protected void Button1_Click(object sender, EventArgs e)
    {
s.AppendLine("<table style='width:800px' border=1>");

    
        for (int i = 0; i < GridView1.Rows.Count; i++)
        {
 

            CheckBox ck = (CheckBox)GridView1.Rows[i].Cells[0].FindControl("CheckBox1");
            if (ck.Checked)
            {
                //GridView1.Columns 
                s.AppendLine(" <tr>");
                for (int j = 0; j < GridView1.Columns.Count; j++)
                {
                
                   s.AppendFormat("<td>{0} </td>", ((Label)GridView1.Rows[i].Cells[j].FindControl("Label")).Text);

// 提示出错：System.NullReferenceException: 未将对象引用设置到对象的实例。

                }
                s.AppendLine(" </tr>");

            }
        } 


        s.Append("</table>");
        Export("application/ms-excel", "excel.xls", s.ToString());

}

private void Export(string FileType, string FileName, string s)
    {

        Response.Charset = "UTF-8";
        Response.ContentEncoding = System.Text.Encoding.UTF8;
        Response.AppendHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(FileName, Encoding.UTF8).ToString());
        Response.ContentType = FileType;
        Response.Write(s);
        Response.End();

    }

for (int i = 0; i < GridView1.Rows.Count; i++)
        {
 

            CheckBox ck = GridView1.Rows[i].FindControl("CheckBox1") as CheckBox;
            if (ck!=null&&ck.Checked)
            {        
                s.AppendLine(" <tr>");        
                Label lbl=GridView1.Rows[i].FindControl("Label1") as Label;
                if(lbl!=null)
                {
                   s.AppendFormat(" <td>{0} </td>", lbl.Text);
                } 
                lbl=GridView1.Rows[i].FindControl("Label2") as Label;
                if(lbl!=null)
                {
                   s.AppendFormat(" <td>{0} </td>", lbl.Text);
                } 
                ...
                s.AppendLine(" </tr>");
            }
        }