Question

I have a combobox which is called combobox1, which I want to populate it with id as value and Name as display name. I searched and read some tutorial and found this code to use in Form load event, but it doesn’t populate the list. I see an empty dropdown. Any ideas of where I am wrong?

In my database class I have this function.

public static void FillDropDownList(string Query, System.Windows.Forms.ComboBox DropDownName)
{
   SqlDataReader dr;

   SqlConnection myConnection = new SqlConnection(CONNECTION_STRING);
   try
   {
      myConnection.Open();
   }
   catch (Exception e)
   {
      Console.WriteLine(e.ToString());
   }

   // Check whether the Drop Down has existing items. If YES, empty it.
   if (DropDownName.Items.Count > 0)
      DropDownName.Items.Clear();

   SqlCommand cmd = new SqlCommand(Query, myConnection);
   dr = cmd.ExecuteReader();

   while (dr.Read())
      DropDownName.Items.Add(dr[0].ToString());

   Console.Write(DropDownName.Items.Add(dr[0].ToString()));
   dr.Close();
}

In my form i call it as

private void sales_record_Load(object sender, EventArgs e)
{
    SqlConnection con = new SqlConnection(DBUtils.CONNECTION_STRING);
    DBUtils.FillDropDownList("select id,Name from Farms", comboBox1);
}

Answer 1

My advice – use .NET built-in functionality as much as possible, and don’t handle data binding manually (which is what you are trying to do in your code):

1. Use ExecuteQuery to pull a DataTable from database.
2. Set DropDownName.DataSource = yourDataTable.
3. Set DropDownName.ValueMember = “id”.
4. Set DropDownName.DisplayMember = “Name”.

So your code would look similar to this:

public static void FillDropDownList(string Query, System.Windows.Forms.ComboBox DropDownName)
{
  DataTable dt;

  using (var cn = new SqlConnection(CONNECTION_STRING))
  {
    cn.Open();

    try
    {
      SqlCommand cmd = new SqlCommand(Query, cn);
      dt = cmd.ExecuteQuery();
    }
    catch (SqlException e)
    {
      Console.WriteLine(e.ToString());
      return;
    }
  }

  DropDownName.DataSource = dt;
  DropDownName.ValueMember = "id";
  DropDownName.DisplayMember = "Name";
}

Notice how I changed exception type to SqlException, so we are only looking for database errors. Everything else will blow up. I don’t remember any situation when myConnection.Open(); would throw an exception, so your try block is not very useful. Notice in my try clause – it has ExecuteQuery inside it, which is likely to fail.

EDIT: There is no need to close connection in the finally block when using the using construct. So it can be removed – and your code becomes more compact as a result.