C# Casting using 'as'

Question

I have the following code:

public class BaseClass {}
public class Class1 : BaseClass {}
public class Class2 : BaseClass {}

public class BaseClassList : List<BaseClass> {}
public class Class1List : List<Class1> {}
public class Class2List : List<Class2> {}

    public void Setup()
    {
        BaseClassList b = new BaseClassList();
        b.Add(new BaseClass());
        b.Add(new BaseClass());
        b.Add(new BaseClass());

        var list = from a in b
                   select a;

        Class1List c1 = new Class1List();

        foreach (BaseClass a in b)
        {
            var o = a as Class1;
            if (o != null)
                c1.Add(o);
        }
    }

However, when casting a as Class1, o is always null. What is the problem?

You are only adding `BaseClass`and no `Class1` to `b`. Every dog is an animal but not all a nimals are dogs. — Tim Schmelter, Feb 10 '14 at 11:18
http://stackoverflow.com/questions/10172767/cast-base-instance-to-derived-class-downcast-in-c-sharp — Konrad Morawski, Feb 10 '14 at 11:18
As doesn't convert one class to the other, it only assigns it if it is an instance of that type — Liath, Feb 10 '14 at 11:19

score 5 · Answer 1 · answered Feb 10 '14 at 11:17

5

The problem is that a is not Class1 object. That's all. Read your code carefully again - why would you think a BaseClass object can be casted to a class, that derives from it?

answered Feb 10 '14 at 11:17

Tarec

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score 3 · Answer 2 · answered Feb 10 '14 at 11:17

3

becasue a is a BaseClass according to your code:

...
 b.Add(new BaseClass());
 b.Add(new BaseClass());
 b.Add(new BaseClass());
...

and you can not cast base class to child class, you have to do it vice versa.

Example:

  ...

    b.Add(new Class1());
    b.Add(new Class1());
   ...

and after this will be correct:

foreach (var a in b)
{
    var o = a as Class1;//CORRECT.      
}

answered Feb 10 '14 at 11:17

Tigran

61,654
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3

All Class1s are BaseClasses but not all BaseClasses are Class1s – Liath Feb 10 '14 at 11:17

score 0 · Answer 3 · answered Feb 10 '14 at 11:26

Because you're casting your base class to a super class. It's like telling the compiler that a "Person" is always "Software Developer", it should be the other way around. So, a "Software Developer" is always a "Person" and an "Accountant" is also a "Person."

Thus, "BaseClass" is to "Person"; "Class1" is to "Software Developer"; "Class2" is to "Accountant"

So you should add an instance of either a "Software Developer/Class1" or "Accountant/Class2" to the list of "Person" which is the BaseClassList.

Then mark you BassClass as abstract.

C# Casting using 'as'

3 Answers3