Workaround for flicker/flashing when programmatically activating MDI child forms

    public partial class Form1 : Form
    {
        [System.Runtime.InteropServices.DllImport("user32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto)]
        public static extern int SetWindowLong(IntPtr hwnd, int iIndex, int iValue);
 
        [System.Runtime.InteropServices.DllImport("user32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto)]
        public static extern int GetWindowLong(IntPtr hwnd, int iIndex);
 
        [System.Runtime.InteropServices.DllImport("user32.dll", ExactSpelling = true)]
        public static extern int SetWindowPos(IntPtr hwnd, IntPtr hwndInsertAfter, int X, int Y, int cx, int cy, uint uFlags);
 
        /// <summary> Wrapper enum for the Win32 WS_(...) constants. </summary>
        public enum EWindowStyle : int
        {
            Border = 0x00800000,
            ClientEdge = 0x00000200
        }
 
        /// <summary> Wrapper enum for the Win32 GWL_(...) constants. </summary>
        public enum EGetWindowLongFlag : int
        {
            Style = -16,
            ExStyle = -20
        }
 
        /// <summary> Wrapper enum for the Win32 SWP_(...) constants. </summary>
        public enum ESetWindowPosFlag : uint
        {
            NoSize = 0x0001,
            NoMove = 0x0002,
            NoZOrder = 0x0004,
            NoRedraw = 0x0008,
            NoActivate = 0x0010,
            NoCopyBits = 0x0100,
            NoOwnerZOrder = 0x0200,
            NoSendChanging = 0x0400,
            FrameChanged = 0x0020,
            ShowWindow = 0x0040,
            HideWindow = 0x0080,
        }
 
        public Form1()
        {
            InitializeComponent();
 
            this.IsMdiContainer = true;
 
            #region Adjust MdiClient
 
            // Get MdiClient
            MdiClient mdiclient = null;
            foreach (Control control in Controls)
            {
                mdiclient = control as MdiClient;
                if (mdiclient != null)
                {
                    // Set some properties
                    mdiclient.BackColor = SystemColors.Control;
                    // mdiclient.BackgroundImage = etc.
 
                    // Adjust BorderStyle
                    int iStyle = GetWindowLong(mdiclient.Handle, (int)EGetWindowLongFlag.Style);
                    int iExStyle = GetWindowLong(mdiclient.Handle, (int)EGetWindowLongFlag.ExStyle);
                    // ... to BorderStyle.None
                    iStyle &= ~((int)EWindowStyle.Border);
                    iExStyle &= ~((int)EWindowStyle.ClientEdge);
                    SetWindowLong(mdiclient.Handle, (int)EGetWindowLongFlag.Style, iStyle);
                    SetWindowLong(mdiclient.Handle, (int)EGetWindowLongFlag.ExStyle, iExStyle);
 
                    // Previouse changes were made to the NonClient area of the Form. So calling Invalidate() won't redraw the borders.
                    // Calling SetWindowPos does an implicit redraw of the NonClient area - thats the trick.
                    SetWindowPos(mdiclient.Handle, IntPtr.Zero, 0, 0, 0, 0,
                       (uint)(ESetWindowPosFlag.NoActivate | ESetWindowPosFlag.NoMove | ESetWindowPosFlag.NoSize | ESetWindowPosFlag.NoZOrder | ESetWindowPosFlag.NoOwnerZOrder | ESetWindowPosFlag.FrameChanged));
 
                    break;
                }
            }
 
            #endregion // Adjust MdiClient
        }
    }

protected override void WndProc(ref Message m)
{
  if (m.Msg == WM_MDIACTIVATE)
  {
    if (this.parentForm.ActiveMdiChild.Handle != m.WParam)
    {
      int pos = -1;
      for (int i = 0; i < this.mdiClient.Controls.Count; i++)
      {
        if (mdiClient.Controls[i].Handle == m.WParam)
	{
	  pos = i;
	  break;
	}
      }
      if (pos < 0)
	throw new InvalidOperationException("MDIChild form not found");
 
      Control form = null;  // next or previous MDIChild form
      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);
   }
   return;
   }
   base.WndProc(ref m);
}

const int WM_MDIACTIVATE = 0x0222;

private MDIClientController mdiClientController;
public MDIForm()
{
  InitializeComponent();
  this.mdiClientController = new MdiClientController(this);
}

Private Const WM_MDINEXT As Integer = &H224
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hWnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByRef lParam As Integer) As Integer
'
'Flicker free form activation (MDI child forms)
    Public Sub ActivateChild(ByVal chldForm As Form)
        On Error Resume Next
        Dim moMDIClient As MdiClient, iPos As Integer, frmTemp As Form, iLastIdX As Integer
        '
        moMDIClient = GetMDIClient(chldForm)
        iPos = moMDIClient.Controls.IndexOf(chldForm)
        iLastIdX = moMDIClient.Controls.Count - 1
        '
        If iPos = iLastIdX Then
            frmTemp = moMDIClient.Controls(0)
        Else
            frmTemp = moMDIClient.Controls(iPos + 1)
        End If
        SendMessage(moMDIClient.Handle, WM_MDINEXT, frmTemp.Handle, 0)        
    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

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