Suppose you want to have a primitive type with a null (or an unknown) value. This is where you would use a nullable type.

Nullable types have the following characteristics:

• They 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 System.Nullable&ltT&gt, where T is a value type. The two forms are interchangeable.
• You can assign a value to a nullable type in the same way you’d assign a value for an ordinary value type. For example:

  int? x = 10; or double? d = 4.108;

• Use the System.Nullable.GetValueOrDefault property 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 a nullable type variable sets HasValue to false. The Value is undefined.
• 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> n;