Type casting vs as operator in C#

Question

What is the difference between Type casting and as operator in C#. For example in this code:

class LivingThing
{
        public int NumberOfLegs { get; set; }
}
class Parrot : LivingThing
{

}

interface IMover<in T>
{
        void Move(T arg);
}

class Mover<T> : IMover<T>
{
    public void Move(T arg)
    {
       // as operator
       Console.WriteLine("Moving with" + (arg as Parrot).NumberOfLegs + " Legs");
       // Type casting
       Console.WriteLine("Moving with" + (Parrot)arg.NumberOfLegs + " Legs");
    }
}

Why the Type casting is not valid in the second situation and what is the difference of those?

Did you mean `((Parrot)arg).NumberOfLegs`? Both lines are not the same, one refers to `arg` and the other refers to `arg.NumberOfLegs`. Looks to me like a typo — Cleptus, Aug 05 '23 at 18:51

Dmitry Bychenko · Answer 1 · 2023-08-05T18:53:26.013

Well,

((Parrot) arg) cast : either Parrot instance or exception (invalid cast) thrown. Note, that you should cast arg first and only then use NumberOfLegs: Console.WriteLine("Moving with" + ((Parrot)arg).NumberOfLegs + " Legs"); note extra (...)
arg as Parrot: either Parrot instance or null (note you can't use as if type is struct and thus can't be null)
arg is Parrot: either true if arg can be cast to Parrot or false otherwise

The last case (is) can be used in pattern matching, e.g.

Console.WriteLine($"Moving with {(arg is Parrot parrot ? parrot.NumberOfLegs : 0)} Legs");

score -1 · Answer 2 · answered Aug 05 '23 at 18:59

The type casting is invalid because you should do this

 // ((Parrot)arg).NumberOfLegs
 Console.WriteLine("Moving with" + ((Parrot)arg).NumberOfLegs + " Legs");

Difference are :

Type Casting (Explicit Casting): (Parrot) if arg is not a Parrot or a type derived from Parrot. It does not perform any safety checks.
Operator: The as operator performs a conversion between reference types or nullable types, and it includes a safety check. If the cast is not valid, it returns null rather than throwing an exception. So (arg as Parrot).NumberOfLegs will attempt to convert arg to Parrot, and if arg is not a Parrot, it will return null. If you then try to access NumberOfLegs, you'll get a NullReferenceException.

Note: It's also a good practice to add generic type constraint : https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/where-generic-type-constraint

Type casting vs as operator in C#

2 Answers2