[DXCategory("Editor")]
public event EventHandler ShownEditor

<DXCategory("Editor")>
Public Event ShownEditor As EventHandler

using DevExpress.XtraEditors;
using DevExpress.XtraGrid.Views.Base;
using System;
using System.Collections.Generic;
using System.Linq;

namespace DxSample {
    public partial class MainForm : XtraForm {
        public MainForm() {
            InitializeComponent();
        }

        private void MainForm_Load(object sender, EventArgs e) {
            this.PhonesSource.DataSource = DataContext.GetPhones();
            this.CountriesSource.DataSource = DataContext.GetCountries();
            this.CitiesSource.DataSource = DataContext.GetAllCities();
        }

        private void GridView_ShownEditor(object sender, EventArgs e) {
            ColumnView view = (ColumnView)sender;
            if (view.FocusedColumn.FieldName == "CityCode") {
                LookUpEdit editor = (LookUpEdit)view.ActiveEditor;
                string countryCode = Convert.ToString(view.GetFocusedRowCellValue("CountryCode"));
                editor.Properties.DataSource = DataContext.GetCitiesByCountryCode(countryCode);
            }
        }

        // In certain scenarios you may want to clear the secondary editor's value
        // You can use the RepositoryItem.EditValueChanged event for this purpose
        private void CountryEditor_EditValueChanged(object sender, EventArgs e) {
            this.GridView.PostEditor();
            this.GridView.SetFocusedRowCellValue("CityCode", null);
        }
    }
}

Imports DevExpress.XtraEditors
Imports DevExpress.XtraGrid.Views.Base
Imports System
Imports System.Collections.Generic
Imports System.Linq

Namespace DxSample
    Partial Public Class MainForm
        Inherits XtraForm

        Public Sub New()
            InitializeComponent()
        End Sub

        Private Sub MainForm_Load(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Load
            Me.PhonesSource.DataSource = DataContext.GetPhones()
            Me.CountriesSource.DataSource = DataContext.GetCountries()
            Me.CitiesSource.DataSource = DataContext.GetAllCities()
        End Sub

        Private Sub GridView_ShownEditor(ByVal sender As Object, ByVal e As EventArgs) Handles GridView.ShownEditor
            Dim view As ColumnView = DirectCast(sender, ColumnView)
            If view.FocusedColumn.FieldName = "CityCode" Then
                Dim editor As LookUpEdit = CType(view.ActiveEditor, LookUpEdit)
                Dim countryCode As String = Convert.ToString(view.GetFocusedRowCellValue("CountryCode"))
                editor.Properties.DataSource = DataContext.GetCitiesByCountryCode(countryCode)
            End If
        End Sub

        ' In certain scenarios you may want to clear the secondary editor's value
        ' You can use the RepositoryItem.EditValueChanged event for this purpose
        Private Sub CountryEditor_EditValueChanged(ByVal sender As Object, ByVal e As EventArgs) Handles CountryEditor.EditValueChanged
            Me.GridView.PostEditor()
            Me.GridView.SetFocusedRowCellValue("CityCode", Nothing)
        End Sub
    End Class
End Namespace

gridView1.DoubleClick -= gridView1_DoubleClick;
gridView1.ShownEditor -= gridView1_ShownEditor;
gridView1.HiddenEditor -= gridView1_HiddenEditor;

view.RowHeight = slider.Size.Height;
    view.ShownEditor += view_ShownEditor;
}

RemoveHandler gridView1.DoubleClick, AddressOf gridView1_DoubleClick
RemoveHandler gridView1.ShownEditor, AddressOf gridView1_ShownEditor
RemoveHandler gridView1.HiddenEditor, AddressOf gridView1_HiddenEditor

view.RowHeight = slider.Size.Height
    AddHandler view.ShownEditor, AddressOf view_ShownEditor
End Sub