Generic class with the same class inside

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1.00/5 (1 vote)

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I am trying to create a generic class having a variable of the same generic type inside:

C#

public class MyClass<T>
{
    MyClass<T1> a = new MyClass<T1>();
}

The idea is to have a variable of the same generic type and with a different parameter type T1. And I cannot do that because of a compilation error.

Any ideas? Thank you.

Sergey

Posted 26-Feb-13 6:12am

serguru604

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Andreas Gieriet 26-Feb-13 12:20pm

What error?
The code above does not compile since the assignment is not in a method.
Please clarify and fix the example. In addition, is the T1 class a concrete type or a generic parameter? Please give the full example code. See also my solution #1 below.
Andi

serguru604 26-Feb-13 12:49pm

Hello Andi,
Thank you for your response. Actually the only thing I am trying to find out is: how to declare a variable inside a class MyClass<t> of the same type MyClass<t> but with a different generic parameter, i.e. not T but T1.

In your example you declared a variable T Value. And how to declare a variable MyClass<t1> Value?

Thank you.
Sergey

Andreas Gieriet 26-Feb-13 17:21pm

What is the concrete design you want to have? I.e. what do you expect to happen? You can not directly do that.
Andi

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Andreas Gieriet · Answer 1 · 2013-02-26T06:25:00

Solution 1

This code compiles:

C#

class MyClass<T>
{
    public T Value { get; private set; }
    public MyClass(T value)
    {
        Value = value;
    }

    public static int Func(int x)
    {
        MyClass<int> inst = new MyClass<int>(x);
        ...
        return inst.Value;
    }

    public static U OtherFunc<U>(U x)
    {
        MyClass<U> inst = new MyClass<U>(x);
        ...
        return inst.Value;
    }

}

[EDIT]

There is a solution to your question. You define MyClass to have two generic types. E.g. The second one is for your field type (below named V). You may then pass any type that fulfils the constraint (the where clause). e.g.:

C#

public class MyClass<U, V>
    where V: new()
{
    // note: public only to show the effect here, otherwise use private for fields
    public U _uField = default(U); // could also leave away: "= default(U)"
    public V _vField = new V();    // only parameter less constructor supported
    ...
}
...
MyClass<string, int> obj1 = new MyClass<string, int>();
Console.WriteLine(obj._uField);
Console.WriteLine(obj._vField);
...
MyClass<int, MyClass<string, bool>> obj2 = new MyClass<int, MyClass<string, bool>>();
Console.WriteLine(obj2._uField);
Console.WriteLine(obj2._vField);

So, you pass the type from outside.

[/EDIT]

Cheers
Andi

Posted 26-Feb-13 6:25am

Andreas Gieriet

Updated 26-Feb-13 12:44pm

v6

Comments

Sergey Alexandrovich Kryukov 26-Feb-13 13:59pm

Andi,

I voted 4 this time, because Func and OtherFunc should be static, if you look at their bodies. Having them as instance functions won't cause any problem, it's just a redundancy, because "this" is not used. For example, FxCop will give you a performance warning.

Besides, there is more to it. Please see my answer.

—SA

Andreas Gieriet 26-Feb-13 17:12pm

Hello Sergey,
Thanks for your 4. I thought of it while I posted it but did not have a focus on that. You can assume some ... in the function body ;-).
Cheers
Andi

Sergey Alexandrovich Kryukov 26-Feb-13 17:17pm

Yes I can, but that needs some explanation. You see, improper choice of static vs. instance is one of the usual beginner's problems; I'm sure you do understand the matter, but we should take more care when we answer questions, to avoid teaching wrong techniques. Minor problem, anyway...
—SA

Andreas Gieriet 26-Feb-13 17:43pm

Updated.
Cheers
Andi

Sergey Alexandrovich Kryukov 26-Feb-13 17:49pm

You apparently are not satisfied with my 4, and I was picky enough. Well, I think both things only worked out for common good :-). My 5 now!
—SA

Andreas Gieriet 26-Feb-13 17:51pm

;-) Thanks for the 5!
Andi

Sergey Alexandrovich Kryukov 26-Feb-13 18:01pm

My pleasure. :-)
—SA

Andreas Gieriet 26-Feb-13 18:45pm

BTW: see my updated answer.
Andi

Sergey Alexandrovich Kryukov 26-Feb-13 18:53pm

Well, yes, you could note that I mentioned in my answer, that OP might mean two generic type parameters he named T and T1, but mentioned only one (but I doubt it was the case, probably it was some lack of understanding) and briefly described how the declaration should look... Yes, should be mentioned for completeness...
—SA

Sergey Alexandrovich Kryukov · Answer 2 · 2013-02-26T07:56:00

Sergey,

It looks like, at this moment, your head is messes up very well where it comes to generics. Otherwise you would not write this T1. This is a type which is not defined anywhere. In contrast, T is a generic type. It actually means "that very complete type which will ultimately be substituted instead of the generic type parameter". If you meant T instead T1, you should have written so, if this is meant to be another type, you should have added it as a generic parameter in class MyClass<T, T1> { /* ... */ }.

Now, using the variable if the same class is a very usual thing; please see Solution 1. However, you need a big warning about what you tried to do.

This is allowed by syntax, but will cause infinite recursion:

C#

    class MyClass<t> {
        MyClass<t> myInstance = new MyClass<t>(); // DON'T DO IT, will cause infinite recursion!
        // ...
    }
</t></t></t>

And it has nothing to do with generics; you would have exact same problem without then:

C#

class MyClass {
    MyClass myInstance = new MyClass(); // DON'T DO IT, will cause infinite recursion!
    // ...
}

Do I even have to explain why? If it's not clear, just think just a bit.

—SA