Note: This article was originally published in 2010. Some steps, commands, or software versions may have changed. Check the current .Net documentation for the latest information.
Recently when working with the Entity Framework I had a parameter marked as ‘Guid?’, after reasearching the Internet this is what I came up with as to what a type is with a ’?’ (question mark) character at the end: ? is use to assign null for premitive data types like int, string, Guid, etc. So for example, you could do the following: int? i = null; However not this: int i = null; The last one would cause a compile error. So, why would you use this? Well, for the entity framework it makes sense as a non-initialized property is hard to identify. For example, in the case of an integer (Say property age) zero could be a valid value (just born), however, how do you distinguish between a valid zero and a non initialized integer? That’s when the null is helpful.