c# - 为什么 EventInfo.RemoveEventHandler 抛出 NullReferenceException？

Question

我有一些代码使用反射在 Internet Explorer BHO 中的 MSHTML 对象上安装 .NET 事件处理程序，它似乎工作正常。但是，当我尝试删除事件处理程序时遇到了麻烦。

这是事件处理程序安装和删除的代码:

public class HandlerExample {
    private static void Handler(IHTMLEventObj e) { ... }
    private static Delegate handlerDelegate;

    public static void InstallHandler(IHTMLElement target, string eventName)
    {
        // FindInterface() finds the correct event interface for the particular subclass of
        // IHTMLElement that target really is
        Type eventInterface = FindInterface(target);
        EventInfo eInfo = eventInterface.GetEvent(eventName);
        Type tHandler = eInfo.EventHandlerType;

        handlerDelegate = Delegate.CreateDelegate(tHandler, typeof(HandlerExample), "Handler");

        eInfo.AddEventHandler(target, handlerDelegate);
    }

    public static void RemoveHandler(IHTMLElement target, string eventName)
    {
        Type eventInterface = FindInterface(target);
        EventInfo eInfo = eventInterface.GetEvent(eventName);
        eInfo.RemoveEventHandler(target, handlerDelegate); // THIS LINE CRASHES
    }
}

对 InstallEventHandler 的调用工作正常，然后在浏览器中触发事件时调用 Handler。当我使用与 InstallEventHandler 调用相同的参数调用 RemoveEventHandler 时，最后一行抛出 TargetInvocationException，内部异常为 NullReferenceException 。我不知道我在这里做错了什么，堆栈跟踪也没有太大帮助。

编辑:我已经在调试器中单步调试了代​​码，我直接引用的对象变量都不是空的。

我也试过在 InstallHandler 方法中同时调用 AddEventHandler 和 RemoveEventHandler，并且工作正常。

堆栈跟踪如下:

System.Reflection.TargetInvocationException occurred  Message="Exception has been thrown by the target of an invocation."  Source="mscorlib"  StackTrace:       at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)       at System.RuntimeMethodHandle.InvokeMethodFast(Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner)       at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)       at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)       at System.Reflection.EventInfo.RemoveEventHandler(Object target, Delegate handler)       at RemoveHandler(IHTMLElement target, String eventName)  InnerException: System.NullReferenceException       Message="Object reference not set to an instance of an object."       Source="Microsoft.mshtml"       StackTrace:            at mshtml.HTMLTextContainerEvents2_EventProvider.remove_onclick(HTMLTextContainerEvents2_onclickEventHandler )            at mshtml.HTMLTextAreaElementClass.HTMLTextContainerEvents2_Event_remove_onclick(HTMLTextContainerEvents2_onclickEventHandler )       InnerException: 

可能是什么原因造成的，我该如何解决？

Answer 1

当您调用 RemoveHandler 时，看起来您的 handlerDelegate 为 null。我不确定为什么会发生这种情况，但您是否尝试过在调用 RemoveEventHandler 之前实例化 handlerDelegate？