Use Nullable Types to Assign a Null Value to Value-type Variables

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;
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