Pass delegate/event as ref generic parameter

Sergio Romero

We are dealing with a lot of legacy WinForms code and are slowly refactoring, breaking down dependencies, implementing the MVP pattern and moving logic from forms and user controls into the presenter.

We've added Castle Windsor, which creates a ViewFactory that returns a form/user control with the Presenter and all its dependencies attached.

A user control is used on almost every screen and we have refactored it but haven't finished refactoring all the screens that use it, so some of them are using the ViewFactory and some are still calling the control's constructor , but of course doesn't create a presenter.

Although what I can fix is ​​to check if the events the control has now are null, if so it means no presenter has subscribed to them, so I create it manually. This works, but since there are some events that can fire at different times, it means I have to do the following:

private void RaiseEventForPresenter(ref EventHandler action, EventArgs e)
{
   if(action == null) CreatePresenter();

   action.Invoke(this, e);
}

private void RaiseEventForPresenter(ref CustomDelegate1 action, CustomDelegate1EventArgs e)
{
   if(action == null) CreatePresenter();

   action.Invoke(this, e);
}

private void RaiseEventForPresenter(ref CustomDelegate2 action, CustomeDelegate2EventArgs e)
{
   if(action == null) CreatePresenter();

   action.cInvoke(this, e);
}

private void CreatePresenter()
{
    new Presenter(this, new Bl(new Dal(new ConnectionManager())));
}

Note that the delegate needs to be passed as a ref in order for it to be successfully called after the presenter is created and subscribed to view events.

Instead of using a different overloaded method for each event, I want to do this:

private void RaiseEventForPresenter<T1, T2>(ref T1 action, T2 e)
{
    if (action == null) CreatePresenter();

    action.Invoke(this, e); //Does not compile
}

Of course, T1 does not contain the definition of the Invoke method . So far I have tried:

Convert action to action:

((Action<object, T2)action).Invoke(this, e);

Redefine T1 to represent:

private void RaiseEventForPresenter<T>(ref Delegate action, T e) ...

Redefine T1 as an action:

private void RaiseEventForPresenter<T>(ref Action<object, T> action, T e) ...

All these options give me different compile errors, so my question is, can this be done?

Wu

This problem is hard to solve because generics do not allow delegates as type constraints , so we cannot write generic methods that will accept any specific type of delegate.

But we can specify type constraints on the type parameters in the delegate . So the solution is to get rid CustomDelegate1of and CustomDelegate2and instead use a generic version of such as , CustomDelegate<T>and pass the event arg type as a generic parameter.

Here's what the delegate and event parameter definitions look like:

public class CustomEventArgs1 : EventArgs { }


public class CustomEventArgs2 : EventArgs { }

public delegate void CustomDelegate<T>(object sender, T e) where T : EventArgs;

Here is your method:

public void RaiseEventForPresenter<T>(ref CustomDelegate<T> action, T args) where T : EventArgs
{
    if (action == null) CreatePresenter();
    action(this, args);
}

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