Nested class in C#

Question

I am trying to study about nested class in c#. After reading many documents and goggling, I still not yet clear about when to use nested classes. But as far as I understand I did a small sample program. I am pasting my code below. Is this nested class program implemented in correct logic? . What actually a nested class using for ?. and also I have a doubt arise in this program and I specified that doubt in the program. Please help me ...

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        Bank bankObj = new Bank();
        bankObj.CreateAccount();
        bankObj.ShowMyAccountNumber();
    }
}

class Bank
{
    static int accountNumber; //  here if I just declare this as int accountNumber without static it showing an error in the CreatePersonalAccount(int accNo) method's first line ie accountNumber = accNo; as "Cannot access a non-static member of outer type." What actually this error mean ?

    public class BankAccountSection
    {
        public bool CreatePersonalAccount(int accNo)
        {
            accountNumber = accNo;
            return true;
        }
    }

    public void CreateAccount()
    {
        bool result = new BankAccountSection().CreatePersonalAccount(10001);
    }

    public void ShowMyAccountNumber()
    {
        MessageBox.Show(accountNumber.ToString());
    }
}

Your code doesn't show a nested class at all. It has two separate top-level classes. — Jon Skeet, Feb 27 '13 at 06:42

score 7 · Accepted Answer · edited May 23 '17 at 12:25

Nested classes are usually used for small utility classes that have no use outside the enclosing (outer) class. For that reason, nested classes are usually private. (There's even an FxCop rule for that.)

Your code

In your case, the nested class BankAccountSection is not really useful, since it has no state by itself. CreatePersonalAccount might as well just be a method of the outer class.

Regarding static int accountNumber;: This will make accountNumber a shared field across all Bank objects, which defeats the whole purpose. Don't do that. If you really need to set a field of the Bank object inside the inner class, you need to pass a reference of the Bank object to the inner class. (This is different to Java, where such a reference is available automatically under some circumstances.) In your particular case, just get rid of the inner class.

Examples for legitimate use cases

You have a large algorithm inside a method. You realize that extracting this algorithm into its own class using many small methods and instance variables would increase readability. Since the algorithm is very specific and probably not useful for other classes, you put the algorithm into an inner class. Thus, you avoid cluttering your outer class with instance variables only used by that algorithm.
You create a List data structure, which is internally implemented as a linked list. Since you don't expose the list nodes to the outside world, you make the nodes an inner class.

Why/when should you use nested classes in .net? Or shouldn't you?

I would suggest that if a `Thing` defines a type whose instances will exist to act on a particular instance, having the type be a `Thing.Frobber` conveys that relationship much more clearly than does `ThingFrobber`. This is especially true in many situations involving generics. `List.Enumerator` is clearer than would be `ListEnumerator`. Further, in some cases involving things like adapters, a `Name1.Name2` may express the relationships among the types more cleanly than would `Name`. — supercat, Jul 30 '13 at 19:55
Note also that if one wants to invoke a generic method in such fashion as to specify some parameters but not others, one may do that by having a static class `ClassName` contain a method `MethodName(U param)`. I know of no better way to allow specification of some but not all of a method's generic types. — supercat, Jul 30 '13 at 20:19

score 0 · Answer 2 · answered Feb 27 '13 at 06:45

You seem to think that nested classes in C# behave how they do in Java. That in other words, unless a nested class is declared as static, that it will share the instance of the enclosing class. In C# this is not the case. There is no such thing as that sort of thing in C# -- all nested classes are implicitly static.

This is why you cannot access accountNumber from the nested class unless that field is declared static. (Since the nested class has no access to any particular instance) The idomatic solution to this problem in C# is to pass the instance of the enclosing class into the nested class (presumably by passing this via a constructor argument when instantiating it).

Are you saying that `BankAccountSection` is implicitly static - i.e. it can only have static members? The `CreatePersonalAccount` isn't declared as `static` in the example given. — Kirk Broadhurst, Feb 27 '13 at 06:48
No, I'm saying that it's not an "instance class" (which has no analog in C#, but *does* exist in Java) — Kirk Woll, Feb 27 '13 at 06:49

score 0 · Answer 3 · answered Feb 27 '13 at 06:45

First, that's not a nested class, they are just two classes in one file.

Now, even if it were a nested class, this would probably be an example of when NOT to use nested classes. You should definitely separate your logic from your GUI logic.

I'm don't really think you should be using nested classes anyway, they are in my opinion hard to mantain, but I might be wrong. If I really needed to use nested classes I'd probably do so only when the child class is tightly related.

tariq · Answer 4 · 2013-02-27T06:57:57.323

0

The error is because you can not access a member of a non static class without its object. if you do so then it must be declared static.

edited Feb 27 '13 at 06:57

answered Feb 27 '13 at 06:49

tariq

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Nested class in C#

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