C#•4mo ago

Abstract events implemented, but compiler warns they aren't used

I have an abstract class that declares abstract events that derived classes are required to implement. That allows owners of the class who have only a reference to the base type to subscribe to those events. But not all of the derived classes fire those events. Now the compiler is complaining those events aren't being used. Any suggestions on what I can do about those warnings?

public abstract class T : System.Windows.Controls.UserControl
{
  public abstract event System.Windows.RoutedEventHandler evtCanMoveOnChanged;
}
public class G : T
{
  public override event System.Windows.RoutedEventHandler evtCanMoveOnChanged; // Warning here
}

public abstract class T : System.Windows.Controls.UserControl
{
  public abstract event System.Windows.RoutedEventHandler evtCanMoveOnChanged;
}
public class G : T
{
  public override event System.Windows.RoutedEventHandler evtCanMoveOnChanged; // Warning here
}

9 Replies

wasabi•4mo ago

"not being used" means you have nobody using it. which might be perfectly valid.

Will PittengerOP•4mo ago

It's like the compiler is suggesting I can remove the event, but then my base class isn't completely implemented.

wasabi•4mo ago

Why override it?

Will PittengerOP•4mo ago

It's abstract.

wasabi•4mo ago

And not just make it virtual?

Will PittengerOP•4mo ago

Only the derived class can ever fire it. If it's virtual, then only the base class has that option.

wasabi•4mo ago

That's why the pattern is to make On* methods.

wasabi•4mo ago

https://learn.microsoft.com/en-us/dotnet/standard/events/ Typically, to raise an event, you add a method that is marked as protected and virtual (in C#) or Protected and Overridable (in Visual Basic). Name this method OnEventName; for example, OnDataReceived. The method should take one parameter that specifies an event data object, which is an object of type EventArgs or a derived type. You provide this method to enable derived classes to override the logic for raising the event. A derived class should always call the OnEventName method of the base class to ensure that registered delegates receive the event.

Handling and Raising Events - .NET

Learn to handle and raise .NET events, which are based on the delegate model. This model lets subscribers register with or receive notifications from providers.

Will PittengerOP•4mo ago

I ended up providing protected Fire* methods. On* is taken by event handlers. This is a sender, not a handler. The On* methods you mentioned do fire the event, but they let the derived class handle the event too. Those are virtual.

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Abstract events implemented, but compiler warns they aren't used