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();
    }
}