Generics contraints "or?"

Ista

What I want to do is say "Only Add classes that implement interfaceA or interfaceB" so saying public List<T> where T: interfaceA, interfaceB {} but that says they must implement both interface A and B. How can I say A or B?

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Richard Deeming

You can't. Probably the best option would be to use a factory method or a singleton, with a derived class for each interface. This is the same method used by the Comparer<T>.Default property. For example:

public abstract class MyClass<T>
{
private static readonly MyClass<T> _instance = CreateInstance();

public static MyClass<T> Instance
{
    get { return \_instance; }
}

private static MyClass<T> CreateInstance()
{
    Type itemType = typeof(T);
    Type resultType = null;
    
    if (typeof(InterfaceA).IsAssignableFrom(itemType))
    {
        resultType = typeof(MyClassA<>).MakeGenericType(itemType);
    }
    else if (typeof(InterfaceB).IsAssignableFrom(itemType))
    {
        resultType = typeof(MyClassB<>).MakeGenericType(itemType);
    }
    
    if (null == resultType) throw new ArgumentException("Type parameter \\"T\\" must implement InterfaceA or InterfaceB.");
    return (MyClass<T>)Activator.CreateInstance(resultType);
}

}

internal sealed class MyClassA<T> : MyClass<T> where T : InterfaceA
{
...
}

internal sealed class MyClassB<T> : MyClass<T> where T : InterfaceB
{
...
}

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Ennis Ray Lynch Jr

That both A and B implement then you can check for it instead. public interface IShape; public interface IEvenPolygon : IShape; public interface IOddPolygon : IShape; public List where T: IShape; The problem is what you want isn't type safe.

On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question. - Charles Babbage

Judah Gabriel Himango

Do you have the source code to interfaceA and interfaceB? If so:

interface IAmDaddy
{
}

interface interfaceA : IAmDaddy
{
}

interface interfaceB : IAmDaddy
{
}

public List<T> where T : IAmDaddy

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