null in C#

From Microsoft MDSN:

The null keyword is a literal that represents a null reference, one that does not refer to any object. null is the default value of reference-type variables. Ordinary value types cannot be null. However, C# 2.0 introduced nullable value types. See Nullable Types (C# Programming Guide).

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There are three things in C# that "null" can be. A reference, a pointer, and a nullable type.

• The implementation of C# on the CLR represents a null reference by zero bits
• The a null pointer is represented also by  zero bits
•  A null nullable value type is also implemented by zeroes, though in a different way. When you say

int? j = null;
what we actually create is the moral equivalent of:
struct NullableInt
{
    int value;
    bool hasValue;
}


The following example demonstrates use of nullable data types:

using System;
namespace CalculatorApplication
{
   class NullablesAtShow
   {
      static void Main(string[] args)
      {
         int? num1 = null;
         int? num2 = 45;
         double? num3 = new double?();
         double? num4 = 3.14157;
         
         bool? boolval = new bool?();

         // display the values
         
         Console.WriteLine("Nullables at Show: {0}, {1}, {2}, {3}", 
                            num1, num2, num3, num4);
         Console.WriteLine("A Nullable boolean value: {0}", boolval);
         Console.ReadLine();

      }
   }
}

When the above code is compiled and executed, it produces following result:

Nullables at Show: , 45,  , 3.14157
A Nullable boolean value:            

The Null Coalescing Operator (??)

The null coalescing operator is used with the nullable value types and reference types. It is used for converting an operand to the type of another nullable( or not) value type operand, where an implicit conversion is possible.

If the value of the first operand is null, then the operator returns the value of the second operand, otherwise it returns the value of the first operand. The following example explains this:
using System;
namespace CalculatorApplication
{
   class NullablesAtShow
   {
         
      static void Main(string[] args)
      {
         
         double? num1 = null;
         double? num2 = 3.14157;
         double num3;
         num3 = num1 ?? 5.34;      
         Console.WriteLine(" Value of num3: {0}", num3);
         num3 = num2 ?? 5.34;
         Console.WriteLine(" Value of num3: {0}", num3);
         Console.ReadLine();

      }
   }
}

When the above code is compiled and executed, it produces following result:
Value of num3: 5.34
Value of num3: 3.14157

DBNull

DBNull.Value is what the .NET Database providers return to represent a null entry in the database. DBNull.Value is not null and comparissons to null for column values retrieved from a database row will not work, you should always compare to DBNull.Value.

DBNull Class