Implement generic interface with event multiple times

Question

I have a generic interface, which looks like this:

public interface IObjectProvider<out T>
{
    event Action<T> ValueOccured;
}

Now I want to make a class, which implements this interface two times like this:

public class GlobalReceiver : IObjectProvider<Foo>, IObjectProvider<Bar>
{
    public event Action<Foo> IObjectProvider<Foo>.ValueOccured;

    public event Action<Bar> IObjectProvider<Bar>.ValueOccured;
}

and can't get it working, because for this compiler says that I need to use proper syntax and if I make non-explicit implementation I (as expected) get an error:

Member is already declared.

How can I figure out this problem?

Try this : GlobalReceiver : IObjectProvider, IObjectProvider — rducom, Mar 02 '15 at 11:43
Well the class as shown **doesn't** implement the interface shown - it should be something like `public class GlobalReceiver : IObjectProvider, IObjectProvider` — RobV, Mar 02 '15 at 11:44
Your question is awfuly unclear. You're not even implementing your own interface type. What do you except should happen? — Yuval Itzchakov, Mar 02 '15 at 11:51
@YuvalItzchakov sorry, i provided interface, which implements those meantioned. Edited my question — Alex Voskresenskiy, Mar 02 '15 at 12:21
Don't translate or interpret compiler errors if you want help. Post the full, exact, English error the compiler gives you. — CodeCaster, Mar 02 '15 at 12:27
It says PROPERTY syntax not "proper" syntax e.g. https://msdn.microsoft.com/en-us/magazine/cc163533.aspx use the second code snippet style syntax — Charleh, Mar 02 '15 at 12:28

score 8 · Accepted Answer · answered Mar 02 '15 at 12:28

8

You have to explicitly tell how to implement the add and remove on the event, since it is an explicit interface implementation:

public class GlobalReceiver : IObjectProvider<Foo>, IObjectProvider<Bar>
{
    private event Action<Foo> ActionFoo;

    event Action<Foo> IObjectProvider<Foo>.ValueOccured { add { ActionFoo += value; } remove { ActionFoo -= value; } }

    private event Action<Bar> ActionBar;

    event Action<Bar> IObjectProvider<Bar>.ValueOccured { add { ActionBar += value; } remove { ActionBar -= value; } }
}

I added a default implementation that you might use for the actual events to make them accessible from your class.

Why? Probably because you can't call the event from just the member name in your class, so it will be very hard to call the event handler.

answered Mar 02 '15 at 12:28

Patrick Hofman

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your code works, but i still dont' get, why compiler cannot find out, what event to use based on type (like he does in methods with different generic type arguments)? – Alex Voskresenskiy Mar 02 '15 at 12:36
You have to cast `this` every time you want to call the event handler, that is not cool. That's why you have to tell how to implement it. – Patrick Hofman Mar 02 '15 at 12:37
3

You also [may want to use a lock](http://stackoverflow.com/questions/2205711/should-i-lock-event). – CodeCaster Mar 02 '15 at 12:38
casting this doesn't look for me as trouble as i'll make an event invocator anyway, but this code looks a little ugly :( – Alex Voskresenskiy Mar 02 '15 at 12:39
@AlexVoskresenskiy: Blame the design team. Not me ;) – Patrick Hofman Mar 02 '15 at 12:41

score 2 · Answer 2 · answered Mar 02 '15 at 12:29

Explicit interface implementations can't be public. Remove the public accessor.

Also, as explained in How to: Implement Interface Events (C# Programming Guide):

When you write an explicit interface implementation for an event, you must also write the add and remove event accessors.

Implement generic interface with event multiple times

2 Answers2