Bala's Blog

Nullable in .Net 2.0, helps to determine whether variable has been assigned a value or not. Example: Quiz application having option yes/no, but it should displayed “Not Attempted” when user does not make any choice. This will work for both Data types and data base nullables values.NameSpace use for this purpose is system.nullable.
 
In .Net Framework 2.0, value types can be extended to take either their normal values or a null value. Such an extension is called a nullable type.

Declaring a variable as nullable enables the HasValue and Value members.

-OR-
// Shorthand notation for declaring nullable type, only for C#

bool? b = null;



Example of Nullable Type
Use HasValue to detect whether or not a value has been set:

if (b.HasValue)
Console.WriteLine("User has Attempted Given Question");
else
Console.WriteLine("User has not Attempted Given Question");

Output
User has not Attempted Given Question


C# 2.0 allows you to assign null values to premitive types such as int, long, and bool. The following code shows how to define an integer as nullable type and checks if the value of the integer is null or not.

Nullable types

public static void TestNullableTypes()
{
// Syntax to define nullable types
int? counter;
counter = 100;
if (counter != null)
{
// Assign null to an integer type
counter = null;
Console.WriteLine("Counter assigned null.");
}
else
{
Console.WriteLine("Counter cannot be assigned null.");
Console.ReadLine();
}
If you remove the "?" from int? counter, you will see a warning as following: "Cannot convert null to int besause it is a value type "

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, 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> n;