Nullable types (C# Programming Guide)

Nullable types are instances of the System.Nullable<T> struct. Nullable types can represent all the values of an underlying type T, and an additional null value. The underlying type T can be any non-nullable value type. T cannot be a reference type.

For example, you can assign null or any integer value from Int32.MinValue to Int32.MaxValue to a Nullable<int> and true, false, or null to a Nullable<bool>.

You use a nullable type when you need to represent the undefined value of an underlying type. A Boolean variable can have only two values: true and false. There is no "undefined" value. In many programming applications, most notably database interactions, a variable value can be undefined or missing. For example, a field in a database may contain the values true or false, or it may contain no value at all. You use a Nullable<bool> type in that case.

Nullable types have the following characteristics:

  • Nullable types represent value-type variables that can be assigned the null value. 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<T>. The two forms are interchangeable.

  • Assign a value to a nullable type just as you would for an underlying value type: int? x = 10; or double? d = 4.108;. You also can assign the null value: int? x = null;.

  • Use the Nullable<T>.HasValue and Nullable<T>.Value readonly properties to test for null and retrieve the value, as shown in the following example: if (x.HasValue) y = x.Value;

  • You can also use the == and != operators with a nullable type, as shown in the following example: if (x != null) y = x.Value;. If a and b are both null, a == b evaluates to true.

  • Beginning with C# 7.0, you can use pattern matching to both examine and get a value of a nullable type: if (x is int xValue) y = xValue;.

  • The default value of T? is an instance whose HasValue property returns false.

  • Use the GetValueOrDefault() method to return either the assigned value, or the default value of the underlying value type if the value of the nullable type is null.

  • Use the GetValueOrDefault(T) method to return either the assigned value, or the provided default value if the value of the nullable type is null.

  • Use the null-coalescing operator, ??, to assign a value to an underlying type based on a value of the nullable type: int? x = null; int y = x ?? -1;. In the example, since x is null, the result value of y is -1.

  • If a user-defined conversion is defined between two data types, the same conversion can also be used with the nullable versions of these data types.

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

For more information, see the Using nullable types and How to: Identify a nullable type topics.

See also

System.Nullable<T>
System.Nullable
?? Operator
C# Programming Guide
C# Guide
C# Reference
Nullable Value Types (Visual Basic)