C# Signature:

[DllImport("user32.dll")]
static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X,
int Y, int cx, int cy, uint uFlags);

VB Signature:

<DllImport("user32.dll", SetLastError:=True)> _
Private Shared Function SetWindowPos(ByVal hWnd As IntPtr, _
                      ByVal hWndInsertAfter As IntPtr, _
                      ByVal X As Integer, _
                      ByVal Y As Integer, _
                      ByVal cx As Integer, _
                      ByVal cy As Integer, _
                      ByVal uFlags As UInt32) As Boolean
End Function

VB.NET Signature:

Declare Auto Function SetWindowPos Lib "user32" (ByVal hWnd As IntPtr, _
                      ByVal hWndInsertAfter As IntPtr, _
                      ByVal X As Integer, _
                      ByVal Y As Integer, _
                      ByVal cx As Integer, _
                      ByVal cy As Integer, _
                      ByVal uFlags As UInteger) As Boolean

User-Defined Types:

C#:

static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);
static readonly IntPtr HWND_NOTOPMOST = new IntPtr(-2);
static readonly IntPtr HWND_TOP = new IntPtr(0);
static readonly IntPtr HWND_BOTTOM = new IntPtr(1);

VB:

ReadOnly HWND_BOTTOM As New IntPtr(1)
ReadOnly HWND_NOTOPMOST As New IntPtr(-2)
ReadOnly HWND_TOP As New IntPtr(0)
ReadOnly HWND_TOPMOST As New IntPtr(-1)

Values for the uFlags parameter

C#:

// From winuser.h
const UInt32 SWP_NOSIZE = 0x0001;
const UInt32 SWP_NOMOVE = 0x0002;
const UInt32 SWP_NOZORDER = 0x0004;
const UInt32 SWP_NOREDRAW = 0x0008;
const UInt32 SWP_NOACTIVATE = 0x0010;
const UInt32 SWP_FRAMECHANGED = 0x0020;  /* The frame changed: send WM_NCCALCSIZE */
const UInt32 SWP_SHOWWINDOW = 0x0040;
const UInt32 SWP_HIDEWINDOW = 0x0080;
const UInt32 SWP_NOCOPYBITS = 0x0100;
const UInt32 SWP_NOOWNERZORDER = 0x0200;  /* Don't do owner Z ordering */
const UInt32 SWP_NOSENDCHANGING = 0x0400;  /* Don't send WM_WINDOWPOSCHANGING */

VB Conversion - If you want to paste into VB.

VB:

Shared ReadOnly SWP_NOSIZE As UInt32 = Convert.ToUInt32(&H1)
Shared ReadOnly SWP_NOMOVE As UInt32 = Convert.ToUInt32(&H2)
Shared ReadOnly SWP_NOZORDER As UInt32 = Convert.ToUInt32(&H4)
Shared ReadOnly SWP_NOREDRAW As UInt32 = Convert.ToUInt32(&H8)
Shared ReadOnly SWP_NOACTIVATE As UInt32 = Convert.ToUInt32(&H10)
Shared ReadOnly SWP_FRAMECHANGED As UInt32 = Convert.ToUInt32(&H20)     '/* The frame changed: send WM_NCCALCSIZE */
Shared ReadOnly SWP_SHOWWINDOW As UInt32 = Convert.ToUInt32(&H40)
Shared ReadOnly SWP_HIDEWINDOW As UInt32 = Convert.ToUInt32(&H80)
Shared ReadOnly SWP_NOCOPYBITS As UInt32 = Convert.ToUInt32(&H100)
Shared ReadOnly SWP_NOOWNERZORDER As UInt32 = Convert.ToUInt32(&H200)    '/* Don't do owner Z ordering */
Shared ReadOnly SWP_NOSENDCHANGING As UInt32 = Convert.ToUInt32(&H400)    '/* Don't send WM_WINDOWPOSCHANGING */

Notes:

HWND_TOP will bring a window to the front of the Z-Order only if the thread is in the foreground. Otherwise it will bring the window to the front of the thread's Z-order.

