从dropDownlist控件返回字符串

Question

I have what is a very elementary problem I realize, I am trying to return a string value from a selected value of a DropDownList upon selected index change but for some or other reason it is not happening.

protected void drpMinisters_SelectedIndexChanged(object sender, EventArgs e)
{
    name = drpMinisters.SelectedValue;
    LabMessage.Text = name;

}

When I try to add name to a database I get a NullReferenceException.

protected void butSubmitMinister_Click(object sender, EventArgs e)
{
    int index = drpMinisters.SelectedIndex;
    if (index == 0)
    {
        LabMessage.Text = "Please select a minister";
        return;
    }
    try
    {
        OleDbCommand cmd = conn.CreateCommand();
        cmd.CommandText = @"INSERT INTO MinisterTable(MinisterName)VALUES(" + name + "')";
        cmd.ExecuteNonQuery();

        LabMessage.Text = "The record was successfully added";
        conn.Close();
    }
    catch (Exception ex)
    {
        LabMessage.Text = ex.ToString();
    }


}

Advice perhaps.

Answer 1

I assume that you get the NullRefernceException on your connection object conn on this line:

OleDbCommand cmd = conn.CreateCommand();

Remember that all objects (also your name variable) are disposed at the end of the current page-lifecycle. You can use the control's ViewState to maintain values across postbacks, fe the SelectedValue property of your DropDownList .

You should also create the connection where you use it and always dispose it as soon as you're finished, best by using the using -statement. Otherwise other threads(requests) would need to create a new physical connection whenever this is still open:

string sql = "INSERT INTO MinisterTable(MinisterName) VALUES(@MinisterName);";
using(var con = new OleDbConnection(connectionString))
using(var cmd = new OleDbCommand(sql, con))
{
    cmd.Parameters.AddWithValue("@MinisterName", drpMinisters.SelectedValue);
    con.Open();
    cmd.ExecuteNonQuery();
}

Answer 2

well i dont think your problem is with the database part, it actually lies in how you are getting the selected value from your dropdown list. if you want to get the string value that actually appears in the dropdown list, then you should use the following .

string name= (string)drpMinisters.SelectedItem;

in case you want the text in the text area of combobox then use SelectedText property