C# Inheritance

Question

I had a coworker see this the other day and I'm not quite sure why this happened. Can anyone explain it?

We have class A:

using System;
using System.Data;

public class A
{
  protected DataTable _table;

  public A()
  {

  }
}

We have class B that inherits from class A (assume there in the same namespace):

using System;

public class B : A
{
  public B()
  {

  }
}

In the constructor of class B, if I try this._table, it doesn't compile? Why does it not?

But to get it to work, I have to add using System.Data; and everything works fine.

Why does .NET not do this for me? So that when I try to access the protected member, it knows that its a System.Data.DataTable?

forgive me if the classes are 100% correct...

Answer 1

Other than the semi-colon missing after the declaration of _table, it's fine. On the other hand, I personally avoid having fields with an access modifier of anything other than private.

Answer 2

This is not a language issue - if anything, you could call it an IDE issue, for not adding the using statement when creating the new class. Even then, the IDE does not know you will be accessing this member, and I prefer to keep my "usings" as minimal as possible. In any case, the fact that B extends A does not tell the compiler anything about which namespaces it must search for the types referenced in B.

Also, I would strongly suggest you change your access of the _table member to use a getter property.

Answer 3

Hmm

Is this exact code? I don't see a semi-colon after your declaration of _table...

Answer 4

Are the classes in the same assembly? I tried your sample with the classes in the same assembly and it compiled fine.

Answer 5

It should work. I just tried it and it does work.

Check if both classes are in the same namespace. If they are not, don't forget to include the namespace of class A with the using keyword.

Check if both classes are in the same assembly. If they are not, don't forget to include a reference to the assembly of class A in the assembly of class B.

Answer 6

I agree with ya'll that they should be using "getters/setters" and just set the _table to private.

I was just curious to why the .Net language wouldn't even let it compile....

Answer 7

Actually I don't think the compiler even looks at _table, as soon as it sees DataTable and sees that it's an undefined type it will skip forward to the next line that looks syntatically correct. I think even if you had some junk variable after DataTable in class B you would still get the same error.

Answer 8

Shouldn't we be fully qualifing our types in a class anyway? It was the mantra driven into my head while learning c++ anwyay.

Answer 9

I'm agreeing with ya'll now.

I thought I'd post the question because I believed my coworker knew that he was talking about. I tried the following and had now problems:

using System.Data;

namespace WindowsApplication1
{
    public class classA
    {
        protected DataTable _table;

        public classA()
        {

        }
    }
}

next file:

namespace WindowsApplication1
{
    public class classB : classA
    {
        public classB()
        {
            this._table = new System.Data.DataTable();
        }
    }
}

and that compiled just fine. I thought there was going to be a problem with a linkage between all the .Net namespaces but there's not.

Thanks for all the comments/answers.