Consider the program:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class EMP
    {
        private string name ;
        private int age     ;
        private int salary  ;

        public void setEmp(string name, int a = 18, int salary = 20000) 
        {
            this.name   = name      ;
            this.age    = a       ;
            this.salary = salary    ;


        }

        public void printEmp()
        {
            Console.WriteLine("\nEmployee Record: ");
            Console.WriteLine("\tName  : " + name   );
            Console.WriteLine("\tAge   : " + age    );
            Console.WriteLine("\tSalary: " + salary );

        }		
    }
    class Program
    {
        static void Main()
        {
            EMP E1 = new EMP();

            E1.setEmp("Sandy",25, salary: 48500);
            E1.printEmp();

            EMP E2 = new EMP();

            E2.setEmp("Mark", a:33,34000);
            E2.printEmp();
        }
    }
}

Output

Employee Record:
        Name  : Sandy
        Age   : 25
        Salary: 48500

Employee Record:
        Name  : Mark
        Age   : 33
        Salary: 34000

In above program, we are creating a class named EMP, it contains method setEmp() which has two optional or default argument (age,salary).

With E1 object, we are using salary parameter with colon( : ) operator to assign value. While with E2 object we are using a parameter with colon( : ) to set age of employee.