Nullable Types

Nullable types are instances of the Nullable struct. A nullable type can represent the correct range of values for its underlying value type, plus an additional null value. For example, a Nullable<Int32>, pronounced "Nullable of Int32," can be assigned any value from -2147483648 to 2147483647, or it can be assigned the null value. A Nullable<bool> can be assigned the values true false, or null. The ability to assign null to numeric and Boolean types is especially useful when you are dealing with databases and other data types that contain elements that may not be assigned a value. For example, a Boolean field in a database can store the values true or false, or it may be undefined.

C#

class NullableExample
{
    static void Main()
    {
        int? num = null;

        // Is the HasValue property true? 
        if (num.HasValue)
        {
            System.Console.WriteLine("num = " + num.Value);
        }
        else
        {
            System.Console.WriteLine("num = Null");
        }

        // y is set to zero 
        int y = num.GetValueOrDefault();

        // num.Value throws an InvalidOperationException if num.HasValue is false 
        try
        {
            y = num.Value;
        }
        catch (System.InvalidOperationException e)
        {
            System.Console.WriteLine(e.Message);
        }
    }
}

The example will display the output:

num = Null

Nullable object must have a value.

Nullable Types Overview

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, 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. A nullable type can also be assigned the value null: int? x = null.
• Use the Nullable.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, as shown in the following 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 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, as shown in the following 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;