curious declaration

Hello.
I've seen in a Microsoft Workshop a declaration:

Code:

---

decimal? averagePriceInRange = 0;

---

I've searched for andwers but I couldn't find what the question mark (?) is for.
Any info is appreciate.

Thanks.

It's a nullable struct. Exactly the same as writing: Nullable<decimal>

http://msdn.microsoft.com/en-us/library/b3h38hb0.aspx

Many thanks.