returning generic class

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I am running into a compiler error "cannot implicitly convert type T to T..." when I have code similar to this:

C#

class A<T>
{
   private T[] stuff;
   public A(int n) { stuff = new T[n]; }
   public T get(int i) { return stuff[i]; }
}
class B<T>
{
   private A<T> things;
   public B() { things = new A<T>(10); }
   public T get<T>() { return things.get(1); } 
}

error is on things.get(1) above.

The error goes away if B's get is not generic:

C#

public T get(){ return things.get(1)};

However, I do have a need to use a generic method here.
any suggestions/help is greatly appreciated.

Thanks.

Posted 26-Sep-15 5:06am

binsafir

Updated 26-Sep-15 5:35am

Wendelius

v3

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Wendelius 26-Sep-15 11:31am

What else would the return type of get be than T defined in class B?

Andreas Gieriet 27-Sep-15 6:13am

Why do you need a generic function? This need seems odd to me.
B.Get() is already generic through B being generic. I guess you are mixing up something here.
Andi

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OriginalGriff · Answer 1 · 2015-09-26T05:45:00

The problem is that you are are redefining a new T type separate from the one declared on the class level. Try with one less generic:

C#

class A<T>
    {
    private T[] stuff;
    public A(int n) { stuff = new T[n]; }
    public T get(int i) { return stuff[i]; }
    }
class B<T>
    {
    private A<T> things;
    public B() { things = new A<T>(10); }
    public T get() { return things.get(1); }
    }

"I mention this solution in my question and as I say, I have a need to use a generic method. So in essence class A has other uses outside of B and so I need to keep it separate. Since A's data, T[] stuff, is private B has to call its' public method. Generally speaking can a generic method in a generic class call a method in the inner class that returns type T?"

B.Get is already a generic method that inherits its generic type from the class definition of B when it is instantiated. If you try to add a second generic to it, then the new B.Get-specific declaration of T masks the B-class-specific declaration of T and the compiler (rightly) assumes that it's a different type - because you can otherwise legitimately say

C#

B<int> b = new B<int>();
string s = b.Get<string>();

Because you can't add any constraints on what type A.Get or B.Get can return, the compiler won't allow you to mask T and return the unmasked type - because it can't guarantee any conversion between them is possible.
Try it in VS with the code as I showed:

C#

A<int> a = new A<int>(7);
int i = a.get(3);
B<int> b = new B<int>();
string s = b.get();

VS will complain you can't convert the int to a string, and Intellisense will display b as being of type B<int> automatically.