Nullable Types (C# Programming Guide)

Source- MSDN.Miscrosoft.com

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. 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. The ability to assign null to numeric and Boolean types is particularly useful when dealing with databases and other data types containing 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;
        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.

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 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 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<Nullable<int>> n;