C# Signature:

/// <summary>
/// Retrieves the show state and the restored, minimized, and maximized positions of the specified window.
/// </summary>
/// <param name="hWnd">
/// A handle to the window.
/// </param>
/// <param name="lpwndpl">
/// A pointer to the WINDOWPLACEMENT structure that receives the show state and position information.
/// <para>
/// Before calling GetWindowPlacement, set the length member to sizeof(WINDOWPLACEMENT). GetWindowPlacement fails if lpwndpl-> length is not set correctly.
/// </para>
/// </param>
/// <returns>
/// If the function succeeds, the return value is nonzero.
/// <para>
/// If the function fails, the return value is zero. To get extended error information, call GetLastError.
/// </para>
/// </returns>
[DllImport("user32.dll", SetLastError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
internal static extern bool GetWindowPlacement(IntPtr hWnd, ref WINDOWPLACEMENT lpwndpl);

VB.NET Signature:

<DllImport("user32.dll")> _
Private Shared Function GetWindowPlacement(ByVal hWnd As IntPtr, ByRef lpwndpl As WINDOWPLACEMENT) As Boolean
End Function

VB Signature:

Public Declare Function GetWindowPlacement Lib "user32" _
          (ByVal hWnd As Long, _
           ByRef lpwndpl As WINDOWPLACEMENT) As Long

User-Defined Types:

WINDOWPLACEMENT

Notes:

None.

Tips & Tricks:

For C# only: It seems you have to do rcNormalPosition.Width - rcNormalPosition.X to get the actual width because "Width" is actually a "Right", same goes for height.

C# Sample Code:

public class Sample
{

    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    static extern bool GetWindowPlacement(IntPtr hWnd, ref WINDOWPLACEMENT lpwndpl);

    private struct WINDOWPLACEMENT
    {
        public int length;
        public uint flags;
        public uint showCmd;
        public System.Drawing.Point ptMinPosition;
        public System.Drawing.Point ptMaxPosition;
        public System.Drawing.Rectangle rcNormalPosition;
    }

    const uint SW_HIDE                = 0;
    const uint SW_MAXIMIZE            = 3;
    const uint SW_MINIMIZE            = 6;
    const uint SW_RESTORE            = 9;
    const uint SW_SHOW                = 5;
    const uint SW_SHOWMAXIMIZED        = 3;
    const uint SW_SHOWMINIMIZED        = 2;
    const uint SW_SHOWMINNOACTIVE    = 7;
    const uint SW_SHOWNA                = 8;
    const uint SW_SHOWNOACTIVATE        = 4;
    const uint SW_SHOWNORMAL            = 1;

...

    private void GetPlacement()
    {
       WINDOWPLACEMENT placement = new WINDOWPLACEMENT();
       placement.length = Marshal.SizeOf(placement);
       GetWindowPlacement(this.Handle, ref placement);
    }
}

VB Sample Code:

Public Class Form1

    Private Declare Function GetWindowPlacement Lib "user32" (ByVal hwnd As IntPtr, ByRef lpwndpl As WINDOWPLACEMENT) As Integer

    Private Structure POINTAPI
         Public x As Integer
         Public y As Integer
    End Structure

    Private Structure RECT
         Public Left As Integer
         Public Top As Integer
         Public Right As Integer
         Public Bottom As Integer
    End Structure

    Private Structure WINDOWPLACEMENT
         Public Length As Integer
         Public flags As Integer
         Public showCmd As Integer
         Public ptMinPosition As POINTAPI
         Public ptMaxPosition As POINTAPI
         Public rcNormalPosition As RECT
    End Structure


    Private Sub Form1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Click

        Dim intRet As Integer
        Dim wpTemp As WINDOWPLACEMENT

        wpTemp.Length = System.Runtime.InteropServices.Marshal.SizeOf(wpTemp)
        intRet = GetWindowPlacement(Me.Handle.ToInt32, wpTemp)

        If wpTemp.showCmd = 1 Then
            'normal
        ElseIf wpTemp.showCmd = 2 Then
            'minimized
        ElseIf wpTemp.showCmd = 3 Then
            'maximized
        End If
        'Stop

    End Sub

End Class

Alternative Managed API:

The ManagedWindowsApi project (http://mwinapi.sourceforge.net) provides a

class ManagedWinapi.SystemWindow that has a Position property.