Using generic type from other generic parameter

Question

I have a question about .net generics. Consider the following code:

public abstract class Test<TKey>
{
    TKey Key { get; set; }
}

public class Wrapper<TValue, TKey>
    where TValue : Test<TKey>
{
    public TValue Value { get; set; }
}

Now, when using this code, I could do something like this:

Wrapper<Test<int>, int> wrapper = new Wrapper<Test<int>, int>();

The int type parameter has to be provided twice. Is it possible to modify the Wrapper definition, to require TValue to be a generic type, and use this 'nested' generic type parameter insted of the TKey type parameter?

Can Test<TKey> could be SomethingInheritingFromTest<TKey> instead? If that's the case I don't see a way around specifying the type parameter, since you don't have a type you can feed the same T to, since you don't know what it is, even if you created a base/wrapper...smarter generic guys here than I though, hopefully someone educates me :) — Nick Craver
– Nick Craver, Commented Jun 5, 2010 at 21:48

Dan Tao · Accepted Answer · 2010-06-05 21:53:39Z

1

I'd say this depends on whether or not you really need to expose your Value property as a specific TValue where TValue derives from Test<T>. In other words, do you need to expose functionality available only to derived classes, or could you simply expose a Test<T>, with all the functionality of the base class?

In the latter case, you could simplify your class definition as:

public class Wrapper<TKey>
{
    public Test<TKey> Value { get; set; }
}

As for the precise functionality you're seeking: I don't believe anything quite like that is available in the current version of C#.

That said, another option in your case might be to actually use your Wrapper class itself as a base class:

public abstract class Test<TKey>
{
    TKey Key { get; set; }
}

public abstract class Wrapper<TValue, TKey>
    where TValue : Test<TKey>
{
    public TValue Value { get; set; }
}

public class TestWrapper<TKey> : Wrapper<Test<TKey>, TKey>
{ }

// ... some code somewhere
var tw = new TestWrapper<int>();
Test<int> value = tw.Value;

answered Jun 5, 2010 at 21:53

Dan Tao

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1 Comment

Nick Craver Over a year ago

Test is an abstract class though, based on comments in a deleted question you can't see quite yet (if I remember my rep limits!), the OP wants classes that inherit from Test<T>, not the actual class itself. He wants something like Wrapper<Test, int> or Wrapper<TestChild, int> and TestChild actually be TestChild<T>...I believe the answer is the same though, this doesn't exist. But hey, maybe there's a way to get this I'm unaware of...I'm sure the IL can do it, but not sure about C#.

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