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Workaround for flicker/flashing when programmatically activating MDI child forms

, 8 Jul 2007
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This short class demonstrates how to activate an MDI child form without the fireworks that you see if you just set the ActiveMdiChild property of the MDI parent form.

Introduction

This article shows how to activate an MDI child form maximized, without the fireworks that erupt when you activate an MDI child form using the Active() or Select() methods of the child form.

Background

If you've used MDI in WinForms or Win32, you've probably come across this very annoying issue, where MDI child forms that are activated through code, paint themselves in the restored window state they always have when they are non-active, just before they are activated and maximized (this happens only when the current MDI child form is maximized).

Because this issue was so annoying, and resulted in a very unprofessional 'look and feel', I became determined to find a workaround. The one I found to work best involves no subclassing or message handling. I discovered this when I noticed that if I activate the next MDI child form via Ctrl+F6 or the MDI system menu, there was none of the usual fireworks that I was seeing when a child form is activated by calling its Activate() method.

That lead me to the WM_MDINEXT Windows message which, at first glance, seemed only useful for activating the next MDI child form in the sequence. However, it can be used to activate any MDI child form by passing the handle of the next or previous MDI child in the Z-order along with a flag telling it whether to activate the next or previous MDI child in the Z-order.

Using the code

The following sample MDI parent Form class implements the workaround via a method that can be called and passed the MDI child form to be activated:

//  
//  MyMDIParent.cs
//

using System;
using System.Data;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace MyNamespace
{
   public partial class MyMDIParent : Form
   {
      public MyMDIParent()
      {
         InitializeComponent();
      }

      public void ActivateMdiChild( Form childToActivate )
      {
         if( this.ActiveMdiChild != childToActivate )
         {
            MdiClient mdiClient = GetMDIClient();
            int count = this.MdiChildren.Length;
            Control form = null;  // next or previous MDIChild form

            int pos = mdiClient.Controls.IndexOf( childToActivate );
            if( pos < 0 )
               throw new InvalidOperationException( "MDIChild form not found" );
            if( pos == 0 )
               form = mdiClient.Controls[1];  // get next and activate previous

            else
               form = mdiClient.Controls[pos - 1];  // get previous and activate next


            // flag indicating whether to activate previous or next MDIChild
            IntPtr direction = new IntPtr( pos == 0 ? 1 : 0 );
            
            // bada bing, bada boom
            SendMessage( mdiClient.Handle, WM_MDINEXT, form.Handle, direction );
         }
      }

      public MdiClient GetMDIClient()
      {
         foreach( Control c in this.Controls )
         {
            if( c is MdiClient )
               return (MdiClient) c;
         }
         throw new InvalidOperationException( "No MDIClient !!!" );
      }

      [System.Security.SuppressUnmanagedCodeSecurity]
      [DllImport( "user32.dll", CharSet = CharSet.Auto )]
      public static extern IntPtr SendMessage( IntPtr hWnd, int msg, 
                                  IntPtr wParam, IntPtr lParam );

      public const int WM_MDINEXT = 0x224;
   }
}

Points of interest

This solution works fine for me, when I need to activate a child window via code, but I haven't looked at how to get the MDI child form menu entries inserted into the menu by the framework, to use it. Any insight or advice on that would be welcome.

History

  • Initial submission - 7/09/07.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

tonyt

United States United States
No Biography provided

Comments and Discussions

 
GeneralMy vote of 5 Pinmemberzerochangjj4-Nov-12 16:07 
QuestionMemory management in MDI forms PingroupAli91Asadi28-Sep-12 5:40 
GeneralThank you Pinmemberajhuddy28-Oct-09 10:34 
GeneralMy vote of 1 PinmemberJon Burchel31-Aug-09 10:50 
General[My vote of 1] CRAP PinmemberJon Burchel31-Aug-09 10:50 
Questionchanging vs form look PinmemberMember 46116445-Mar-09 18:44 
GeneralSimilar Problem Pinmemberjohannesnestler7-May-08 22:13 
GeneralRe: Similar Problem Pinmembers.t.a.v.o8-Dec-08 3:48 
GeneralRe: Similar Problem Pinmemberjohannesnestler8-Dec-08 22:51 
QuestionRe: Similar Problem PinmemberMember 46116445-Mar-09 18:47 
AnswerRe: Similar Problem Pinmemberjohannesnestler6-Mar-09 5:25 
GeneralRe: Similar Problem Pinmemberjasonhh14-Apr-09 20:05 
QuestionI have a question please Pinmembersdev5-Oct-07 8:13 
GeneralFW: &quot;A flicker issue in MDI applications&quot; PinmemberMarcus Deecke17-Aug-07 11:54 
GeneralVery good - looking forward to the MDI menu working PinmemberMartin Berriman26-Jul-07 1:34 
GeneralRe: Very good - looking forward to the MDI menu working [modified] Pinmembertonyt18-Aug-07 14:37 
Generalstill MDI Questions PinmemberSHartmann22-Aug-07 14:59 
GeneralRe: Very good - looking forward to the MDI menu working Pinmemberjefrubio19-Feb-09 8:01 
GeneralRe: Very good - looking forward to the MDI menu working Pinmembersprice8629-May-12 0:20 
Generalconverted to VB and adjusted Pinmemberdmbf1b517-Jul-07 10:27 
GeneralRe: converted to VB and adjusted PinmemberJason Newland16-Feb-09 23:52 
GeneralRe: converted to VB and adjusted [modified] PinmemberJordan Deyton15-Jul-09 3:38 
I had a look at this yesterday while trying to get rid of that annoying flicker and learned some things.
 
