使用C＃.ASP中的动态/编程创建的网格视图启用分页

Question

I have a problem in enabling paging in a dynamically created gridview in C# .ASP 2010.

  protected void ButtonViewFiles_Click(object sender, EventArgs e)
  {
     ClearPanelConfigure();

        /* 
        logic to populate variable "gridviewlist" which is a type of List<string>
        ...

        */

     gv = new GridView();
     gv.ShowHeader = false;
     gv.BorderWidth = 0;
     gv.ForeColor = CONTROL_FONT_COLOR;
     gv.PageSize = 10;
     gv.RowDataBound += new GridViewRowEventHandler(GridView_OnRowDataBound);
     gv.AllowPaging = true;
     gv.PageIndexChanging += new GridViewPageEventHandler(GridView_PageIndexChanging);

     if (gridviewlist.Count > 0)
     {
        gv.DataSource = gridviewlist;
        gv.DataBind();
        PanelConfigure.Controls.Add(gv);
        mySession.SaveSessionObject(gridviewlist, "GridViewList");
     }
  }


  protected void GridView_PageIndexChanging(object sender, GridViewPageEventArgs e)
  {        
     gv.PageIndex = e.NewPageIndex;
     gv.DataBind();
  }


  // Enable HTML decode for each cell in the gridview rows.
  protected void GridView_OnRowDataBound(object sender, GridViewRowEventArgs e)
  {
     if (e.Row.RowType == DataControlRowType.DataRow)
     {
        for (int c = 0; c < e.Row.Cells.Count; c++)
        {
           e.Row.Cells[c].Text = Server.HtmlDecode(e.Row.Cells[c].Text);
        }
     }
  }

Unlike most people drag and drop the GridView control at design time, the Gridview in my case has to be programmatically created at the run time. That is why I have "gv = new GridView();" In other words, if user does not click a button and fires the button event, I will not show the GridView with all the URL links to files.

Once "gv" is created and enabled with paging, gv is added to PanelConfigure (which is type of Panel). Then user will be able to view all the file links.

"gv" is a global variable, otherwise "gv" will not compile in

  protected void GridView_PageIndexChanging(object sender, GridViewPageEventArgs e)
  {        
     gv.PageIndex = e.NewPageIndex;
     gv.DataBind();
  }

For people with drag and drop GridView at design time, this is not a problem. Because "gv" is defined with global scope.

If I don't enable paging in "gv". The web application at least runs and shows all the files in one shot. However, if I enable paging at run time with "gv.AllowPaging = true;", exception is thrown which points to "gv.DataBind();" in button event handler function

  protected void ButtonViewFiles_Click(object sender, EventArgs e)
  {        
     ...

     gv.DataBind();

     ...
  }

The exception says: NullReferenceException was unhandled by user code. Object reference not set to an instance of an object.

System.NullReferenceException was unhandled by user code

  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=System.Web
  StackTrace:
       at System.Web.UI.WebControls.GridView.get_StateFormatter()
       at System.Web.UI.WebControls.GridView.BuildCallbackArgument(Int32 pageIndex)
       at System.Web.UI.WebControls.GridView.CreateNumericPager(TableRow row, PagedDataSource pagedDataSource, Boolean addFirstLastPageButtons)
       at System.Web.UI.WebControls.GridView.InitializePager(GridViewRow row, Int32 columnSpan, PagedDataSource pagedDataSource)
       at System.Web.UI.WebControls.GridView.CreateRow(Int32 rowIndex, Int32 dataSourceIndex, DataControlRowType rowType, DataControlRowState rowState, Boolean dataBind, Object dataItem, DataControlField[] fields, TableRowCollection rows, PagedDataSource pagedDataSource)
       at System.Web.UI.WebControls.GridView.CreateChildControls(IEnumerable dataSource, Boolean dataBinding)
       at System.Web.UI.WebControls.CompositeDataBoundControl.PerformDataBinding(IEnumerable data)
       at System.Web.UI.WebControls.GridView.PerformDataBinding(IEnumerable data)
       at System.Web.UI.WebControls.DataBoundControl.OnDataSourceViewSelectCallback(IEnumerable data)
       at System.Web.UI.DataSourceView.Select(DataSourceSelectArguments arguments, DataSourceViewSelectCallback callback)
       at System.Web.UI.WebControls.DataBoundControl.PerformSelect()
       at System.Web.UI.WebControls.BaseDataBoundControl.DataBind()
       at System.Web.UI.WebControls.GridView.DataBind()
       at DataSearcher.advanced.ButtonViewFiles_Click(Object sender, EventArgs e) in C:\Users\fgui\documents\visual studio 2010\Projects\DataSearcher\DataSearcher\Advanced.aspx.cs:line 1853
       at System.Web.UI.WebControls.Button.OnClick(EventArgs e)
       at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
       at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
       at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
       at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
       at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
  InnerException: 

I searched this web site.

how to create paging in a dynamic gridview

The above post does not provide an answer to dynamically created GridView. It only works for GridView drag and drop at design time. I am not quite familiar with GridView. So any help is appreciated.

Thanks.

Answer 1

Creating such complicated controls dynamically and maintaining their state during postbacks could be extemely difficult. I really suggest that instead you create the control statically in the declarative code and only sets its Visible to true/false when necessary.

Answer 2

A dynamically added control will last for the current request only. If you use paging or any other action (!) on this page , a postback will be send to the server. The new request will recreate the page-object again without creating the dynamically added control. This behaviour is by purpose, because a postback request might be sent to another server in the server farm. Since you do not have stored the existence-information about the gridview somewhere else (eg in the ViewState), the server is not able to reconstruct it on a new request.

In conclusion, you will have to maintain the state and reconstruction of the control by yourself (very tricky if you are not familiar with page cycle, view state etc. ). Alternativly you follow Wiktor and create the grid view statically in the declarative code and use the visibility flag (highly recommended).