How to place a ComboBoxBase control within a PopupControlContainer such that the PopupControlContainer does not close when the ComboBoxBase’s popup is displayed

You can do this by deriving from our PopupControlContainer, overriding the OnPopup method, and setting the focus to the derived control. This ensures that the derived PopupControlContainer does not lose focus and close prematurely. The customized PopupControlContainer code should look as shown below:

[C#]

public class CustomPopupControlContainer : Syncfusion.Windows.Forms.PopupControlContainer
{

   public CustomPopupControlContainer()
   {       }

   public CustomPopupControlContainer(IContainer container):this()
   {
     container.Add(this);
   }

  protected override void OnPopup(EventArgs args)
  {
    base.OnPopup(args);
    this.Focus();
  }

}

[VB.NET]

Public Class CustomPopupControlContainer
   Inherits Syncfusion.Windows.Forms.PopupControlContainer
   Public  Sub New()
     End Sub
   Public  Sub New(ByVal container As IContainer) : Me()
    container.Add(Me)
   End Sub
     Protected Overrides  Sub OnPopup(ByVal args As EventArgs)
    MyBase.OnPopup(args)
    Me.Focus()
   End Sub
   End Class

It is also necessary to specify the parent-child relationship between the ComboBoxBase’s popup and the PopupControlContainer. This could be done by handling the ComboBoxBase’s drop-down event as shown in the code sample below:

[C#]

private void comboBoxBase1_DropDown(object sender, System.EventArgs e)
{
        /* Setup the relationship between the ComboBoxBase’s dropdown and it's parent
                PopupControlContainer, so that the popup will not close when the ComboBoxBase’s
            dropdown is shown */

  this.comboBoxBase1.PopupContainer.PopupParent = this.popupControlContainer1;
   this.popupControlContainer1.CurrentPopupChild = this.comboBoxBase1.PopupContainer;
}

[VB.NET]

Private  Sub comboBoxBase1_DropDown(ByVal sender As Object, ByVal e As System.EventArgs)

'         Setup the relationship between the ComboBoxBase’s dropdown and it's parent
         '        PopupControlContainer, so that the popup will not close when the ComboBoxBase’s 
    '        dropdown is shown
      Me.comboBoxBase1.PopupContainer.PopupParent = Me.popupControlContainer1
   Me.popupControlContainer1.CurrentPopupChild = Me.comboBoxBase1.PopupContainer
 End Sub