private void btndel_Click(object sender, System.EventArgs e)
{
string sqlco= ConfigurationSettings.AppSettings["strCon"];
SqlConnection sqlcon=new SqlConnection(sqlco);
string sqlstr3="delete from ReadRoom1 where ReadRoomName=@roomname";
sqlcon.Open();
SqlCommand cm2=new SqlCommand(sqlstr3,sqlcon);
cm2.Parameters.Add(new SqlParameter("@roomname",SqlDbType.VarChar,20));
cm2.Parameters["@roomname"].Value=listroom.SelectedItem.ToString();
cm2.ExecuteNonQuery();
sqlcon.Close();
BindGrid();
//这里,将在querrynote1表中删除该列
string sqlstr4="alter table Querrynote1 drop column @room ";
SqlCommand cm3=new SqlCommand(sqlstr4,sqlcon);
cm3.Parameters.Add(new SqlParameter("@room",SqlDbType.VarChar,20));
cm3.Parameters["@room"].Value=listroom.SelectedItem.ToString();
sqlcon.Open();
cm3.ExecuteNonQuery();
sqlcon.Close();
}
错误显示:cm3.Parameters["@room"].Value=listroom.SelectedItem.ToString();这里有问题,
System.NullReferenceException: 未将对象引用设置到对象的实例。但是我在注释前面的代码中都可以得到listroom.SelectedItem.ToString()的值啊,为什么一加注释后的代码就不行了
{
string sqlco= ConfigurationSettings.AppSettings["strCon"];
SqlConnection sqlcon=new SqlConnection(sqlco);
string sqlstr3="delete from ReadRoom1 where ReadRoomName=@roomname";
sqlcon.Open();
SqlCommand cm2=new SqlCommand(sqlstr3,sqlcon);
cm2.Parameters.Add(new SqlParameter("@roomname",SqlDbType.VarChar,20));
cm2.Parameters["@roomname"].Value=listroom.SelectedItem.ToString();
cm2.ExecuteNonQuery();
sqlcon.Close();
BindGrid();
//这里,将在querrynote1表中删除该列
string sqlstr4="alter table Querrynote1 drop column @room ";
SqlCommand cm3=new SqlCommand(sqlstr4,sqlcon);
cm3.Parameters.Add(new SqlParameter("@room",SqlDbType.VarChar,20));
cm3.Parameters["@room"].Value=listroom.SelectedItem.ToString();
sqlcon.Open();
cm3.ExecuteNonQuery();
sqlcon.Close();
}
错误显示:cm3.Parameters["@room"].Value=listroom.SelectedItem.ToString();这里有问题,
System.NullReferenceException: 未将对象引用设置到对象的实例。但是我在注释前面的代码中都可以得到listroom.SelectedItem.ToString()的值啊,为什么一加注释后的代码就不行了
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改成上面的代码试试。
这个肯定跟注释没关系的
listroom.SelectedItem.text.ToString();
rickjelly2004(rick & jelly) ( ) 信誉:100
我注释前面的代码里也用的是listroom.SelectedItem.ToString();,为什前面可以得到呢?后面就得不到了呢?
另外listroom.SelectedItem.ToString();
要写成listroom.SelectedItem.value.ToString();或者
listroom.SelectedItem.text.ToString();
如果你要使用多次,最好取一次,存到string
string str = listroom.SelectedItem.value.ToString();
cm2.Parameters["@roomname"].Value=str;
cm2.ExecuteNonQuery();
sqlcon.Close();
BindGrid();
//这里,将在querrynote1表中删除该列
string sqlstr4="alter table Querrynote1 drop column @room ";
SqlCommand cm3=new SqlCommand(sqlstr4,sqlcon);
cm3.Parameters.Add(new SqlParameter("@room",SqlDbType.VarChar,20));
cm3.Parameters["@room"].Value=str;
sqlcon.Open();
cm3.ExecuteNonQuery();
sqlcon.Close();
private void Page_Load(object sender, System.EventArgs e)
{ if(!IsPostBack)
{
BindGrid();
}
}
void BindGrid()
{
string sqlco= ConfigurationSettings.AppSettings["strCon"];
SqlConnection sqlcon=new SqlConnection(sqlco);
string sqlstr="select ReadRoomName from ReadRoom1";
SqlDataAdapter da=new SqlDataAdapter(sqlstr,sqlcon);
DataSet ds=new DataSet();
da.Fill(ds);
listroom.DataSource=ds;
listroom.DataBind();
}
private void btndel_Click(object sender, System.EventArgs e)
{
string sqlco= ConfigurationSettings.AppSettings["strCon"];
SqlConnection sqlcon=new SqlConnection(sqlco);
string sqlstr3="delete from ReadRoom1 where ReadRoomName=@roomname";
sqlcon.Open();
string str=listroom.SelectedItem.Value.ToString();
SqlCommand cm2=new SqlCommand(sqlstr3,sqlcon);
cm2.Parameters.Add(new SqlParameter("@roomname",SqlDbType.VarChar,20));
cm2.Parameters["@roomname"].Value=str;
cm2.ExecuteNonQuery();
sqlcon.Close();
BindGrid();
string sqlstr4="alter table Querrynote1 drop column @room ";
SqlCommand cm3=new SqlCommand(sqlstr4,sqlcon);
cm3.Parameters.Add(new SqlParameter("@room",SqlDbType.VarChar,20));
cm3.Parameters["@room"].Value=str;
sqlcon.Open();
cm3.ExecuteNonQuery();
sqlcon.Close();
}
{
string tem;
string sqlco= ConfigurationSettings.AppSettings["strCon"];
SqlConnection sqlcon=new SqlConnection(sqlco);
string sqlstr3="delete from ReadRoom1 where ReadRoomName=@roomname";
sqlcon.Open();
SqlCommand cm2=new SqlCommand(sqlstr3,sqlcon);
cm2.Parameters.Add(new SqlParameter("@roomname",SqlDbType.VarChar,20));
cm2.Parameters["@roomname"].Value=listroom.SelectedItem.ToString();
cm2.ExecuteNonQuery();
sqlcon.Close();
tem=listroom.SelectedItem.value.ToString();
BindGrid();
string sqlstr4="alter table Querrynote1 drop column @room ";
SqlCommand cm3=new SqlCommand(sqlstr4,sqlcon);
cm3.Parameters.Add(new SqlParameter("@room",SqlDbType.VarChar,20));
cm3.Parameters["@room"].Value=tem
sqlcon.Open();
cm3.ExecuteNonQuery();
sqlcon.Close();
}