When should you use the as keyword in C#

Question

When you want to change types most of the time you just want to use the traditional cast.

var value = (string)dictionary[key];

It's good because:

• It’s fast
• It’ll complain if something is wrong (instead of giving object is null exceptions)

So what is a good example for the use of as I couldn't really find or think of something that suits it perfectly?

Note: Actually I think sometimes there are cases where the complier prevents the use of a cast where as works (generics related?).

Answer 1

Use as when it's valid for an object not to be of the type that you want, and you want to act differently if it is. For example, in somewhat pseudo-code:

foreach (Control control in foo)
{
    // Do something with every control...

    ContainerControl container = control as ContainerControl;
    if (container != null)
    {
        ApplyToChildren(container);
    }
}

Or optimization in LINQ to Objects (lots of examples like this):

public static int Count<T>(this IEnumerable<T> source)
{
    IList list = source as IList;
    if (list != null)
    {
        return list.Count;
    }
    IList<T> genericList = source as IList<T>;
    if (genericList != null)
    {
        return genericList.Count;
    }

    // Okay, we'll do things the slow way...
    int result = 0;
    using (var iterator = source.GetEnumerator())
    {
        while (iterator.MoveNext())
        {
            result++;
        }
    }
    return result;
}

So using as is like an is + a cast. It's almost always used with a nullity check afterwards, as per the above examples.

Answer 2

Every time when you need to safe cast object without exception use as:

MyType a = (MyType)myObj; // throws an exception if type wrong

MyType a = myObj as MyType; // return null if type wrong

Answer 3

As is used to avoid double casting logic like in:

if (x is MyClass)
{
  MyClass y = (MyClass)x;
}

Using

MyClass y = x as MyClass;
if (y == null)
{
}

FYI, IL generated for case #1:

  // if (x is MyClass)
  IL_0008:  isinst     MyClass
  IL_000d:  ldnull
  IL_000e:  cgt.un
  IL_0010:  ldc.i4.0
  IL_0011:  ceq
  IL_0013:  stloc.2
  IL_0014:  ldloc.2
  IL_0015:  brtrue.s   IL_0020
  IL_0017:  nop
  // MyClass y = (MyClass)x;
  IL_0018:  ldloc.0
  IL_0019:  castclass  MyClass
  IL_001e:  stloc.1

and for case #2:

  // MyClass y = x as MyClass;
  IL_0008:  isinst     MyClass
  IL_000d:  stloc.1
  // if (y == null)
  IL_000e:  ldloc.1
  IL_000f:  ldnull
  IL_0010:  ceq
  IL_0012:  stloc.2
  IL_0013:  ldloc.2
  IL_0014:  brtrue.s   IL_0018