为什么DataSet 的值是空??
public partial class Report1 : System.Web.UI.Page
{
DataSet ds = new DataSet();

protected void btn_find_Click(object sender, EventArgs e)
{
string strconn = ConfigurationManager.AppSettings[ "dsn "];
//连接本地计算机的WMS数据库
SqlConnection cn = new SqlConnection(strconn);
string mysql = "SELECT a.vin, d.name AS operate, a.operate_time, a.warehouse, b.name AS driver, c.name AS operator FROM Transations a INNER JOIN Drivers b ON a.driver = b.code INNER JOIN Users c ON a.operator = c.Uname INNER JOIN TransationCode d ON a.operate = d.code where (operate= 'movein ') ";
//如果仓库有输入
if (ddl_warehouse.SelectedIndex != 0)
{
mysql = mysql + "and a.warehouse= ' " + ddl_warehouse.SelectedItem.Text.ToString() + " ' ";
}
if (tbx_time1.Text.ToString() != " ")
{
mysql = mysql + "and a.operate_time> = ' " + tbx_time1.Text.ToString() + " ' ";
}
if (tbx_time2.Text.ToString() != " ")
{
mysql = mysql + "and a.operate_time <= ' " + tbx_time2.Text.ToString() + " ' ";
}
mysql = mysql + " ORDER BY a.operate_time ";

SqlDataAdapter da = new SqlDataAdapter(mysql, cn);

da.Fill(ds);

GridView1.DataSource = ds;

GridView1.DataBind();

cn.Close();
//查询得到一个ds
}

public void CreateExcel(DataSet myds)//导出数据到execl的函数
{
..........
}
protected void btn_export_Click(object sender, EventArgs e)//导出数据
{
CreateExcel(this.ds)
}
}

在btn_export_Click事件中调用this.ds,但是this.ds的值是空?请问是甚么原因呢

[解决办法]
btn_find_Click 点击以后DataSet的值并没有被 保存

楼主要清除asp.net是无状态的 每次访问的状态的不被保存的(除非楼主使用了缓存cache)

再你的btn_export_Click中
需要重新获取ds 的值
[解决办法]
看你的实际需要，你可以保存ds结果，也可以保存条件去查询。
可以用viewstate, 也可以用session. 下面的用viewstate. 自己权衡性能上的要求。

protected void btn_find_Click(object sender, EventArgs e)
{
string strconn = ConfigurationManager.AppSettings[ "dsn "];
//连接本地计算机的WMS数据库
SqlConnection cn = new SqlConnection(strconn);
string mysql = "SELECT a.vin, d.name AS operate, a.operate_time, a.warehouse, b.name AS driver, c.name AS operator FROM Transations a INNER JOIN Drivers b ON a.driver = b.code INNER JOIN Users c ON a.operator = c.Uname INNER JOIN TransationCode d ON a.operate = d.code where (operate= 'movein ') ";
//如果仓库有输入
if (ddl_warehouse.SelectedIndex != 0)
{
mysql = mysql + "and a.warehouse= ' " + ddl_warehouse.SelectedItem.Text.ToString() + " ' ";
}
if (tbx_time1.Text.ToString() != " ")
{
mysql = mysql + "and a.operate_time> = ' " + tbx_time1.Text.ToString() + " ' ";
}
if (tbx_time2.Text.ToString() != " ")
{
mysql = mysql + "and a.operate_time <= ' " + tbx_time2.Text.ToString() + " ' ";
}
mysql = mysql + " ORDER BY a.operate_time ";

SqlDataAdapter da = new SqlDataAdapter(mysql, cn);

da.Fill(ds);

GridView1.DataSource = ds;

GridView1.DataBind();

cn.Close();
//查询得到一个ds
ViewState[ "ds "] = ds
}

protected void btn_export_Click(object sender, EventArgs e)//导出数据
{ if (ViewState[ "ds "] != null)
CreateExcel((DataSet)ViewState[ "ds "])
}