/// 

/// 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 "