CODEWITHSUNDEEP
c#listinheritancecasting
Ivan-Mark Debono
Ivan-Mark Debono

Reputation: 16310

C# Casting using 'as'

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?

Upvotes: 0

Views: 124

Answers (3)

marce
marce

Reputation: 106

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.

Upvotes: 0

Tigran
Tigran

Reputation: 62256

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.      
}

Upvotes: 3

Tarec
Tarec

Reputation: 3255

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?

Upvotes: 5

Related Questions