From Wikibooks, open books for an open world

The else keyword identifies a else clause of an if statement with the following syntax:

if-statement ::= "if" "(" condition ")" if-body "else" else-body

condition ::= boolean-expression

if-body ::= statement-or-statement-block

else-body ::= statement-or-statement-block

An else clause immediately follows an if-body. It provides code to execute when the condition is false. Making the else-body another if statement creates the common cascade of if, else if, else if, else if, else statements:

using System;

public class IfStatementSample
{
    public void IfMyNumberIs()
    {
        int myNumber = 5;
        if (myNumber == 4)
            Console.WriteLine("This will not be shown because myNumber is not 4.");
        else if(myNumber < 0)
        {
            Console.WriteLine("This will not be shown because myNumber is not negative.");
        }
        else if(myNumber%2 == 0)
            Console.WriteLine("This will not be shown because myNumber is not even.");
        else
        {
            Console.WriteLine("myNumber does not match the coded conditions, so this sentence will be shown!");
        }
    }
}

The above example only checks whether myNumber is less than 0, if myNumber is not 4. It in turn only checks whether myNumber%2 is 0, if myNumber is not less than 0. Since none of the conditions are true, it executes the body of the final else clause.