C# Signature:

[DllImport("user32.dll", SetLastError=true)]
static extern int GetWindowLong(IntPtr hWnd, int nIndex);

VB Signature:

<DllImport("user32.dll", SetLastError:=True)> _
Private Shared Function GetWindowLong( _
     ByVal hWnd As IntPtr, _
     ByVal nIndex As Integer) As Integer
End Function

Constants:

GWL_ID = (-12)
GWL_STYLE = (-16)
GWL_EXSTYLE = (-20)

// Window Styles
const UInt32 WS_OVERLAPPED = 0;
const UInt32 WS_POPUP = 0x80000000;
const UInt32 WS_CHILD = 0x40000000;
const UInt32 WS_MINIMIZE = 0x20000000;
const UInt32 WS_VISIBLE = 0x10000000;
const UInt32 WS_DISABLED = 0x8000000;
const UInt32 WS_CLIPSIBLINGS = 0x4000000;
const UInt32 WS_CLIPCHILDREN = 0x2000000;
const UInt32 WS_MAXIMIZE = 0x1000000;
const UInt32 WS_CAPTION = 0xC00000;      // WS_BORDER or WS_DLGFRAME  
const UInt32 WS_BORDER = 0x800000;
const UInt32 WS_DLGFRAME = 0x400000;
const UInt32 WS_VSCROLL = 0x200000;
const UInt32 WS_HSCROLL = 0x100000;
const UInt32 WS_SYSMENU = 0x80000;
const UInt32 WS_THICKFRAME = 0x40000;
const UInt32 WS_GROUP = 0x20000;
const UInt32 WS_TABSTOP = 0x10000;
const UInt32 WS_MINIMIZEBOX = 0x20000;
const UInt32 WS_MAXIMIZEBOX = 0x10000;
const UInt32 WS_TILED = WS_OVERLAPPED;
const UInt32 WS_ICONIC = WS_MINIMIZE;
const UInt32 WS_SIZEBOX = WS_THICKFRAME;

// Extended Window Styles
const UInt32 WS_EX_DLGMODALFRAME = 0x0001;
const UInt32 WS_EX_NOPARENTNOTIFY = 0x0004;
const UInt32 WS_EX_TOPMOST = 0x0008;
const UInt32 WS_EX_ACCEPTFILES = 0x0010;
const UInt32 WS_EX_TRANSPARENT = 0x0020;
const UInt32 WS_EX_MDICHILD = 0x0040;
const UInt32 WS_EX_TOOLWINDOW = 0x0080;
const UInt32 WS_EX_WINDOWEDGE = 0x0100;
const UInt32 WS_EX_CLIENTEDGE = 0x0200;
const UInt32 WS_EX_CONTEXTHELP = 0x0400;
const UInt32 WS_EX_RIGHT = 0x1000;
const UInt32 WS_EX_LEFT = 0x0000;
const UInt32 WS_EX_RTLREADING = 0x2000;
const UInt32 WS_EX_LTRREADING = 0x0000;
const UInt32 WS_EX_LEFTSCROLLBAR = 0x4000;
const UInt32 WS_EX_RIGHTSCROLLBAR = 0x0000;
const UInt32 WS_EX_CONTROLPARENT = 0x10000;
const UInt32 WS_EX_STATICEDGE = 0x20000;
const UInt32 WS_EX_APPWINDOW = 0x40000;
const UInt32 WS_EX_OVERLAPPEDWINDOW = (WS_EX_WINDOWEDGE | WS_EX_CLIENTEDGE);
const UInt32 WS_EX_PALETTEWINDOW = (WS_EX_WINDOWEDGE | WS_EX_TOOLWINDOW | WS_EX_TOPMOST);
const UInt32 WS_EX_LAYERED = 0x00080000;
const UInt32 WS_EX_NOINHERITLAYOUT = 0x00100000; // Disable inheritence of mirroring by children
const UInt32 WS_EX_LAYOUTRTL = 0x00400000; // Right to left mirroring
const UInt32 WS_EX_COMPOSITED = 0x02000000;
const UInt32 WS_EX_NOACTIVATE = 0x08000000;

Notes:

This function is NOT 64-bit safe when retrieving handle or pointer values. Use GetWindowLongPtr instead when retrieving an IntPtr.

Tips & Tricks:

Please add some!

Sample Code:

C#:

  static public class WndInfo
  {
    [DllImport("user32.dll", SetLastError = true)]
    static extern int GetWindowLong(IntPtr hWnd, int nIndex);

    ...
    public static bool IsWindowTopMost(IntPtr Handle)
    {
      return (GetWindowLong(Handle, GWL_EXSTYLE) & WS_EX_TOPMOST) != 0;
    }
    ...
  }

Alternative Managed API:

Do you know one? Please contribute it!