In C#, what is the `?` in the type `DateTime?` [duplicate]

Question

I just ran across some code while working with System.DirectoryServices.AccountManagement

public DateTime? LastLogon { get; }

What is the ? after the DateTime for.

I found a reference for the ?? Operator (C# Reference), but it's not the same thing. (280Z28: Here is the correct link for Using Nullable Types.)

Answer 1

The ? makes it a nullable type (it's shorthand for the Nullable<T> Structure and is applicable to all value types).

Nullable Types (C#)

Note:

The ?? you linked to is the null coalescing operator which is completely different.

Answer 2

The ? is not an operator in this case, it's part of the type. The syntax

DateTime?

is short for

Nullable<DateTime>

so it declares that LastLogon is a property that will return a Nullable<DateTime>. For details, see MSDN.

The ?? that you linked to is somewhat relevant here. That is the null-coalescing operator which has the following semantics. The expression

x ?? y

evaluates to y if x is null otherwise it evaluates to x. Here x can be a reference type or a nullable type.