How to treat an instance variable as an instance of another type in C#

Question

I have a simple inheritance heirarchy with MyType2 inheriting from MyType1.

I have an instance of MyType1, arg, passed in as an argument to a method. If arg is an instance of MyType2, then I'd like to perform some logic, transforming the instance. My code looks something like the code below.

Having to create a new local variable b feels inelegant - is there a way of achieving the same behavior without the additional local variable?

public MyType1 MyMethod(MyType1 arg) 
{
    if(arg is MyType2)
    {
        MyType2 b = arg as MyType2;
        //use b (which modifies "arg" as "b" is a reference to it)...
    }

    return arg;
}

Answer 1

No; you need to create a separate variable.

Also, your code performs more casts than necessary; it would be a little bit faster like this:

MyType2 b = arg as MyType2;
if(b != null)
{
    //Use b 
}

Answer 2

Note that the "is" and "as" is duplicating the test; either use is and then (once you know) just cast - or use as in the first place and test for null.

Re your issue; if you only want to do one thing - then cast:

if(arg is MyType2)
{
    ((MyType2)arg).SomeSpecialMethod();
}

Otherwise - perhaps a virtual method (on the base-type), or just refactor the logic out into another method, so you have:

if(arg is MyType2)
{
    StuffThatTakesType2((MyType2)arg);
}

But persoanlly, I'd just use the extra variable:

MyType2 whatever = arg as MyType2;
if(whatever != null) {
    whatever.Foo = 123;
    whatever.Bar();
}

Answer 3

I would say option 1 is to put all the modifying code (your comment line) as a function of MyType2, and then do (arg as MyType2).Foo() on it.

Option 2 is to implement MyMethod on both MyType1 and as an override in MyType2, and just call it without worrying. The MyType1 implementation may do nothing, which isn't the best design.

edit: technically not a violation of LSP since that only applies in the reverse of this situation (MyType2 shouldn't break behavior of MyType1).