Direct casting vs 'as' operator?

Question

Consider the following code:

void Handler(object o, EventArgs e)
{
   // I swear o is a string
   string s = (string)o; // 1
   //-OR-
   string s = o as str         


        

Answer 1

'as' is based on 'is', which is a keyword that checks at runtime if the object is polimorphycally compatible (basically if a cast can be made) and returns null if the check fails.

These two are equivalent:

Using 'as':

string s = o as string;

Using 'is':

if(o is string) 
    s = o;
else
    s = null;

On the contrary, the c-style cast is made also at runtime, but throws an exception if the cast cannot be made.

Just to add an important fact:

The 'as' keyword only works with reference types. You cannot do:

// I swear i is an int
int number = i as int;

In those cases you have to use casting.

Answer 2

Since nobody mentioned it, the closest to instanceOf to Java by keyword is this:

obj.GetType().IsInstanceOfType(otherObj)

Answer 3

Use direct cast string s = (string) o; if in the logical context of your app string is the only valid type. With this approach, you will get InvalidCastException and implement the principle of Fail-fast. Your logic will be protected from passing the invalid type further or get NullReferenceException if used as operator.

If the logic expects several different types cast string s = o as string; and check it on null or use is operator.

New cool feature have appeared in C# 7.0 to simplify cast and check is a Pattern matching:

if(o is string s)
{
  // Use string variable s
}

or

switch (o)
{
  case int i:
     // Use int variable i
     break;
  case string s:
     // Use string variable s
     break;
 }

Answer 4

It seems the two of them are conceptually different.

Direct Casting

Types don't have to be strictly related. It comes in all types of flavors.

• Custom implicit/explicit casting: Usually a new object is created.
• Value Type Implicit: Copy without losing information.
• Value Type Explicit: Copy and information might be lost.
• IS-A relationship: Change reference type, otherwise throws exception.
• Same type: 'Casting is redundant'.

It feels like the object is going to be converted into something else.

AS operator

Types have a direct relationship. As in:

• Reference Types: IS-A relationship Objects are always the same, just the reference changes.
• Value Types: Copy boxing and nullable types.

It feels like the you are going to handle the object in a different way.

Samples and IL

    class TypeA
    {
        public int value;
    }

    class TypeB
    {
        public int number;

        public static explicit operator TypeB(TypeA v)
        {
            return new TypeB() { number = v.value };
        }
    }

    class TypeC : TypeB { }
    interface IFoo { }
    class TypeD : TypeA, IFoo { }

    void Run()
    {
        TypeA customTypeA = new TypeD() { value = 10 };
        long longValue = long.MaxValue;
        int intValue = int.MaxValue;

        // Casting 
        TypeB typeB = (TypeB)customTypeA; // custom explicit casting -- IL:  call class ConsoleApp1.Program/TypeB ConsoleApp1.Program/TypeB::op_Explicit(class ConsoleApp1.Program/TypeA)
        IFoo foo = (IFoo)customTypeA; // is-a reference -- IL: castclass  ConsoleApp1.Program/IFoo

        int loseValue = (int)longValue; // explicit -- IL: conv.i4
        long dontLose = intValue; // implict -- IL: conv.i8

        // AS 
        int? wraps = intValue as int?; // nullable wrapper -- IL:  call instance void valuetype [System.Runtime]System.Nullable`1<int32>::.ctor(!0)
        object o1 = intValue as object; // box -- IL: box [System.Runtime]System.Int32
        TypeD d1 = customTypeA as TypeD; // reference conversion -- IL: isinst ConsoleApp1.Program/TypeD
        IFoo f1 = customTypeA as IFoo; // reference conversion -- IL: isinst ConsoleApp1.Program/IFoo

        //TypeC d = customTypeA as TypeC; // wouldn't compile
    }