in MM .NET Developer's Guide
Jump Start: Creating Business Objects - C#
Step 8: Enhancing the OrderDetail Business Object

There's some code lines:

public DataTable GetOrderDetail(int? orderID)

and twice

this.GetOrderDetail((int?)e.PrimaryKeyValue);

I'm not familiar with the ?, a typo yes?

[Edit]
Ah, not a typo. Allows the null check somehow. I'm not finding this idiom/operator on the net or MSDN. Can someone tell me the name? Give me a pointer to this? Thanks!

{Edit]
Found it: C# 2.0 Nullable type modifier.