Use of Is and As operators in C#

Introduction

Type casting is one of the unavoidable things in software development. In many situations we need to convert one object (Type) to another object (Type) and some times we get an exception like this: "Cannot implicitly convert type 'Object one' to 'object two'". To avoid this type of exceptions and check object compatibility, C# provides two operators namely is and as.

is operator

The is operator in C# is used to check the object type and it returns a bool value: true if the object is the same type and false if not.

For null objects, it returns false.

Syntax:

bool isobject = (Object is Type);

Example:

namespace IsAndAsOperators
{
    // Sample Student Class
    class Student
    {
        public int stuNo { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }

    }
    // Sample Employee Class
    class Employee
    {
        public int EmpNo { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
        public double Salary { get; set; }

    }
    class Program
    {

        static void Main(string[] args)
        {
            Student stuObj = new Student();
            stuObj.stuNo = 1;
            stuObj.Name = "Siva";
            stuObj.Age = 15;

            Employee EMPobj=new Employee();
            EMPobj.EmpNo=20;
            EMPobj.Name="Rajesh";
            EMPobj.Salary=100000;
            EMPobj.Age=25;

            // Is operator

            // Check Employee EMPobj is Student Type

            bool isStudent = (EMPobj is Student);
            System.Console.WriteLine("Empobj is a Student ?: {0}", isStudent.ToString());

            // Check Student stiObj is Student Typoe
            isStudent = (stuObj is Student);
            System.Console.WriteLine("Stuobj is a Student ?: {0}", isStudent.ToString());

            stuObj = null;
            // Check  null object Type
            isStudent = (stuObj is Student);
            System.Console.WriteLine("Stuobj(null) is a Student ?: {0}", isStudent.ToString());
            System.Console.ReadLine();
        }
    }

Output

Empobj is a Student ?: False
Stuobj is a Student ?: True 
Stuobj(null) is a Student ?: False

as operator

The as operator does the same job of is operator but the difference is instead of bool, it returns the object if they are compatible to that type, else it returns null.

Syntax:

Type obj = Object as Type;

Example:

namespace IsAndAsOperators
{
    // Sample Student Class
    class Student
    {
        public int stuNo { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }

    }
    // Sample Employee Class
    class Employee
    {
        public int EmpNo { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
        public double Salary { get; set; }

    }
    class Program
    {
        static void Main(string[] args)
        {
            Student stuObj = new Student();
            stuObj.stuNo = 1;
            stuObj.Name = "Praveen";
            stuObj.Age = 15;

            Employee EMPobj=new Employee();
            EMPobj.EmpNo=20;
            EMPobj.Name="Rajesh";
            EMPobj.Salary=100000;
            EMPobj.Age=25;     

            System.Console.WriteLine("Empobj is a Student ?: {0}", CheckAndConvertobject(EMPobj));

            System.Console.WriteLine("StuObj is a Student ?: {0}", CheckAndConvertobject(stuObj));
            System.Console.ReadLine();

        }

        public static string CheckAndConvertobject(dynamic obj)
        {
           // If obj is Type student it asign value to Stuobj else it asign null
            Student stuobj = obj as Student;
            if (stuobj != null)
                return "This is a Student and his name is " + stuobj.Name;

                return "Not a Student";         


        }
    }   
}

Output:

Empobj is a Student ?: Not a Student 
StuObj is a Student ?: This is a Student and his name is Praveen

Advantage of 'as' over 'is

In the case of is operator, to type cast, we need to do two steps:

1. Check the Type using is
2. If it’s true then Type cast

Actually this affects the performance since each and every time the CLR will walk the inheritance hierarchy, checking each base type against the specified type. To avoid this, use as it will do it in one step. Only for checking the type should we use the is operator.

History

• Created on 23-01-2013.