Source: Microsoft Support

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You are using a Visual Basic 6.0 ActiveX control (OCX) containing constituent controls on a Microsoft .NET Windows Form.

The LostFocus event of the constituent control is not fired when changing focus from the constituent control to a control on the Windows Form.

When exiting the constituent control, the AXHost wrapper which contains the ActiveX control never receives focus. Therefore the constituent control never lost focus and does not fire its LostFocus event.

The following sample code demonstrates how to deactivate the ActiveX control during the AXHost Leave event, which allows the constituent control's LostFocus event to fire. In the sample code, axALostFocusCTL21 is the AxHost wrapper for the COM control which is placed on the Windows Form.


C# Sample Code
=============

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace WindowsApplication1
{
   [ComImport(), Guid("00000113-0000-0000-C000-000000000046"),
   InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]

   public interface IOleInPlaceObject
   {
      [PreserveSig]
      int GetWindow([Out]out IntPtr hwnd);
      void ContextSensitiveHelp(int fEnterMode);
      void InPlaceDeactivate();

      [PreserveSig]
      int UIDeactivate(); 
      void SetObjectRects([In] IntPtr lprcPosRect, [In] IntPtr lprcClipRect);
      void ReactivateAndUndo(); 
   }

   public partial class Form1 : Form
   {
      AxHost axControl; 
      public Form1()
      {
         InitializeComponent();
         this.axALostFocusCTL21.Leave += new System.EventHandler(this.axALostFocusCTL21_Leave);
      }
   
      private void axALostFocusCTL21_Leave(object sender, System.EventArgs e)
      {
         axControl = (AxHost)sender;
         IOleInPlaceObject oipo = (IOleInPlaceObject)axControl.GetOcx(); 
        oipo.UIDeactivate();
      }

      private void Form1_FormClosing(object sender, FormClosingEventArgs e)
      {
         if(axControl != null)
         axControl.Dispose();
         if (this.axALostFocusCTL21 != null)
            this.axALostFocusCTL21.Dispose(); 
      } 
   }
}

Visual Basic .NET Sample Code
========================

Imports System
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Data
Imports System.Drawing
Imports System.Text
Imports System.Windows.Forms
Imports System.Runtime.InteropServices

   <ComImport(), Guid("00000113-0000-0000-C000-000000000046"), InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)> _
   Public Interface IOleInPlaceObject 

      <PreserveSig()> _
      Sub GetWindow(<OutAttribute()> ByRef phwnd As IntPtr)
      Sub ContextSensitiveHelp(<InAttribute()> ByVal fEnterMode As Integer) 
      Sub InPlaceDeactivate()

      <PreserveSig()> _
      Sub UIDeactivate()
      Sub SetObjectRects(<InAttribute()> ByVal lprcPosRect As IntPtr, <InAttribute()> ByVal lprcClipRect As IntPtr)
      Sub ReactivateAndUndo()
   End Interface

   Public Class Form1
      Dim axControl As AxHost
      Private Sub axALostFocusCTL21_Leave(ByVal sender As Object, ByVal e As System.EventArgs)
         axControl = CType(sender, AxHost)
         Dim oipo As IOleInPlaceObject = CType(axControl.GetOcx(), IOleInPlaceObject)
         oipo.UIDeactivate()
      End Sub

      Private Sub Form1_FormClosing(ByVal sender As Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
         If Not axControl Is Nothing Then
            axControl.Dispose()
         End If
         If Not AxALostFocusCTL21 Is Nothing Then
            AxALostFocusCTL21.Dispose()
         End If
      End Sub
     
      Public Sub New()
         ' This call is required by the Windows Form Designer.
         InitializeComponent()

        ' Add any initialization after the InitializeComponent() call.
        AddHandler Me.AxALostFocusCTL21.Leave, AddressOf Me.axALostFocusCTL21_Leave
      End Sub
   End Class

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