Generic type parameter constraint based on other type parameter in the same definition

Question

I have type hierarchy defined like this:

interface IMyClass
{
}

interface IBase1<T>
{
}

interface IBase2<T>
{
}

interface IMyDerived1 : IBase1<IMyClass>
{
}

class Base1<T, U> : IBase1<T>
    where U : IBase2<T>
{
}

class Base2<T, U> : IBase2<T>
    where U : IBase1<T>
{
}

class Derived1<T, U> : Base1<T, U>, IMyDerived1
    where T : IMyClass
    where U : IBase2<T>
{
}

class Derived2<T, U> : Base2<T, U*>
    where T : IMyClass
    where U : IMyDerived1
{
}

but Visual Studio 2008 (.net 3.5 SP1) says that parameter U in parent specifier of Derived2 (marked with *) is not convertible to IBase1<T>. Is this solvable?

EDIT:

It indeed looks like generics overuse but allows Base1,2 and Derived1,2 to apply operations on supplied types without a casts. Something like this:

class MyClass : IMyClass
{}

class MySpecific1 : Derived1<MyClass, MySpecific2>
{
    // use inherited properties and methods of type MyClass here
    // use properties of MySpecific2 returning MyClass without casts
}

class MySpecific2 : Derived2<MyClass, MySpecific1>
{
    // use inherited properties and methods of type MyClass here
    // use properties of MySpecific1 returning MyClass without casts
}

Probably this can be solved more elegantly with variance in .net4 but I'm stuck with 3.5 for now.

At first glance this looks like a variant of co- and contra-variance, though I might be wrong. — Lasse V. Karlsen
– Lasse V. Karlsen, Commented Jun 30, 2010 at 13:44
I think it's not solvable. In principle it could be solvable via covariance (in C# 4, not 3.5 anyway), but it would imply making a covariant class, which is not allowed anyway. — Mau
– Mau, Commented Jun 30, 2010 at 13:50
By the way, covariance can be used in .NET 3.5 as well. You just have have to use targeted compiler from .NET 4. — Ivan G.
– Ivan G., Commented Jun 30, 2010 at 13:55
C# is not the same thing as .NET. C# 4.0 can be used with .NET 3.5. I don't know if variance requires CLR support or not. If it does, then .NET 4.0 would be required. — John Saunders
– John Saunders, Commented Jun 30, 2010 at 19:22
please don't repeat tags ("C#") in the title. That's what the tags are for. — John Saunders
– John Saunders, Commented Jun 30, 2010 at 19:22

Henrik · Accepted Answer · 2010-06-30 13:58:53Z

3

class Derived2<T, U>: Base2<T, U>
        where T: IMyClass
        where U: IMyDerived1, IBase1<T>
    {
    }

answered Jun 30, 2010 at 13:58

Henrik

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2 Comments

Oleg Kolosov Over a year ago

Great solution! I didn't thought of something so simple. Can you explain a little bit? It seems to me that compiler do not consider other constraints while evaluating expression in 'where U'. Is that right?

Henrik Over a year ago

As Stefan noted, the constraints in Derived2 don't imply, that U is derived from IBase1<T>. But this is required by Base2.

Matthew Abbott · Accepted Answer · 2010-06-30 13:56:57Z

That hurt my head!

Having a look at it, the problem lies with this definition:

interface IMyDerived1 : IBase1<IMyClass>
{
}

You've specialised that generic implementation, and then attempted to use with generic arguments later on:

class Derived2<T, U> : Base2<T, U>
    where T : IMyClass
    where U : IMyDerived1
{
}

Which is invalid. Not sure if this is correct, but can you make either this change:

interface IMyDerived1<T> : IBase1<T>
{
}

class Derived2<T, U> : Base2<T, U>
    where T : IMyClass
    where U : IMyDerived1<T>
{
}

That's a complicated hierarchy you're designing there, what will be its use?

Stefan Steinegger · Accepted Answer · 2010-06-30 13:59:56Z

0

The problem is, in Derived2, T is not IMyClass, it could be some other class implementing this interface. In Base1, it is specified to be exactly IMyClass. Types with different generic arguments are not compatible in C# 3.0.

For me, this looks a bit like generic overuse. But I can't see the context.

edited Jun 30, 2010 at 13:59

answered Jun 30, 2010 at 13:53

Stefan Steinegger

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