Base / Derived Type Generics Parameter

Question

I'd like to have an abstract method inside an abstract class and method signature like this:

public abstract IEnumerable<SynchronizeItem<IDataItem>> Sync(IEnumerable<SynchronizeItem<IDataItem>> clientSyncItems);

In the derived class I want to define the method type safety:

public override IEnumerable<SynchronizeItem<AdItem>> Sync(IEnumerable<SynchronizeItem<AdItem>> clientSyncItems)
{
    // Do something with the AdItems
    _adHandler.Foo(clientItems):
}

This is not possible. Any other possibilities to achieve the same behavior? I don't want to cast each usage of IDataItem to it's specific implementation within AdHandler class.

Answer 1

Try to define your abstract method like:

public  IEnumerable<SynchronizeItem<T>> Sync<T>(IEnumerable<SynchronizeItem<T>> clientSyncItems)  where T : IDataItem;

Your other method, the implementation, is correct.

Answer 2

Renen's answer will not do quite what you want, as it appears to me you want the subclass to define the concrete type, not the caller . To do this, make the type argument a part of the base class , not the method:

public class Base<T> where T : IDataItem
{
    public abstract IEnumerable<SynchronizeItem<T>> Sync(
        IEnumerable<SynchronizeItem<IDataItem>> 
        clientSyncItems);
}

And your derived class:

public class Derived : Base<AdItem>
{
    public override IEnumerable<SynchronizeItem<AdItem>> Sync(
        IEnumerable<SynchronizeItem<AdItem>> 
        clientSyncItems) 
    { ... }
}