is operator

In this chapter you will learn:

Get to know is operator
How to use is operator with interface
Use 'is' to avoid an invalid cast
Runtime check with in keyword

Using is operator

is operator tells if a reference is a certain class. We can determine whether an object is of a certain type by using the "is" operator.

Its general form is shown here:

expr is type

using System;//  ja  va2  s  . c om
class Person
{
    public string name;
}
class Employee : Person
{
    public string companyName;
}

class Program
{
    static void Main(string[] args)
    {
        Person p = new Employee();

        Console.WriteLine(p is Employee);
    }
}

The output:

is interface

The following code shows how to use the is Keyword to Work with an Interface.

using System;/* j  a  v  a  2  s .  com*/
   
public interface IPrintMessage
{
    void Print();
};
   
class Class1
{
    public void Print()
    {
        Console.WriteLine("Hello from Class1!");
    }
}
   
class Class2 : IPrintMessage
{
    public void Print()
    {
        Console.WriteLine("Hello from Class2!");
    }
}
   
class MainClass
{
    public static void Main()
    {
        PrintClass   PrintObject = new PrintClass();
   
        PrintObject.PrintMessages();
    }
}
   
class PrintClass
{
    public void PrintMessages()
    {
        Class1      Object1 = new Class1();
        Class2      Object2 = new Class2();
   
        PrintMessageFromObject(Object1);
        PrintMessageFromObject(Object2);
    }
   
    private void PrintMessageFromObject(object obj)
    {
        if(obj is IPrintMessage)
        {
            IPrintMessage PrintMessage;
   
            PrintMessage = (IPrintMessage)obj;
            PrintMessage.Print();
        }
    }
}

is for invalid casting

using System; //from  j a va2s. c  o m
 
class A {} 
class B : A {} 
 
class CheckCast { 
  public static void Main() { 
    A a = new A(); 
    B b = new B(); 
 
    if(a is B)
      b = (B) a; 
  } 
}

Runtime check with in keyword

The following code show to choose between two overloaded methods at run-time using the 'is' keyword.

using System;//from   j  a  va 2  s. co m

public class BankAccount {
    virtual public void Withdraw() {
        Console.WriteLine("Call to BankAccount.Withdraw()");
    }
}

public class SavingsAccount : BankAccount {
    override public void Withdraw() {
        Console.WriteLine("Call to SavingsAccount.Withdraw()");
    }
}

public class MainClass {

    public static void Main(string[] strings) {
        BankAccount ba = new BankAccount();
        Test(ba);

        SavingsAccount sa = new SavingsAccount();
        Test(sa);
    }
    public static void Test(BankAccount baArgument) {
        if (baArgument is SavingsAccount) {
            SavingsAccount saArgument = (SavingsAccount)baArgument;
            saArgument.Withdraw();
        } else {
            baArgument.Withdraw();
        }
    }
}