使用类型约束实现泛型类时出错

Question

I have a Generic Base Class that I want to allow one of two types ITest or IBoldface .

My Base Class looks like this:

public abstract class BaseTestingCollections<T> where T : ITest, IBoldface
{
...
}

One of the classes that inherit it looks like this:

public class TestCollection : BaseTestingCollections<ITest>, ITestCollection
{
...
}

When I compile I get this error:

The type DomainLogic.ITest' cannot be used as type parameter 'T' in the generic type or method 'DomainLogic.BaseTestingCollections'. There is no implicit reference conversion from 'DomainLogic.ITest' to 'DomainLogic.IBoldface'.

Answer 1

Such an either/or restriction can't be done (as I'm sure you've noticed, the comma is more like && than || ). You can either make two different abstract classes with different names (one BaseTestingCollectionsTest<T> where T : ITest , the other BaseTestingCollectionsBoldface<T> where T : IBoldface ), or remove the static restriction and put the check at runtime. Or make one of ITest or IBoldface extend the other, or extend a common interface, if they share members.

Here's an example of checking at runtime:

public abstract class BaseTestingCollections<T>
{
    public BaseTestingCollections()
    {
        if (!typeof(ITest).IsAssignableFrom(typeof(T)) && !typeof(IBoldface).IsAssignableFrom(typeof(T)))
            throw new Exception();
    }
}

Answer 2

Well you're not satistfying the constraint you've specified. T has to extend/implement both ITest and IBoldFace .

Your constraint doesn't mean that it has to extend/implement one of the types - it has to do both . A type argument has to satisfy all the type constraints in order to be valid.

See MSDN on generic constraints for more information.

Answer 3

Per your own generic constraint, only those types which implement both ITest and IBoldface are suitable generic arguments for your BaseTestingCollection class.

It's not either-or, how could it be? What happens when you call method Bar from the ITest interface on a T which only implements IBoldFace , which in turn defines no method Bar ? What is a compiler to do when faced with such a scenario?

There is no reasonable action to take. Your constraints apply all at the same time to the generic argument.

Answer 4

You can't express an "either-or" constraint like this with C# generics. Use a common base interface of some sort:

interface ITestOrBold {}
interface ITest : ITestOrBold {}
interface IBoldface : ITestOrBold {}

class BaseTestingCollections<T> where T : ITestOrBold {}

Obviously instead of ITestOrBold you should have some sensible abstraction.