Nullable Types in C#

Nullable Types:

•  Nullable types are instances of the System. Nullable struct.
•  A Nullable type can represent the normal range of values for its underlying value type, plus an additional null value.
• 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 System. Nullable<T>, where T is a value type. The two forms are interchangeable.
• Assign a value to a Nullable type in the same way as for an ordinary value type, for example int? x = 10; or double? d = 4.108;
• 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<Nullable<int>> n;
• 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 or false, or null.

Example:

Class NullableExample

{

    Static void Main ()

    {

        int? num = null;

        If (num.HasValue == true)

        {

            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 above will display the output:

num = Null

Nullable object must have a value.

Null coalescing operator (??):

The ?? Operator is called the null-coalescing operator and is used to define a default value for a nullable value types as well as reference types. It returns the left-hand operand if it is not null; otherwise it returns the right operand.

Example:

int b = a ?? -1;   

it means if the value of a is null then b=a otherwise the value of b will be -1.

Simply this means

If (a == null)    
     b = a; 
Else 
   b = -1; 

//The?? Operator also works with reference types:

    //message = param, unless param is null

    //in which case message = "No message"

    String message = param?? "No message";

To use the null-coalescing-operator, there are some compile-time ground rules.

• The left-hand-side must evaluate to a reference or nullable type.
• All evaluated statements must be of matching type, unless they can be implicitly converted.