How to insert 'Empty' field in ComboBox bound to DataTable

Question

I have a combo box on a WinForms app in which an item may be selected, but it is not mandatory. I therefore need an 'Empty' first item to indicate that no value has been set.

The combo box is bound to a DataTable being returned from a stored procedure (I offer no apologies for Hungarian notation on my UI controls :p ):

 DataTable hierarchies = _database.GetAvailableHierarchies(cmbDataDefinition.SelectedValue.ToString()).Copy();//Calls SP
 cmbHierarchies.DataSource = hierarchies;
 cmbHierarchies.ValueMember = "guid";
 cmbHierarchies.DisplayMember = "ObjectLogicalName";

How can I insert such an empty item?

I do have access to change the SP, but I would really prefer not to 'pollute' it with UI logic.

Update: It was the DataTable.NewRow() that I had blanked on, thanks. I have upmodded you all (all 3 answers so far anyway). I am trying to get the Iterator pattern working before I decide on an 'answer'

Update: I think this edit puts me in Community Wiki land, I have decided not to specify a single answer, as they all have merit in context of their domains. Thanks for your collective input.

Answer 1

There are two things you can do:

1. Add an empty row to the DataTable that is returned from the stored procedure.

   DataRow emptyRow = hierarchies.NewRow();
   emptyRow["guid"] = "";
   emptyRow["ObjectLogicalName"] = "";
   hierarchies.Rows.Add(emptyRow);
   

   Create a DataView and sort it using ObjectLogicalName column. This will make the newly added row the first row in DataView.

   DataView newView =           
        new DataView(hierarchies,       // source table
        "",                             // filter
        "ObjectLogicalName",            // sort by column
        DataViewRowState.CurrentRows);  // rows with state to display
   

   Then set the dataview as DataSource of the ComboBox.

2. If you really don't want to add a new row as mentioned above. You can allow the user to set the ComboBox value to null by simply handling the "Delete" keypress event. When a user presses Delete key, set the SelectedIndex to -1. You should also set ComboBox.DropDownStyle to DropDownList. As this will prevent user to edit the values in the ComboBox.