Nullable Types

• 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 type already support the n ull value.)
  
• The syntax T? is shorthand for Nullable<T>, 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.
  eg:- int?X = 10; OR double?D = 4.190.
  
• A nullable type can also be assigned the value null.
  eg:- int?X = null.
  
• Use the Nullable<T>.GetValueOrDefault method to return either the assigned value, or the default value for the underlying type if the value is null.
  eg:- int?X = 10;
          int J = X.GetValueOrDefault();
  
• Use the Hashvalue and Value read-only properties to test for null and retrieve the value.
  eg:- if(X.Hashvalue)J = X.Value;
  
  - The Hashvalue 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 Hashvalue is false. The value property has no default value.
  
• You can also use the '==' and '!=' operators with nullable type.
  eg:- 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.
  eg:- int?X = null';
         int Y = X ?? -1;
  
• Nested nullable types are not allowed. The following line will not compile:
  Nullable<Nullable<T>>n;