Add combo box items from data layer c#

Question

I have a data layer class where I create all the SQL statements inside methods. In my UI layer i have a combo box. What I want to do is, I want to fill the combo box from the data layer, not from the UI layer . So far I have typed this code inside data layer....

public void ComboImageList()
{
    try
    {
        SqlCommand command = new SqlCommand("SELECT ImageName FROM [ImageWithTags]", con);
        con.Open();
        SqlDataReader reader = command.ExecuteReader();

        while(reader.Read())
        {
          string name = reader.GetString(reader.GetOrdinal("ImageName"));
        }
        con.Close();
    }
    catch(Exception ex)
    {
        MessageBox.Show(ex.Message);
    } 
}

Since I cannot access UI layer directly from Data layer, I dont know how to pass the values. I dont know if I can pass the combo box to the data layer or I have to pass the string to the UI layer.

can someone help?

Answer 1

You shouldn't do that, because such design leads to numerous problems: awkward testing, hard to refactor, relying on particular database and UI controls where you shouldn't, it will be hard to reuse DAL from other places.

Diagram

User -> UI -> DAL -> Database
                         |
User <- UI <-  DAL <------

Proper way of doing this will be to have a method in DAL like

IEnumerable<string> GetNames()
{
    // execute your SQL and return result as some abstract collection
    using( /* connection setup */
    {
        using(var command = new SqlCommand("SELECT ImageName FROM [ImageWithTags]", con))
        {
            con.Open();  // check this, maybe it could be opened in first using
            SqlDataReader reader = command.ExecuteReader();
            while(reader.Read())
            {
                yield return  reader.GetString(reader.GetOrdinal("ImageName"));
            }
        }
    }
}

And in UI layer you just call it like so

public void FillCombos()
{
    var repo = new DALRepo();
    var names = repo.GetNames();

    // now you can assign names to your combobox
    // ...
}