null C# with Example

A variable of a reference type can hold either a valid reference to an instance or a null reference. The null reference 
is the default value of reference type variables, as well as nullable value types. 
null is the keyword that represents a null reference. 
As an expression, it can be used to assign the null reference to variables of the aforementioned types: 
object a = null; 
string b = null; 
int? c = null; 
List d = null; 
Non-nullable value types cannot be assigned a null reference. All the following assignments are invalid: 
int a = null; 
float b = null; 
decimal c = null; 
The null reference should not be confused with valid instances of various types such as: 
an empty list (new List()) 
an empty string ("") 
0
the number zero ( , 0f, 0m) 
the null character ( '\0' ) 
Sometimes, it is meaningful to check if something is either null or an empty/default object. The 
System.String.IsNullOrEmpty(String) method may be used to check this, or you may implement your own equivalent 
method. 
private void GreetUser(string userName) 
{ 
if (String.IsNullOrEmpty(userName)) 
{ 
//The method that called us either sent in an empty string, or they sent us a null reference. 
Either way, we need to report the problem. 
throw new InvalidOperationException("userName may not be null or empty."); 
} 
else 
{ 
//userName is acceptable. 
Console.WriteLine("Hello, " + userName + "!"); 
} 
 

}