遇到一个UpdatePanel奇怪问题请教大家

Question

场景：使用UpdatePanel,然后里面放了两个联动的DropDownList,姑且称为dl1、dl2,其中在dl1的SelectedIndexChanged事件中绑定dl2.提交表单时：dl1的SelectedIndexChanged事件先被触发，然后才是提交表单Button的Click事件。重新又做了个小的例子，却未遇到此问题，感觉非常无语。另,网上已有人遇到此问题，但是遗憾的是没有确切明白的解释
参见
http://www.softwelt.com/Know/KnowDetail-3083137.html

Answer 1

<asp:ScriptManager ID="ScriptManager1" runat="server">
  </asp:ScriptManager>
  <div>
  <asp:UpdatePanel ID="UpdatePanel1" runat="server">
  <ContentTemplate>
  <asp:DropDownList ID="ddlLB" runat="server" Width="15%" AutoPostBack="True" OnSelectedIndexChanged="ddlLB_SelectedIndexChanged">
  </asp:DropDownList>
  <asp:DropDownList ID="ddlChild" runat="server" Width="20%">
  </asp:DropDownList>
  </ContentTemplate>
  </asp:UpdatePanel>
  </div>   
protected void ddlLB_SelectedIndexChanged(object sender, EventArgs e)
  {
  if(this.ddlLB.SelectedValue!=null)
  {  }
  }  
if(!Page.IsPostBack) 
{ 
ddlLB绑定数据
} 

Answer 2

但是点击提交按钮时先是触发了ddlLB_SelectedIndexChanged事件，然后才是Button的Click事件

Answer 3

很正常！所有的xxxxChanged类的事件，在所有的xxxxClicked类的事件之前触发（当然通常这只有一个）。asp.net就是如此。

Answer 4

我们把asp.net页面处理的内部过程的源代码看看：private void ProcessRequestMain(bool includeStagesBeforeAsyncPoint, bool includeStagesAfterAsyncPoint)
{
    try
    {
        HttpContext context = this.Context;
        string str = null;
        if (includeStagesBeforeAsyncPoint)
        {
            if (this.IsInAspCompatMode)
            {
                AspCompatApplicationStep.OnPageStartSessionObjects();
            }
            if (this.PageAdapter != null)
            {
                this._requestValueCollection = this.PageAdapter.DeterminePostBackMode();
            }
            else
            {
                this._requestValueCollection = this.DeterminePostBackMode();
            }
            string callbackControlID = string.Empty;
            if (this.DetermineIsExportingWebPart())
            {
                if (!RuntimeConfig.GetAppConfig().WebParts.EnableExport)
                {
                    throw new InvalidOperationException(SR.GetString("WebPartExportHandler_DisabledExportHandler"));
                }
                str = this.Request.QueryString["webPart"];
                if (string.IsNullOrEmpty(str))
                {
                    throw new InvalidOperationException(SR.GetString("WebPartExportHandler_InvalidArgument"));
                }
                if (string.Equals(this.Request.QueryString["scope"], "shared", StringComparison.OrdinalIgnoreCase))
                {
                    this._pageFlags.Set(4);
                }
                string str3 = this.Request.QueryString["query"];
                if (str3 == null)
                {
                    str3 = string.Empty;
                }
                this.Request.QueryStringText = str3;
                context.Trace.IsEnabled = false;
            }
            if (this._requestValueCollection != null)
            {
                if (this._requestValueCollection["__VIEWSTATEENCRYPTED"] != null)
                {
                    this.ContainsEncryptedViewState = true;
                }
                callbackControlID = this._requestValueCollection["__CALLBACKID"];
                if ((callbackControlID != null) && (this._request.HttpVerb == HttpVerb.POST))
                {
                    this._isCallback = true;
                }
                else if (!this.IsCrossPagePostBack)
                {
                    VirtualPath path = null;
                    if (this._requestValueCollection["__PREVIOUSPAGE"] != null)
                    {
                        try
                        {
                            path = VirtualPath.CreateNonRelativeAllowNull(DecryptString(this._requestValueCollection["__PREVIOUSPAGE"]));
                        }
                        catch
                        {
                            this._pageFlags[8] = true;
                        }
                        if ((path != null) && (path != this.Request.CurrentExecutionFilePathObject))
                        {
                            this._pageFlags[8] = true;
                            this._previousPagePath = path;
                        }
                    }
                }
            }
            if (this.MaintainScrollPositionOnPostBack)
            {
                this.LoadScrollPosition();
            }
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "Begin PreInit");
            }
            if (EtwTrace.IsTraceEnabled(5, 4))
            {
                EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_PRE_INIT_ENTER, this._context.WorkerRequest);
            }
            this.PerformPreInit();
            if (EtwTrace.IsTraceEnabled(5, 4))
            {
                EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_PRE_INIT_LEAVE, this._context.WorkerRequest);
            }
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "End PreInit");
            }
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "Begin Init");
            }
            if (EtwTrace.IsTraceEnabled(5, 4))
            {
                EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_INIT_ENTER, this._context.WorkerRequest);
            }
            this.InitRecursive(null);
            if (EtwTrace.IsTraceEnabled(5, 4))
            {
                EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_INIT_LEAVE, this._context.WorkerRequest);
            }
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "End Init");
            }
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "Begin InitComplete");
            }
            this.OnInitComplete(EventArgs.Empty);
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "End InitComplete");
            }
            if (this.IsPostBack)
            {
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "Begin LoadState");
                }
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_LOAD_VIEWSTATE_ENTER, this._context.WorkerRequest);
                }
                this.LoadAllState();
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_LOAD_VIEWSTATE_LEAVE, this._context.WorkerRequest);
                }
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End LoadState");
                    this.Trace.Write("aspx.page", "Begin ProcessPostData");
                }
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_LOAD_POSTDATA_ENTER, this._context.WorkerRequest);
                }
                this.ProcessPostData(this._requestValueCollection, true);
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_LOAD_POSTDATA_LEAVE, this._context.WorkerRequest);
                }
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End ProcessPostData");
                }
            }
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "Begin PreLoad");
            }
            this.OnPreLoad(EventArgs.Empty);
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "End PreLoad");
            }
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "Begin Load");
            }
            if (EtwTrace.IsTraceEnabled(5, 4))
            {
                EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_LOAD_ENTER, this._context.WorkerRequest);
            }
            this.LoadRecursive();
            if (EtwTrace.IsTraceEnabled(5, 4))
            {
                EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_LOAD_LEAVE, this._context.WorkerRequest);
            }
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "End Load");
            }

