>What does <code>DateTime?</code> mean?</p>

It's a nullable DateTime.
```
?
```
after a primitive type/structure indicates that it is the nullable version.
DateTime is a structure that can never be null. From
MSDN
:
The DateTime value type represents dates and times with values ranging from 12:00:00 midnight, January 1, 0001 Anno Domini, or A.D. (also known as Common Era, or C.E.) through 11:59:59 P.M., December 31, 9999 A.D. (C.E.)
```
DateTime?
```
can be null however.