Event fires more and more times

Question

I have a silverlight mvvm application that loads main view with 2 user controls loaded into 2 ContentControls, one with listbox showing items and other with edit button. When i click edit button, 2 new user controls load into the ContentControls, one showing data to edit (EditData) and other having Save and Cancel button (EditAction). When i click save button, it raises an event that is defined in seperate GlobalEvents.cs class like:

public event EventHandler OnSaveButtonClicked;  
public void RaiseSaveButtonClicked()  
{  
  this.OnSaveButtonClicked(this, EventArgs.Empty);  
}

and i subscribe to it in the other user control EditData, because i need to transfer that edited data via custom EventArgs, so i have put in the constructor of it's ViewModel:

this.globalEvents.OnSaveButtonClicked += (s, e) => SaveData();  

and in the Save data:

public void SaveData()  
{  
    globalEvents.RaiseSaveData(EditedGuy);     
}  

which raises another event that loads previous user controls into their ControlContent and shows edited data in list box. Thats all fine, but whenever i click on edit and then save again, it raises the event twice, and again 3 times, then 4 and so on. How can i make it to be raised only ONE time? I thought it could be because every time i click edit, a new instance of the user control is loaded and i dont know, maybe the subscription to the event stays, so i have tried to paste

this.globalEvents.OnSaveButtonClicked -= (s, e) => SaveData(); 

to the Dispose() method but without success. How can i make this work?

Answer 1

You can't use lambdas when you want to unregister from events.

this.globalEvents.OnSaveButtonClicked += (s, e) => SaveData(); 

This will create one instance - let's call it instance A - of type EventHandler and add it as a handler.

this.globalEvents.OnSaveButtonClicked -= (s, e) => SaveData(); 

This will not remove instance A from the event but create a new instance - instance B - and tries to remove it from the event.

To fix this problem, either create a little method or save that anonymous method in a field:

class ViewModel
{

    private EventHandler _saveButtonClickedHandler;
    // ...

    public ViewModel()
    {
        _saveButtonClickedHandler = (s, e) => SaveData();
        this.globalEvents.OnSaveButtonClicked += _saveButtonClickedHandler;
        // ...
    }

    public void Dispose()
    {
        this.globalEvents.OnSaveButtonClicked -= _saveButtonClickedHandler;
        // ...
    }

    // ...
}

Answer 2

this.globalEvents.OnSaveButtonClicked += (s, e) => SaveData();

This line is being called multiple times so you are adding a new event handler every time.

You need to either move that line to somewhere where it's only called once or change the event handler to:

this.globalEvents.OnSaveButtonClicked += SaveData;

public void SaveData(object sender, EventArgs e)  
{  
    globalEvents.RaiseSaveData(EditedGuy);     
    this.globalEvents.OnSaveButtonClicked -= SaveData();
}

So you remove the event handler after dealing with it. This assumes that the handler will be added back next time you go into edit mode.