Advertisement

---

Top

< Prev

Next >

In this tutorial, we are going to discuss an important operator provided by C# language, which is known as the is operator. Similar to Java, the C# language gives us a powerful feature of runtime type identification, which allows us to determine the type of an object by using the is operator during the runtime of a program.

Syntax of the is operator

expression is type

• expression is an object whose type is being tested against the type specified in the is operator expression.

Rules of is operator -


The is operator returns the bool true value in two conditions:
• If an object is being tested against its own class type.
• If an object is being tested against a type of base class of its own class i.e. the class from which its own class is derived.
In all other cases, the is keyword, returns the bool false value.

The is operator example

In the upcoming example, we are going to create two classes A and B, where the class A has inherited from the class B. Next. we are going to make their objects and use the is operator to determine the type of these objects.

//C# is Operator Example

using System;


class A
{
}

//Class B inheriting from class A
class B : A
{
public static void Main()
{
	//Creating an object of class A
	A a = new A();


	//Creating an object of class B
	B b = new B();
	
	
	//The first expression involving the is operator
	bool result = a is A;
	Console.WriteLine("Is a an object of type A : " + result);


	//The second expression involving the is operator
	result = a is B;
	Console.WriteLine("Is a an object of type B : " + result);

	
	//The third xpression involving the is operator
	result = b is A;
	Console.WriteLine("Is b an object of type A : " + result);

	
	//The fourth expression involving the is operator
	result = b is A;
	Console.WriteLine("Is b an object of type B : " + result);
}
}

Output

Is a an object of type A : True
Is a an object of type B : False
Is b an object of type A : True
Is b an object of type B : True

Program Analysis

• bool result = a is A;

  The first is operator expression has returned true because an object of class A is tested against its own class type.





• bool result = a is B;

  The second is operator expression has returned false because an object of class A is tested against its derived class type.





• bool result = b is A;

  The third is operator expression has returned true because an object of class B is tested against its base class type.





• bool result = b is B;

  The fourth is operator expression has returned true because an object of class B is tested against its own class type.

The is operator always returns true if any object is tested against Object class.

//C# The is Operator Example, testing against the Object class i.e. mother of all class in C#

using System;


class A
{
}

//Class B inheriting from class A
class B : A
{
public static void Main()
{
	//Creating an object of class A
	A a = new A();


	//Creating an object of class B
	B b = new B();
	
	
	//The first expression involving the is operator
	bool result = a is Object;
	Console.WriteLine("Is a an object of type Object : " + result);


	//The second expression involving the is operator
	result = b is Object;
	Console.WriteLine("Is b an object of type Object : " + result);
}
}

Output:

Is a an object of type Object : True
Is b an object of type Object : True

In the last code, we have created two objects, each of class A and B and then we have checked their type against the Object class i.e. mother of all classes in C#, which, as expected turns out to be true.

Advertisement

The is operator can only be checked against the concrete initialized object.

In the upcoming example, we are going to show you how using the is operator with an uninitialized object returns a compile error.

//C# The is operator testing against uninitialized objects


using System;

class A
{
public static void Main()
{
	//Creating a reference variable object of class A i.e uninitialized object
	A a;


	//The first expression involving the is operator
	bool result = a is A;
	Console.WriteLine("Is a an object of type A : " + result);
}
}

Compile Error:

A.cs(14,16): error CS0165: Use of unassigned local variable 'a'

The is operator checked on null will always returns false.

In the upcoming example, we are going to show you how using the is operator with null always returns false.

//C# The is oprator example with null

using System;

class A
{
public static void Main()
{	
	//The second expression involving the is operator
	bool result = null is A;
	Console.WriteLine("Is null an object of type A : " + result);


	//The third expression involving the is operator
	result = null is Object;
	Console.WriteLine("Is null an object of type Object : " + result);
}
}

Output:

Is null an object of type A : False
Is null an object of type Object : False

The null is an uninitialized value and therefore checking it against any class type will return the bool false value.

Please share this article -

< Prev

Next >

< Logical Operators in C++

Scope Resolution Operator >

Advertisement