When you declare SendMessage, you need to use ByVal for all your parameters. In dmbf1b5 and Jason Newland's code, the lParam argument uses ByRef, causing it to be ignored. I tried running it with ByRef, and it always activated the child with one better z-order than what I wanted, i.e. the previous child. After changing ByRef to ByVal, you can use lParam as described in the MSDN article at http://msdn.microsoft.com/en-us/library/ms644918(VS.85).aspx[^].
 
In my case, I have two sets of "tabs" called collections and lists. Each list has its own MDI child, but you can only see one collection of lists at a time. Sometimes the z-orders of the lists in each collection get mixed up.
 
For example, suppose you have tabs A, b, C, d with indices 0, 1, 2, 3 in the MDIClient.Controls collection (the index corresponds to the z-order, with 0 being the active MDI child and 3 the last child). Suppose further that b and d are hidden via the form.Visible property. If b (or d) is sent to WM_MDINEXT with lParam 0 (or 1), b (or d) will be activated in the process.
 
So if you want to be sure no hidden MDI children are activated, use WM_MDINEXT with a reference to an MDI child that is visible and enabled. From what I can tell, the MDI system automatically ignores hidden MDI children when WM_MDINEXT is used, provided a hidden MDI child is not passed as wParam.
 
Here is some code I came up with. I borrowed heavily from Jason Newland but wound up changing things a bit to fit my problem. Here, MyMDIChild is the base form I use as MDI children. I use WM_MDINEXT with the "previous" visible tab so that no hidden forms will appear. The nice thing is that you can activate an MDI child simply by calling form.ActivateMe().
 
Class MyMDIChild
 
    Protected Shared moMDIClient As MdiClient   'The MDIClient containing MyMDIChild objects.

    Public Sub ActivateMe()
 
        Dim MyIndex, j, count As Integer
        Dim frmTemp As Form
 
        If (moMDIClient Is Nothing) Then        'Get moMDIClient if necessary.
            moMDIClient = GetMDIClient(Me)
        End If
 
        MyIndex = moMDIClient.Controls.IndexOf(Me)  'Get the z-order of the form-to-activate.
        count = moMDIClient.Controls.Count          'Get the number of MDI forms.

        If (MyIndex <> 0 And count > 1) Then        'If the form is the active form already or if it is the only form, quit.
            j = (MyIndex - 1) Mod count             'Get the "previous" index.
            frmTemp = Nothing
 
            While (j <> MyIndex And frmTemp Is Nothing)     'Find the "previous" visible form.
                frmTemp = moMDIClient.Controls(j)
                If (frmTemp.Visible = False) Then
                    frmTemp = Nothing
                End If
                j = (j - 1 + count) Mod count
            End While
 
            If Not (frmTemp Is Nothing) Then
                SendMessage(moMDIClient.Handle, WM_MDINEXT, frmTemp.Handle, 0)
            End If
        End If
    End Sub
 
    Private Function GetMDIClient(ByVal chldForm As Form) As MdiClient
        On Error Resume Next
        Dim c As Control
        For Each c In chldForm.MdiParent.Controls
            If c.ToString = "System.Windows.Forms.MdiClient" Then
                GetMDIClient = c
                Exit For
            End If
        Next
    End Function
 
    Private Const WM_MDINEXT As Integer = &H224
 
    'For use in VB    
    Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByVal lParam As Integer) As Integer
 
    'For use in VB.NET.  Don't forget to import System.Runtime.InteropServices
    '<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    'Private Shared Function SendMessage(ByVal hWnd As IntPtr, ByVal Msg As UInteger, ByVal wParam As IntPtr, ByVal lParam As IntPtr) As IntPtr
    'End Function
End Class

 
modified on Wednesday, July 29, 2009 9:33 AM

Questionhow to SendMessage in VB.net? Pinmemberdmbf1b517-Jul-07 5:21 
AnswerRe: how to SendMessage in VB.net? PinmemberCalifBreton17-Jul-07 8:33 
GeneralNever seen any fireworks.... PinmemberKellyLeahy9-Jul-07 6:21 
GeneralRe: Never seen any fireworks.... Pinmembertonyt9-Jul-07 7:26 

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