Answer 5

            if (this.IsPostBack)
            {
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "Begin ProcessPostData Second Try");
                }
                this.ProcessPostData(this._leftoverPostData, false);
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End ProcessPostData Second Try");
                    this.Trace.Write("aspx.page", "Begin Raise ChangedEvents");
                }
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_POST_DATA_CHANGED_ENTER, this._context.WorkerRequest);
                }
                this.RaiseChangedEvents();
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_POST_DATA_CHANGED_LEAVE, this._context.WorkerRequest);
                }
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End Raise ChangedEvents");
                    this.Trace.Write("aspx.page", "Begin Raise PostBackEvent");
                }
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_RAISE_POSTBACK_ENTER, this._context.WorkerRequest);
                }
                this.RaisePostBackEvent(this._requestValueCollection);
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_RAISE_POSTBACK_LEAVE, this._context.WorkerRequest);
                }
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End Raise PostBackEvent");
                }
            }
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "Begin LoadComplete");
            }
            this.OnLoadComplete(EventArgs.Empty);
            if (context.TraceIsEnabled)
            {
                this.Trace.Write("aspx.page", "End LoadComplete");
            }
            if (this.IsPostBack && this.IsCallback)
            {
                this.PrepareCallback(callbackControlID);
            }
            else if (!this.IsCrossPagePostBack)
            {
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "Begin PreRender");
                }
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_PRE_RENDER_ENTER, this._context.WorkerRequest);
                }
                this.PreRenderRecursiveInternal();
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_PRE_RENDER_LEAVE, this._context.WorkerRequest);
                }
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End PreRender");
                }
            }
        }
        if ((this._asyncInfo == null) || this._asyncInfo.CallerIsBlocking)
        {
            this.ExecuteRegisteredAsyncTasks();
        }
        this._request.ValidateRawUrl();
        if (includeStagesAfterAsyncPoint)
        {
            if (this.IsCallback)
            {
                this.RenderCallback();
            }
            else if (!this.IsCrossPagePostBack)
            {
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "Begin PreRenderComplete");
                }
                this.PerformPreRenderComplete();
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End PreRenderComplete");
                }
                if (context.TraceIsEnabled)
                {
                    this.BuildPageProfileTree(this.EnableViewState);
                    this.Trace.Write("aspx.page", "Begin SaveState");
                }
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_SAVE_VIEWSTATE_ENTER, this._context.WorkerRequest);
                }
                this.SaveAllState();
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_SAVE_VIEWSTATE_LEAVE, this._context.WorkerRequest);
                }
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End SaveState");
                    this.Trace.Write("aspx.page", "Begin SaveStateComplete");
                }
                this.OnSaveStateComplete(EventArgs.Empty);
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End SaveStateComplete");
                    this.Trace.Write("aspx.page", "Begin Render");
                }
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_RENDER_ENTER, this._context.WorkerRequest);
                }
                if (str != null)
                {
                    this.ExportWebPart(str);
                }
                else
                {
                    this.RenderControl(this.CreateHtmlTextWriter(this.Response.Output));
                }
                if (EtwTrace.IsTraceEnabled(5, 4))
                {
                    EtwTrace.Trace(EtwTraceType.ETW_TYPE_PAGE_RENDER_LEAVE, this._context.WorkerRequest);
                }
                if (context.TraceIsEnabled)
                {
                    this.Trace.Write("aspx.page", "End Render");
                }
                this.CheckRemainingAsyncTasks(false);
            }
        }
    }
    catch (ThreadAbortException exception)
    {
        HttpApplication.CancelModuleException exceptionState = exception.ExceptionState as HttpApplication.CancelModuleException;
        if (((!includeStagesBeforeAsyncPoint || !includeStagesAfterAsyncPoint) || ((this._context.Handler != this) || (this._context.ApplicationInstance == null))) || ((exceptionState == null) || exceptionState.Timeout))
        {
            this.CheckRemainingAsyncTasks(true);
            throw;
        }
        this._context.ApplicationInstance.CompleteRequest();
        Thread.ResetAbort();
    }
    catch (ConfigurationException)
    {
        throw;
    }
    catch (Exception exception3)
    {
        PerfCounters.IncrementCounter(AppPerfCounter.ERRORS_DURING_REQUEST);
        PerfCounters.IncrementCounter(AppPerfCounter.ERRORS_TOTAL);
        if (!this.HandleError(exception3))
        {
            throw;
        }
    }
}
你会看到，首先执行this.RaiseChangedEvents()，然后才是执行this.RaisePostBackEvent(this._requestValueCollection)。

Answer 6

还要请教4楼有什么有效办法确保提交时二级的下拉列表的SelectedIndexChanged事件不会被触发？