Question

Consider these two abstract classes. MyClass2 extends my BaseClass and overrides a virtual method. Note it also defines TValue of MyBaseClass as IList…

public abstract class MyBaseClass<TKey, TValue>
{
    public virtual IList<TValue> DoSomeStuff()
    {
        IList<TValue> result;
        ....
        return restult;
    }
}

public abstract class MyClass2<TKey, TValue>: MyBaseClass<TKey, IList<TValue>>
{
    public override IList<TValue> DoSomeStuff()
    {
        IList<TValue> result;
        ....
        return restult;
    }
}

This code does not compile. The complaint is on the return type of MyClass2.DoSomeStuff. The error is “Cannot change return type when overriding method “IList<TValue> MyClass2<TKey, TValue>.DoSomeStuff()“

I’m not clear as to why this is wrong. Why wouldn’t the compile or .net consider TValue for the overriden method to be that of MyClass2?

Answer 1

The problem’s in your base class part:

public abstract class MyClass2<TKey, TValue>: MyBaseClass<TKey, IList<TValue>>

Replace the IList<TValue> with TValue and it will work – the method already returns an IList.