What is the difference between the is and as operators in C#?

C# is a class-oriented programming language mainly used for backend development. The is and as keywords are operators in C# that are noticeably different than each other.

Key differences

The following table represents the main differences between the two operators:

is

as

Used only for reference, boxing, and unboxing conversions.

Used only for null-able reference, boxing conversions.

Boolean return type.

Object/null return type.

Checks the type of an object with the given type.

Converts an object to a given type if its type is compatible with the given type.

Returns TRUE/FALSE based on the check.

Returns object or null based on the check.

Examples

Let's take a look at these significant differences via code:

The is operator

Let's look at an example of the is operator:

using System;
class HelloWorld
{
    public class obj{}
    public class obj2:obj{}
    public class obj3{}
    static void Main()
    {   
    //For built-in types 
        //     
        string msg = "Hello world!";
        Console.WriteLine(msg is string);
        Console.WriteLine(msg is char);
        Console.WriteLine(msg is int);
        Console.WriteLine(msg is double);
    //For user-defined types
        obj class1 = new obj2();
        Console.WriteLine(class1 is obj);
        Console.WriteLine(class1 is obj2);
        Console.WriteLine(class1 is obj3);
    }
}

Explanation

  • Lines 4–6: We create three classes obj, obj2, and obj3. obj2 is a derived class of obj, while obj3 is a separate non-related class.

  • Lines 11–18: We represent how the is operator works for built-in types. The comparison with the string class returns True as the match was correct, while the comparison with the other classes(char, int, double) returns False as there was a type unmatch.

  • Lines 21–27: After the declaration of the obj object(class1), we check it against obj, obj2, and obj3 types using the is operator. For obj and obj2, it returns True as obj is the same type of class1 object and obj2 is a derived class of obj. Since there is a mismatch in types for obj3, we print false.

The as operator

Let's look at an example of the as operator:

using System;
class HelloWorld
{
    public class temp{}
    static void Main()
    {   
        object[] array = new object[7];
        array[0] = 1;
        array[1] = "Hello world!";
        array[2] = 'H';
        array[3] = 2.3;
        array[4] = new temp();
        array[5] = null;
        array[6] = "Hi!";
        for(int i = 0; i < 7; i++){
            var check = array[i] as string;
            if(check != null){
                Console.WriteLine(check);
            }
            else{
                Console.WriteLine("Null!");
            }
        }
    }
}

Explanation

  • Line 4: We declare a custom type named temp class.

  • Lines 7–14: We define an object array with values of different data types.

  • Lines 16–24: A loop traverses the entire array and has a check that prints Null if the check variable does not have a string type. We print the contained string in check if the as match is correct with the current object of the array.

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