为什么eventinfo.removeeventhandler抛出一个nullreferenceexception?

我有一些代码正在使用反射在InternetExplorerBHO中的MSHTML对象上安装.NET事件处理程序,它看起来工作正常。然而,当我试图删除事件处理程序时,我遇到了麻烦。

事件处理程序安装和删除的代码如下所示:

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
    }
}

呼唤 InstallEventHandler 很好,而且 Handler 然后在浏览器中触发事件时调用。当我呼唤 RemoveEventHandler 与 安装事件处理程序 呼叫,最后一行抛出 TargetInvocationException ,但内部例外 NullReferenceException 。我不知道我做错了什么,堆栈跟踪也没什么帮助。

编辑: 我已经遍历了调试器中的代码,并且我直接引用的对象变量都不是空的。

我也试过给他们打电话 AddEventHandler 和 RemoveEventhandler公司 在installhandler方法中,这是正确的。

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: 

是什么引起的,我该怎么解决?