The if...else

The if...else 

if...else Statement An if statement can be followed by an optional else if...else statement, which is very useful to test various conditions using single if...else if statement. When using if, else if and else statements there are few points to keep in mind.  An if can have zero or one else's and it must come after any else if's.  An if can have zero to many else if's and they must come before the else.  Once an else if succeeds, none of the remaining else if's or else's will be tested.

Syntax 

The syntax of an if...else if...else statement in C# is:
if(boolean_expression 1)
{
   /* Executes when the boolean expression 1 is true */
}
else if( boolean_expression 2)
{
   /* Executes when the boolean expression 2 is true */
}
else if( boolean_expression 3)
{
   /* Executes when the boolean expression 3 is true */
}
else
{
   /* executes when the none of the above condition is true */
}

ex:-

using System; 

namespace DecisionMaking 

{ 

    class Program 

    { 

        static void Main(string[] args) 

        {  

            /* local variable definition */ 

            int a = 100;  

            /* check the boolean condition */ 

            if (a == 10) 

            { 

                /* if condition is true then print the following */ 

                Console.WriteLine("Value of a is 10"); 

            } 

            else if (a == 20) 

            { 

                /* if else if condition is true */ 

                Console.WriteLine("Value of a is 20"); 

            } 

            else if (a == 30) 

            { 

                /* if else if condition is true  */ 

                Console.WriteLine("Value of a is 30"); 

            } 

            else 

          { 

                /* if none of the conditions is true */ 

                Console.WriteLine("None of the values is matching"); 

            } 

            Console.WriteLine("Exact value of a is: {0}", a); 

            Console.ReadLine(); 

        } 

    } 

}