Question Mark is a Nullable Type

One of the questions I keep seeing is what is int? or DatTime? and the like?

Well in .NET 2.0 the Nullable Type was introduced and that is what those type declarations mean. So what is a Nullable type?

 

Nullable types have the following characteristics:

• Nullable types represent value-type variables that can be assigned the value of null. You cannot create a nullable type based on a reference type. (Reference types already support the null value.)

• The syntax T? is shorthand for Nullable<(Of <(T>)>), where T is a value type. The two forms are interchangeable.

• Assign a value to a nullable type just as you would for an ordinary value type, for example int? x = 10; or double? d = 4.108. However, a nullable type can also be assigned the value null: int? x = null.

• Use the Nullable<(Of <(T>)>)..::.GetValueOrDefault method to return either the assigned value, or the default value for the underlying type if the value is null, for example int j = x.GetValueOrDefault();

• Use the HasValue and Value read-only properties to test for null and retrieve the value, for example if(x.HasValue) j = x.Value;

  • The HasValue property returns true if the variable contains a value, or false if it is null.

  • The Value property returns a value if one is assigned. Otherwise, a System..::.InvalidOperationException is thrown.

  • The default value for HasValue is false. The Value property has no default value.

  • You can also use the == and != operators with a nullable type, for example, if (x != null) y = x;

• Use the ?? operator to assign a default value that will be applied when a nullable type whose current value is null is assigned to a non-nullable type, for example int? x = null; int y = x ?? -1;

• Nested nullable types are not allowed. The following line will not compile: Nullable<Nullable<int>> n;

 

Thats about all of it. More information can be found on MSDN