ExecuteReader的问题，请教了！

[astaward]

代码如下：
private void GridViewBind(string Sqlsort)
{
String dknf;
OleDbConnection oledbconn = new OleDbConnection();
oledbconn.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;" + "Data Source=" + Server.MapPath("../App_Data/oil.mdb");
OleDbCommand cmd = oledbconn.CreateCommand();
cmd.CommandText = "SELECT top 1 * FROM kanwu order by dkh desc";
oledbconn.Open();
OleDbDataReader dr = cmd.ExecuteReader();
while (dr.Read())
{
dknf = dr["dknf"].ToString();
}
dr.Close();
oledbconn.Close();
String user = Session["currentUser"].ToString();
OleDbConnection conn = new OleDbConnection();
conn.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;" + "Data Source=" + Server.MapPath("../App_Data/oil.mdb");
DataSet ds = new DataSet();
String SqlStr = "SELECT * FROM kanwu where status='已办结' and dknf='"+dknf+"' ORDER BY dkh ASC";
try
{
if (conn.State.ToString() == "Closed") conn.Open();

OleDbDataAdapter da = new OleDbDataAdapter(SqlStr, conn);
da.Fill(ds, "kanwu");
if (conn.State.ToString() == "Open") conn.Close();

GridView1.DataSource = ds.Tables[0].DefaultView;
GridView1.DataKeyNames = new string[] { "dkh" };
GridView1.DataBind();
if (ds.Tables[0].Rows.Count.ToString() == "0")
{
Response.Write("当前没有已办结订单！");
}


}
catch (Exception ex)
{
Response.Write("数据库错误，错误原因：" + ex.Message);
Response.End();
}
}


为什么dknf值取不出来，执行到“String SqlStr = "SELECT * FROM kanwu where status='已办结' and dknf='"+dknf+"' ORDER BY dkh ASC";”总提示Use of unassigned local variable 'dknf'，但是我在“dknf = dr["dknf"].ToString(); ”后放上response.write("dknf");却又可以把dknf显示出来？

请大家看看是什么原因，万分感谢！

[mngzilin]

用参数化处理

[xray2005]

private void GridViewBind(string Sqlsort)
{
String dknf;


改为==》


private void GridViewBind(string Sqlsort)
{
String dknf=string.Empty;

[Alden]

提示的很清楚，dknf没有被赋值，如果SELECT top 1 * FROM kanwu order by dkh desc
没有找到数据的话，那么就会出错。

[mngzilin]

String dknf=""；

[polarissky]

没看出来哪儿有问题，楼主断点、调试下，一步步运行看看得到的SQL语句是什么样子的

[wuyq11]

String dknf=""或String.Empty;
或isNullOrEmpty(dknf)?"":""

[astaward]

按照mngzilin兄的提示调试成功，万分感谢！