as operator

In this chapter you will learn:

What is as operator and how to use as operator
How to use as operator with interface

Using as operator

as operator does the down cast. Rather than throws exception out it assigns the reference to null.

The general form:

expr as type

expr is the expression being cast to type. On succeed, a reference to type is returned. Otherwise, a null reference is returned.

using System;//from  j  av  a2s.c o m
class Person
{
    public string name;
}
class Employee : Person
{
    public string companyName;
}

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

        Employee e = p as Employee;
       
    }
}

After the as operator we can use if statement to check the result.

using System;// j  a v  a  2 s .co m
class Person
{
    public string name;
}
class Employee : Person
{
    public string companyName;
}

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

        Employee e = p as Employee;

        if (e == null)
        {
            Console.WriteLine("successful");

        }
    }
}

The output:

as an interface

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

using System;/* ja v  a2 s. c om*/
   
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)
    {
        IPrintMessage PrintMessage;
   
        PrintMessage = obj as IPrintMessage;
        if(PrintMessage != null)
            PrintMessage.Print();
    }
}