/ C# INTERMEDIATE

Is And As Operators In C#

Contents

C# Is Operator

C# has an interesting operator know as is. The is operator is used for execution-time type checking. It is a binary operator that returns a Boolean indicating whether or not the instance in question is in fact of the type specified as the second operand.

Here is an example:

C# Is Operator Example

using System;

class Example
{
    static void Main()
    {
        Console.WriteLine("hello, world" is string);  // True
    }
}

Obviously, this is a simple example but I am sure that the reader is capable of extrapolating more practical usages of is.

Problem With C# Is Operator

There is an inherent problem with is operator that many developers are not aware of:

Under the covers, the is operator uses a cast to make its determination.

Why is this a problem?

Because the next step most developers take is to cast the instance to the type. The is operator viewed as a sort of “safe cast” since you know that the instance is of that type. Here is an example of this practice:

using System;

class Dog
{
    public void Speak() { Console.WriteLine("Bark!"); }
}

class Example
{
    static void Main()
    {
        Object obj = new Dog();

        if (obj is Dog)
            ((Dog)obj).Speak();  // Bark!
    }
}

In this example there are actually two separate casts, the is and the explicit cast below it (inside the conditional statement). You shouldn’t cast more then once if you don’t need to do so. Also, the is check does not consider user-defined conversions (such as implicit and explicit cast operators) so is may return false for an instance that could have successfully been casted.

So what should we do instead?

One method would be to use is operator with a variable name. Like this,

using System;

class Dog
{
    public void Speak() { Console.WriteLine("Bark!"); }
}

class Example
{
    static void Main()
    {
        Object obj = new Dog();

        if (obj is Dog dog)
            dog.Speak();  // Bark!
    }
}

Or use C#’s true safe casting operator: as.

C# As Keyword

The as operator allows you to do an explicit cast on a type while avoiding any InvalidCastExceptions that may occur by assigning null to the variable if the cast fails.

Here is an example of how to use as:

As In C# Example

using System;

class Dog
{
    public void Speak() { Console.WriteLine("Bark!"); }
}

class Example
{
    static void Main()
    {
        Object obj = new Dog();

        Dog dog = obj as Dog;
        if (dog != null)
            dog.Speak();
    }
}

This example only contains one cast and also allows you to avoid any exceptions that may occur at execution time.

So, the important thing to remember is that if you feel like you need to use is, most likely you ought to use as.

However,

Be careful that you are not using casting to solve a problem that would be better solved with polymorphism. If you find that you are doing multiple checks for different types, casting, and then calling methods on those types, you should reevaluate your approach to see if a polymorphic solution would be better. Casting is a powerful tool but it can be misapplied.

Is Vs As Operator Keyword In C#

The is operator is used to check if a type of an object is compatible with the given type at the run-time,

if(obj is SomeClass)
{
     ...
}

whereas, as operator is used to perform casting between compatible reference types or Nullable types.

SomeClass someObject = obj as SomeClass;

With the “classic” method of type casting,

SomeClass someObject = (SomeClass) obj;

if the cast fails, an exception is thrown. However, with the as method, it results in null, which can be checked for, and avoid an exception being thrown.

SomeClass someObject = (obj as SomeClass) ?? new SomeClass();

Note: you can only use as with reference types, so if you are typecasting to a value type, you must still use the “classic” method.

Further Reading

  • C# Variable Types - C# is a type-safe language. Variables are declared as being of a particular type, and each variable is constrained to hold only values of its declared type. Variables can hold either value types or reference types, or they can be pointers. This lesson covers the value types and reference types.

  • Generics In C# - This article explains the Generics in C# with the help of a real-time problem and its step by step solution using c# generics. This article serves to be an in-depth post on getting started with generics in c#.

  • Writing Implicit and Explicit C# Conversion Operators by Jason Roberts - When writing custom classes we can provide behavior to allow for both explicit and implicit conversions to other types. Implicit conversion operators are those that don’t require an explicit cast. Explicit conversion operators are those that do require an explicit cast. In this article, Jason explains how to write them.

  • Nullable Types And Null Coalescing Operator C# - A reference type can have an actual value of null, meaning it has no value. A value type can’t have a value of null. This is why Nullables were added to the .Net Framework. This C# article discusses the Nullables with Null Coalescing operator and also explains the unique ways to use the Null Coalescing operator.

shadman_kudchikar

Shadman Kudchikar

I'm a passionate software developer living in Mumbai, India. I am a technology enthusiast and enjoy developing with all technologies, especially with .NET.

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