c# - 尝试取消订阅该 EventHandler 的变量时取消订阅原始事件

Question

我正在尝试取消订阅我的事件的附加方法。我有这段代码:

public event EventHandler MenuItemSelectionChanged;

var eh = MenuItemSelectionChanged;
eh -= TheMethod;

运行上面的代码后结果是:

MenuItemSelectionChanged: Is Not Null

el: Is Null

但我想通过使用 eh(指向 EventHandler 的指针)从 MenuItemSelectionChanged 取消订阅 TheMethod。我如何使用 eh 从 eh 和 MenuItemSelectionChanged

中删除 TheMethod

我需要这个结果:

MenuItemSelectionChanged: Is Null

el: Is Null

我的主要场景是:

在 TargetClass.cs 中:

    public event EventHandler MenuItemSelectionChanged;

    public void UnsubscribeFromEvent(ThisClassEventHandlerNames eventHandlerName, Dictionary<FrameworkElement, MethodInfo> eventHandlerInfos)
    {
        var eh = GetEventHandler(eventHandlerName);
        if (eh == null) return;

        foreach (var eventHandlerInfo in eventHandlerInfos)
        {
            var info = eventHandlerInfo;

            if (eh == null)
                break;
            
                var invocationList = eh.GetInvocationList().Where(x =>
                    Equals(x.Target, info.Key) &&
                    x.Method == info.Value);

            foreach (var myDeligate in invocationList)
            {
                if (eh == null)
                    break;

                // I HAVE A PROBLEM HERE>>
                eh -= (EventHandler) myDeligate;
                // RESULT: eh is null BUT MenuItemSelectionChanged is not null
            }
        }
    }

    public enum ThisClassEventHandlerNames
    {
        MenuItemSelectionChanged
    }

    private EventHandler GetEventHandler(ThisClassEventHandlerNames eventHandlerName)
    {
        EventHandler result = null;
        switch (eventHandlerName)
        {
            case ThisClassEventHandlerNames.MenuItemSelectionChanged:
            {
                result = MenuItemSelectionChanged;
                break;
            }
            default:
            {
                result = null;
                break;
            }
        }
        return result;
    }

我在我的 wpf 用户控件中使用如下代码调用上述代码:

在 MyUserControl.cs 中:

TargetClassObject.UnsubscribeFromEvent(
    TargetClass.ThisClassEventHandlerNames.MenuItemSelectionChanged,
    new Dictionary<FrameworkElement, MethodInfo>
    {
        {
            this,
            Infrastructure.Utility.GetMethodInfo<MyUserControl>(x => x.MenuControl_MenuItemSelectionChanged(null, null))
        }
    });

我真的想通过这个参数调用该类 (targetClass) 中的方法来取消订阅事件的方法:

A) An enum value (to get event by it)

B) A method info (to unsubscribe its methodfrom the event)

注意:我的平台是 C#、WPF、.NET Framework 4.5

Answer 1

当您将 MenuItemSelectionChanged 移动到一个变量然后分配给该变量时，您并没有真正更改原始委托(delegate)的调用列表。 eh -= TheMethod; 与 eh = eh - TheMethod; 相同(如果你在类中)，但这会产生一个新的 MulticastDelegate 并且不修改原始代码。

所以要回答你的第一个代码:

public event EventHandler MenuItemSelectionChanged;

var eh = MenuItemSelectionChanged;
eh -= TheMethod;
MenuItemSelectionChanged = eh;

但是，您想要动态更改事件，因此您可以添加第二个方法来获取 ThisClassEventHandlerNames 和 EventHandler 并将其分配回去。或者您可以修改 GetEventHandler 方法以将引用返回给处理程序:

private delegate void HandlerModifier(ref EventHandler handler);
private bool ModifyEventHandler(ThisClassEventHandlerNames eventHandlerName, HandlerModifier mod)
{
    switch (eventHandlerName)
    {
        case ThisClassEventHandlerNames.MenuItemSelectionChanged:
        {
            mod(ref MenuItemSelectionChanged);
            return true;
        }
        default:
        {
            return false;
        }
    }
}

public void UnsubscribeFromEvent(ThisClassEventHandlerNames eventHandlerName, Dictionary<FrameworkElement, MethodInfo> eventHandlerInfos)
{
    ModifyEventHandler(
        eventHandlerName,
        delegate(ref EventHandler eh)
        {
            if (eh == null) return;

            foreach (var eventHandlerInfo in eventHandlerInfos)
            {
                var info = eventHandlerInfo;

                if (eh == null)
                    break;

                    var invocationList = eh.GetInvocationList().Where(x =>
                        Equals(x.Target, info.Key) &&
                        x.Method == info.Value);

                foreach (var myDeligate in invocationList)
                {
                    if (eh == null)
                        break;

                    // I HAVE A PROBLEM HERE>>
                    eh -= (EventHandler) myDeligate;
                    // RESULT: eh is null BUT MenuItemSelectionChanged is not null
                }
            }
        }
    ); 
}

ModifyEventHandler 现在“返回”对处理程序的引用，因此对它的所有更改都传递给 MenuItemSelectionChanged。