自制可分页的DATALIST WEB 控件 的问题，高手请进

[zhongkeruanjian]

控件名称为SUNDATALIST
代码如下：
using System;
using System.Collections;
using System.Collections.Specialized;
using System.Text;
using System.Text.RegularExpressions;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace Sun
{
public class SunDataList:DataList
{
private const string KEY_ITEM_COUNT = "ItemCount";
private const string KEY_CURRENT_PAGE_INDEX = "CurrentPageIndex";
private ImageButton btnPrev;
private ImageButton btnMore;

private int pageSize;
private IList dataSource;
// private int currentPageIndex;
//private int itemCount;

override public object DataSource
{
set
{
//This try catch block is to avoid issues with the VS.NET designer
//The designer will try and bind a datasource which does not derive from ILIST
try
{
dataSource = (IList)value;
ItemCount = dataSource.Count;
}
catch
{
dataSource = null;
ItemCount = 0;
}
}
}

public int PageSize
{
get { return pageSize; }
set { pageSize = value; }
}

protected int PageCount
{
get { return (ItemCount - 1) / (pageSize*RepeatColumns); }
}


protected int ItemCount
{
get { return (int)ViewState[KEY_ITEM_COUNT]; }
set { ViewState[KEY_ITEM_COUNT] = value; }
}

public int CurrentPageIndex
{
get { return (int)ViewState[KEY_CURRENT_PAGE_INDEX]; }
set { ViewState[KEY_CURRENT_PAGE_INDEX] = value; }
}

public void SetPage(int index)
{
OnPageIndexChanged(new DataGridPageChangedEventArgs(null, index));
}
public event DataGridPageChangedEventHandler PageIndexChanged;

virtual protected void OnPageIndexChanged(DataGridPageChangedEventArgs e)
{
if (PageIndexChanged != null)
PageIndexChanged(this, e);
}


private void PreviousClicked(object sender, ImageClickEventArgs e)
{


OnPageIndexChanged(new DataGridPageChangedEventArgs(sender, CurrentPageIndex - 1));
}

private void MoreClicked(object sender, ImageClickEventArgs e)
{


OnPageIndexChanged(new DataGridPageChangedEventArgs(sender, CurrentPageIndex + 1));
}
override protected void OnInit(EventArgs e)
{



if (!Page.IsPostBack && Visible)
{

CurrentPageIndex = 0;
SetPage(0);
}

}

override protected void CreateControlHierarchy(bool useDataSource)
{
base.CreateControlHierarchy(useDataSource);

btnPrev = new ImageButton();

btnPrev.Click += new ImageClickEventHandler(PreviousClicked);
Controls.Add(btnPrev);

btnMore = new ImageButton();

btnMore.Click += new ImageClickEventHandler(MoreClicked);
Controls.Add(btnMore);
}

override protected void OnPreRender(EventArgs e)
{
btnPrev.Visible = CurrentPageIndex > 0;
btnMore.Visible = CurrentPageIndex < PageCount;
}

override protected void Render(HtmlTextWriter writer)
{


//for (int i = 0, j = Controls.Count - 2; i < j; i++)
// Controls[i].RenderControl(writer);

base.Render(writer);
// if (!Page.IsPostBack && Visible)
// {
// btnPrev.RenderControl(writer);

// btnMore.RenderControl(writer);
// }
// if(Page.IsPostBack && show)
// {
btnPrev.RenderControl(writer);

btnMore.RenderControl(writer);

// }


}

override protected void OnDataBinding(EventArgs e)
{



int start = CurrentPageIndex * pageSize*RepeatColumns;
int size = Math.Min(pageSize*RepeatColumns, ItemCount - start);

IList page = new ArrayList();

//Add the relevant items from the datasource
for (int i = 0; i < size; i++)
page.Add(dataSource[start + i]);

//set the base objects datasource
base.DataSource = page;
//Work out which items we want to render to the page



base.OnDataBinding(e);



}
}
}



问题是控件在第一次显示时正常，但是按“btnMore”图像按钮时出现问题
如下：
[InvalidCastException: 指定的转换无效。]
System.Web.UI.WebControls.DataList.PrepareControlHierarchy() +148
System.Web.UI.WebControls.BaseDataList.Render(HtmlTextWriter writer) +15
Sun.SunDataList.Render(HtmlTextWriter writer) +15
System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +243
System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +72
System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer) +44
System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output) +395
System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +243
System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +72
System.Web.UI.Control.Render(HtmlTextWriter writer) +7
System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +243
System.Web.UI.Page.ProcessRequestMain() +1900

[saucer]

you also need to create the controls again upon postback, see how it is done in the DataBoundTable example provided by the author of Developing Microsoft ASP.NET Server Controls and Components

http://www.nikhilk.net/Content/Presentations/TechEd2003_Code.zip

[zhongkeruanjian]

其实这里面就是两个IMAGEBUTTON什么时候加载到CONTROLS的问题，
但是
[InvalidCastException: 指定的转换无效。]
是在转换什么东西的时候出现这个问题的呢，
我该在DATALIST的哪个阶段加入这两个BUTTON呢？