public class CustomGrid : RadGridView
 {
     static CustomGrid()
     {
         DefaultStyleKeyProperty.OverrideMetadata(typeof(CustomGrid), new FrameworkPropertyMetadata(typeof(CustomGrid)));
     }

     protected override void OnInitialized(EventArgs e)
     {
         base.OnInitialized(e);
                 
     }
 }

 <Grid>
     <my:CustomGrid Grid.Row="0"
                   x:Name="clubsGrid" 
                   AutoGenerateColumns="False"
                   GroupRenderMode="Flat"
                   Margin="5">
         <my:CustomGrid.Columns>
             <telerik:GridViewDataColumn DataMemberBinding="{Binding Name}"/>
             <telerik:GridViewDataColumn DataMemberBinding="{Binding Established}" Header="Est." DataFormatString="{}{0:yyyy}"/>
             <telerik:GridViewDataColumn DataMemberBinding="{Binding StadiumCapacity}" Header="Stadium" DataFormatString="{}{0:N0}"/>
         </my:CustomGrid.Columns>
     </my:CustomGrid>
 </Grid>

 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:PercentComboBox DataContext="{Binding DataContext, RelativeSource={RelativeSource AncestorType={x:Type local:DialogControl}}}"
                           Value="{Binding ScaleFactor,ElementName=pdfViewer, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
                           RadPdfViewer="{Binding ElementName=pdfViewer}" ShowFitToWidth="True" ShowFitToPage="True"
                           telerik:Windows11ThemeSizeHelper.EnableDynamicAnimation="True" Language="de-DE">
      <telerik:PercentComboBox.PercentValues>
          <telerik:Percent Value="200"/>
          <telerik:Percent Value="150"/>
          <telerik:Percent Value="125"/>
          <telerik:Percent Value="110"/>
          <telerik:Percent Value="100"/>
          <telerik:Percent Value="90"/>
          <telerik:Percent Value="75"/>
          <telerik:Percent Value="50"/>
          <telerik:Percent Value="30"/>
      </telerik:PercentComboBox.PercentValues>
  </telerik:PercentComboBox>

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