title: object must implement iconvertible devexpress
body: <p>I have a gridcontrol which is populated from database like this:</p>
  
  <pre><code>OracleConnection connection= new OracleConnection("DATA SOURCE=mydb;PASSWORD=pass;PERSIST SECURITY INFO=True;USER ID=id");
  connection.Open();
  OracleDataAdapter adapter = new OracleDataAdapter("select * from mytable", connection);
  DataTable datatable= new DataTable();
  adapter.Fill(datatable);
  adapter.Dispose();
  connection.Close();
  gridControl1.ItemsSource = datatable.DefaultView;
  </code></pre>
  
  <p>Then , I change the type of gridcolumns like this:</p>
  
  <pre><code>GridColumn colCOMBOBOX = gridControl1.Columns["COMBO_MULTISELECTION"]; 
  ComboBoxEditSettings combo = new ComboBoxEditSettings();
  combo.StyleSettings = new CheckedComboBoxStyleSettings();
  combo.ItemsSource = datatable;
  combo.DisplayMember = "PERSON";
  combo.ValueMember = "COMBO_MULTISELECTION";
  colCOMBOBOX.EditSettings = combo;
  </code></pre>
  
  <p>I have a multiselection combobox in my gridcolumn. Each item from combobox has an id.
  My problem is : after I select multiple items, I get the error: <strong>object must implement iconvertible</strong>. So, I made a list that gets the data from database and also I made it Iconvertible. But the error still remains. Everything goes perfect with a combobox that is outside of gridcontrol.Doesn't gridcontrol supports this? How can I solve the problem? </p>