How would it be possible to remove all event handlers of the 'Click' event of a 'Button'?

c# .net events event-handling

33,061

Solution 1

You can't, basically - at least not without reflection and a lot of grubbiness.

Events are strictly "subscribe, unsubscribe" - you can't unsubscribe someone else's handler, any more than you can change someone else's reference to an object.

Solution 2

Note: Since the question on which I posted my original answer was closed as a duplicate of this question, I'm cross-posting an improved version of my answer here. This answer only applies to WPF. It will not work on Windows Forms or any other UI framework.

The below is a helpful utility method for removing all event handlers subscribed to a routed event on a given element. You can trivially convert this to an extension method if you like.

/// <summary>
/// Removes all event handlers subscribed to the specified routed event from the specified element.
/// </summary>
/// <param name="element">The UI element on which the routed event is defined.</param>
/// <param name="routedEvent">The routed event for which to remove the event handlers.</param>
public static void RemoveRoutedEventHandlers(UIElement element, RoutedEvent routedEvent)
{
    // Get the EventHandlersStore instance which holds event handlers for the specified element.
    // The EventHandlersStore class is declared as internal.
    var eventHandlersStoreProperty = typeof(UIElement).GetProperty(
        "EventHandlersStore", BindingFlags.Instance | BindingFlags.NonPublic);
    object eventHandlersStore = eventHandlersStoreProperty.GetValue(element, null);

    // If no event handlers are subscribed, eventHandlersStore will be null.
    // Credit: https://stackoverflow.com/a/16392387/1149773
    if (eventHandlersStore == null)
        return;

    // Invoke the GetRoutedEventHandlers method on the EventHandlersStore instance 
    // for getting an array of the subscribed event handlers.
    var getRoutedEventHandlers = eventHandlersStore.GetType().GetMethod(
        "GetRoutedEventHandlers", BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
    var routedEventHandlers = (RoutedEventHandlerInfo[])getRoutedEventHandlers.Invoke(
        eventHandlersStore, new object[] { routedEvent });

    // Iteratively remove all routed event handlers from the element.
    foreach (var routedEventHandler in routedEventHandlers)
        element.RemoveHandler(routedEvent, routedEventHandler.Handler);
}

You could then easily call this utility method for your button's Click event:

RemoveRoutedEventHandlers(button, Button.ClickEvent);

Edit: I've copied the bug fix implemented by corona, which stops the method from throwing a NullReferenceException when no event handlers are subscribed. Credit (and upvotes) should go to their answer.

Solution 3

Just wanted to expand on Douglas' routine slightly, which I liked very much. I found I needed to add the extra null check to eventHandlersStore to handle any cases where the element passed didn't have any events attached yet.

/// <summary>
/// Removes all event handlers subscribed to the specified routed event from the specified element.
/// </summary>
/// <param name="element">The UI element on which the routed event is defined.</param>
/// <param name="routedEvent">The routed event for which to remove the event handlers.</param>
public static void RemoveRoutedEventHandlers(UIElement element, RoutedEvent routedEvent)
{
    // Get the EventHandlersStore instance which holds event handlers for the specified element.
    // The EventHandlersStore class is declared as internal.
    var eventHandlersStoreProperty = typeof(UIElement).GetProperty(
        "EventHandlersStore", BindingFlags.Instance | BindingFlags.NonPublic);
    object eventHandlersStore = eventHandlersStoreProperty.GetValue(element, null);

    if (eventHandlersStore == null) return;

    // Invoke the GetRoutedEventHandlers method on the EventHandlersStore instance 
    // for getting an array of the subscribed event handlers.
    var getRoutedEventHandlers = eventHandlersStore.GetType().GetMethod(
        "GetRoutedEventHandlers", BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
    var routedEventHandlers = (RoutedEventHandlerInfo[])getRoutedEventHandlers.Invoke(
        eventHandlersStore, new object[] { routedEvent });

    // Iteratively remove all routed event handlers from the element.
    foreach (var routedEventHandler in routedEventHandlers)
        element.RemoveHandler(routedEvent, routedEventHandler.Handler);
}

Solution 4

I found this answer here on StackOverflow:

How to remove all event handlers from a control

private void RemoveClickEvent(Button b)
{
    FieldInfo f1 = typeof(Control).GetField("EventClick", 
        BindingFlags.Static | BindingFlags.NonPublic);
    object obj = f1.GetValue(b);
    PropertyInfo pi = b.GetType().GetProperty("Events",  
        BindingFlags.NonPublic | BindingFlags.Instance);
    EventHandlerList list = (EventHandlerList)pi.GetValue(b, null);
    list.RemoveHandler(obj, list[obj]);
}

Which the origional poster found here:

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Author by

The Light

I love thinking and writing .NET applications and have over 13 years experience + MCPD, MCTS, MCAD, MCP.

Updated on July 09, 2022

Comments

The Light almost 2 years
I have a button control, and I'd need to remove all the event handlers attached to its Click event.

How would that be possible?
```
Button button = GetButton();
button.Click.RemoveAllEventHandlers();
```
Jon Skeet almost 13 years

That's implementation-specific though - the answer appeared in 2008, I wouldn't even like to say whether it will work on .NET 4. It's a really bad idea to rely on things like this.
The Light almost 13 years

that's exactly what I want; unsubscribing someone else's handler! I'm using a custom button from a thirdparty and would like to remove the author's click event handler...
Jon Skeet almost 13 years

@William: You can't, basically - not without relying on implementation details and reflection, unless the custom button exposes such behaviour. The encapsulation of events is that handlers don't interfere with each other.
The Light almost 13 years

thanks but this line always returns null: typeof(Control).GetField("EventClick", BindingFlags.Static | BindingFlags.NonPublic);
Jon Skeet almost 13 years

@William: 1) You're using the control in a way for which it was not designed, and could thus cause problems very easily. 2) Your workaround could well break in a future version. 3) It won't work in low-trust environments. Basically, how fragile do you want your code to be?
The Light almost 13 years

good points; thinking about design not just implementation. I've created my own custom button instead and copied what I needed (icon, etc) from the original.
Dinis Cruz almost 12 years

I implemented a similar solution that works using a couple more internal methods of the EventHandlerList object. See answer stackoverflow.com/questions/11031149/…
Syaiful Nizam Yahya over 10 years

Is it possible to do this in silverlight? silverlight doesnt have RoutedEventHandlerInfo.
Will about 9 years

... Could this be expanded into an extension method? That would be pretty sweet...