上面这段代码会在ScriptManager的OnInit方法中被调用。请注意红色部分的代码，“_owner”变量是当前页面上的ScriptManager。在页面受到一个真正的异步会送之后，PageRequestManager会响应页面的Error事件，并且将错误信息用它定义的格式输出。如果我们只是修改了ScriptManager的私有field，那么如果在异步回送时出现了一个未捕获的异常，那么页面就会输出客户端未知的内容，导致在客户端解析失败。所以我们必须保证这种情况下的输出和真正的异步回送是相同的，所以我们就可以使用以下的做法来解决错误处理的问题。

internal static class AjaxFileUploadUtility
...{
    internal static bool IsInIFrameAsyncPostBack(NameValueCollection requestBody)
    ...{
        string[] values = requestBody.GetValues("__AjaxFileUploading__");

        if (values == null) return false;

        foreach (string value in values)
        ...{
            if (value == "__IsInAjaxFileUploading__")
            ...{
                return true;
            }
        }

        return false;
    }

    // ...
}

[PersistChildren(false)]
[ParseChildren(true)]
[NonVisualControl]
public class AjaxFileUploadHelper : Control
...{
    // ScriptManager members;
    private static FieldInfo isInAsyncPostBackFieldInfo;
    private static PropertyInfo pageRequestManagerPropertyInfo;

    // PageRequestManager members;
    private static MethodInfo onPageErrorMethodInfo;
    private static MethodInfo renderPageCallbackMethodInfo;


    static AjaxFileUploadHelper()
    ...{
        Type scriptManagerType = typeof(ScriptManager);
        isInAsyncPostBackFieldInfo = scriptManagerType.GetField(
            "_isInAsyncPostBack",
            BindingFlags.Instance | BindingFlags.NonPublic);
        pageRequestManagerPropertyInfo = scriptManagerType.GetProperty(
            "PageRequestManager",
            BindingFlags.Instance | BindingFlags.NonPublic);

        Assembly assembly = scriptManagerType.Assembly;
        Type pageRequestManagerType = assembly.GetType("System.Web.UI.PageRequestManager");
        onPageErrorMethodInfo = pageRequestManagerType.GetMethod(
            "OnPageError", BindingFlags.Instance | BindingFlags.NonPublic);
        renderPageCallbackMethodInfo = pageRequestManagerType.GetMethod(
            "RenderPageCallback", BindingFlags.Instance | BindingFlags.NonPublic);
    }

    public static AjaxFileUploadHelper GetCurrent(Page page)
    ...{
        return page.Items[typeof(AjaxFileUploadHelper)] as AjaxFileUploadHelper;
    }

    private bool isInAjaxUploading = false;

    protected override void OnInit(EventArgs e)
    ...{
        base.OnInit(e);

        if (this.Page.Items.Contains(typeof(AjaxFileUploadHelper)))
        ...{
            throw new InvalidOperationException("One AjaxFileUploadHelper per page.");
        }

        this.Page.Items[typeof(AjaxFileUploadHelper)] = this;

        this.EnsureIsInAjaxFileUploading();
    }

    private void EnsureIsInAjaxFileUploading()
    ...{
        this.isInAjaxUploading =             AjaxFileUploadUtility.IsInIFrameAsyncPostBack(this.Page.Request.Params);

        if (this.isInAjaxUploading)
        ...{
            isInAsyncPostBackFieldInfo.SetValue(
                ScriptManager.GetCurrent(this.Page),
                true);

            this.Page.Error += new EventHandler(Page_Error);
        }
    }

    private void Page_Error(object sender, EventArgs e)
    ...{
        // ...
    }


    private object _PageRequestManager;

    private object PageRequestManager
    ...{
        get
        ...{
            if (this._PageRequestManager == null)
            ...{
                this._PageRequestManager = pageRequestManagerPropertyInfo.GetValue(
                    ScriptManager.GetCurrent(this.Page), null);
            }

            return this._PageRequestManager;
        }
    }

    // ...
}