public partial class MainWindow : Window
 {
     private SqlConnection connection;
     private string dbName = string.Empty;
     
     public MainWindow()
     {
         InitializeComponent();
         LoadData();
     }

     private void LoadData()
     {

         try
         {
             string connectionString = "Data Source=localhost;Initial Catalog=Test_DB;Integrated Security=True";
             using (SqlConnection connection = new SqlConnection(connectionString))
             {
                 connection.Open();
                 string query = "SELECT * FROM dbo.MachineAreas";
                 SqlDataAdapter adapter = new SqlDataAdapter(query, connection);
                 DataTable dataTable = new DataTable();
                 adapter.Fill(dataTable);

                 gridMachineAreas.ItemsSource = dataTable.DefaultView;
             }
         }
         catch (Exception ex)
         {
             MessageBox.Show("Error: " + ex.Message);
         }
     }

     private void GridMachineAreas_AutoGeneratingColumn(object sender, GridViewAutoGeneratingColumnEventArgs e)
     {            

         switch ((e.Column as GridViewDataColumn).DataMemberBinding.Path.Path)
         {
             case "ID":
                 var newColumn1 = new GridViewDataColumn();
                 newColumn1.CopyPropertiesFrom(e.Column);
                 newColumn1.Header = "ID";
                 newColumn1.Width = 60;
                 e.Column = newColumn1;
                 break;

             case "Country":
                 var newColumn2 = new GridViewComboBoxColumn();
                 newColumn2.CopyPropertiesFrom(e.Column);
                 newColumn2.Header = "Country";
                 newColumn2.Width = 60;
                 newColumn2.UniqueName = "Country";
                 newColumn2.IsComboBoxEditable = false;
                 newColumn2.ItemsSource = MakeCountryCollection();                    
                 
                 e.Column = newColumn2;
                 break;
             case "Plant":
                 var newColumn3 = new GridViewComboBoxColumn();
                 newColumn3.CopyPropertiesFrom(e.Column);
                 newColumn3.Header = "Plant";
                 newColumn3.Width = 60;                    
                 e.Column = newColumn3;
                 break;
             case "Area":
                 var newColumn4 = new GridViewComboBoxColumn();
                 newColumn4.CopyPropertiesFrom(e.Column);
                 newColumn4.Header = "Area";
                 newColumn4.Width = 60;
                 e.Column = newColumn4;
                 break;
             case "Machine":
                 var newColumn5 = new GridViewComboBoxColumn();
                 newColumn5.CopyPropertiesFrom(e.Column);
                 newColumn5.Header = "Machine";
                 newColumn5.Width = 60;
                 e.Column = newColumn5;
                 break;
         }

     }

     private List<string> MakeCountryCollection()
     {
         List<string> countryCollection = new List<string>();
         string query = "SELECT Country FROM dbo.MachineAreas Group By Country";
         
         try
         {
             string connectionString = "Data Source=localhost;Initial Catalog=Test_DB;Integrated Security=True";
             using (SqlConnection connection = new SqlConnection(connectionString))
             {
                 connection.Open();


                 using (SqlCommand command = new SqlCommand(query, connection))
                 {
                     using (SqlDataReader reader = command.ExecuteReader())
                     {


                         while (reader.Read())
                         {
                             string value = reader.GetString(0);
                             countryCollection.Add(value);
                         }
                         
                     }
                 }
             }
         }
         catch (Exception ex)
         {
             MessageBox.Show("Error: " + ex.Message);
         }
         return countryCollection;
     }
 }

<telerik:RadGridView x:Name="dg" ItemsSource="{Binding data}"
    ValidatesOnDataErrors="InEditMode" IsPropertyChangedAggregationEnabled="False"
    GroupRenderMode="Flat" ShowGroupPanel="False"AutoGenerateColumns="False"CanUserFreezeColumns="False"RowIndicatorVisibility="Collapsed"CanUserResizeColumns="False">

public class RadGridViewAddCustomFieldDefinitionColumnsBehavior : RadGridViewAddFieldDefinitionColumnsBehavior<CustomFieldDefinition>
{
    protected override void AddFieldsColumns(
        IEnumerable<CustomFieldDefinition> fieldDefinitions,
        GridViewDataControl radGridView,
        RadGridViewAddFieldDefinitionColumnsBehavior<CustomFieldDefinition> radGridViewAddFieldColumnsBehavior)
    {
        var corporationIDToNameConverter = new CorporationIDToNameConverter();
        var contactIDToNameConverter = new ContactIDToNameConverter();

        foreach (var customFieldDefinition in fieldDefinitions)
        {
            var column = new GridViewDataColumn
                         {
                             Header = customFieldDefinition.PropertyName,
                             UniqueName = customFieldDefinition.PropertyName,
                             DataMemberBinding = new Binding("CustomFields")
                                                 {
                                                     Converter = new ValueConverterChain
                                                                 {
                                                                     optionalDictionaryValueConverter,
                                                                 },
                                                     ConverterParameter = new ValueConverterChainParameters
                                                                          {
                                                                              customFieldDefinition.ID,
                                                                          },
                                                 },
                         };

            switch (customFieldDefinition.FieldType)
            {
                case FieldType.Corporation:
                    SetColumnValueConverter(column, customFieldDefinition, corporationIDToNameConverter);
                    break;
                case FieldType.Contact:
                    SetColumnValueConverter(column, customFieldDefinition, contactIDToNameConverter);
                    break;
            }

            CurrentFieldDefinitionColumns.Add(column);
            radGridView.Columns.Add(column);
        }
    }

    protected override void SetColumnValueConverter(
        GridViewDataColumn column,
        CustomFieldDefinition fieldDefinition,
        IValueConverter valueConverter)
    {
        var converterChain = (ValueConverterChain)column.DataMemberBinding.Converter;
        var valueConverterChainParameters = (ValueConverterChainParameters)column.DataMemberBinding.ConverterParameter;

        converterChain.Add(valueConverter);
        valueConverterChainParameters.Add(fieldDefinition.Values);
    }
}