Tips & Tricks:

In .NET, to make a window stay on top you only need to call Me.TopMost = True

Sample Code:

This is a little helper class which can be used with any Form object to make it topmost or not topmost.

C#:

    static public class FormHelper
    {
        [DllImport("user32.dll")]
        static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X,
            int Y, int cx, int cy, uint uFlags);

        static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);
        static readonly IntPtr HWND_NOTOPMOST = new IntPtr(-2);
        static readonly IntPtr HWND_TOP = new IntPtr(0);

        const UInt32 SWP_NOSIZE = 0x0001;
        const UInt32 SWP_NOMOVE = 0x0002;
        const UInt32 SWP_NOZORDER = 0x0004;
        const UInt32 SWP_NOREDRAW = 0x0008;
        const UInt32 SWP_NOACTIVATE = 0x0010;
        const UInt32 SWP_FRAMECHANGED = 0x0020;  /* The frame changed: send WM_NCCALCSIZE */
        const UInt32 SWP_SHOWWINDOW = 0x0040;
        const UInt32 SWP_HIDEWINDOW = 0x0080;
        const UInt32 SWP_NOCOPYBITS = 0x0100;
        const UInt32 SWP_NOOWNERZORDER = 0x0200;  /* Don't do owner Z ordering */
        const UInt32 SWP_NOSENDCHANGING = 0x0400;  /* Don't send WM_WINDOWPOSCHANGING */

        const UInt32 TOPMOST_FLAGS = SWP_NOMOVE | SWP_NOSIZE;

        public static void MakeTopMost (Form form)
        {
            SetWindowPos(form.Handle, HWND_TOPMOST, 0, 0, 0, 0, TOPMOST_FLAGS);
        }

        public static void MakeNormal (Form form)
        {
            SetWindowPos(form.Handle, HWND_NOTOPMOST, 0, 0, 0, 0, TOPMOST_FLAGS);
        }
    }

VB

    Public Class FormHelper
        <DllImport("user32.dll")> _
        Private Shared Function SetWindowPos(ByVal hWnd As IntPtr, ByVal hWndInsertAfter As IntPtr, ByVal X As Integer, _
        ByVal Y As Integer, ByVal cx As Integer, ByVal cy As Integer, ByVal uFlags As UInt32) As Boolean
        End Function

        Private Shared ReadOnly HWND_NOTOPMOST As IntPtr = New IntPtr(-2)
        Private Shared ReadOnly HWND_TOP As IntPtr = New IntPtr(0)
        Private Shared ReadOnly HWND_TOPMOST As IntPtr = New IntPtr(-1)

        Private Const SWP_NOSIZE As UInt32 = &H0001
        Private Const SWP_NOMOVE As UInt32 = &H0002
        Private Const SWP_NOZORDER As UInt32 = &H0004
        Private Const SWP_NOREDRAW As UInt32 = &H0008
        Private Const SWP_NOACTIVATE As UInt32 = &H0010
        Private Const SWP_FRAMECHANGED As UInt32 = &H0020  ' The frame changed: send WM_NCCALCSIZE
        Private Const SWP_SHOWWINDOW As UInt32 = &H0040
        Private Const SWP_HIDEWINDOW As UInt32 = &H0080
        Private Const SWP_NOCOPYBITS As UInt32 = &H0100
        Private Const SWP_NOOWNERZORDER As UInt32 = &H00200  ' Don't do owner Z ordering
        Private Const SWP_NOSENDCHANGING As UInt32 = &H0400  ' Don't send WM_WINDOWPOSCHANGING

        Private Const TOPMOST_FLAGS As UInt32 = SWP_NOMOVE Or SWP_NOSIZE

        Public Shared Sub MakeTopMost(ByVal form As Form)
            FormHelper.SetWindowPos(form.Handle, HWND_TOPMOST, 0, 0, 0, 0, 3)
        End Sub

        Public Shared Sub MakeNormal(ByVal form As Form)
            FormHelper.SetWindowPos(form.Handle, HWND_NOTOPMOST, 0, 0, 0, 0, 3)
        End Sub
    End Class

Alternative Managed API:

Use the System.Windows.Forms.Form.TopMost property.

Documentation

SetWindowPos on MSDN

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