DataList相册分页求教（请各位大侠帮帮忙）

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
public partial class photo : System.Web.UI.Page
{
Class1 showphoto = new Class1();
PagedDataSource ps = new PagedDataSource();
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
lphoto();
isfirstlastpage();

}

}
public void lphoto()
{
ps.DataSource = showphoto.getdataset("select * from photo", "photo").Tables[0].DefaultView;
ps.AllowPaging = true;
ps.PageSize =6;
DLphoto.DataSource = ps;
DLphoto.DataKeyField = "id";
DLphoto.DataBind();
}
public void isfirstlastpage()
{
if (ps.PageCount > 0) //是否荐在数据
{
if (ps.PageCount != 1)
{
if (ps.CurrentPageIndex == 0)
{
prev.Enabled = false;
}
else
{
prev.Enabled=true;
}
if(ps.CurrentPageIndex==ps.PageCount-1)
{
next.Enabled=false ;
}
else
{
next.Enabled=true ;
}
}
}
}
protected void DLphoto_ItemCommand(object source, DataListCommandEventArgs e)
{
string id = DLphoto.DataKeys[e.Item.ItemIndex].ToString();
Server.Transfer("showphoto.aspx?id='"+id+"'");
}
protected void prev_Click(object sender, EventArgs e)
{
ps.CurrentPageIndex -= ps.CurrentPageIndex;
DLphoto.DataSource = ps;
DLphoto.DataBind();
isfirstlastpage();
}
protected void next_Click(object sender, EventArgs e)
{
ps.CurrentPageIndex += ps.CurrentPageIndex;
DLphoto.DataSource = ps;
DLphoto.DataBind(); //错误处
isfirstlastpage();
}
}

其中有错误,错误原因是"未将对象引用设置到对象的实例",代码错在"DLphoto.DataBind()",请问如何修改??
请给出具体的方法！