[DllImport("user32.dll", EntryPoint = "SetWindowLong")]
private static extern int SetWindowLong32(HandleRef hWnd, int nIndex, int dwNewLong);

[DllImport("user32.dll", EntryPoint = "SetWindowLongPtr")]
private static extern IntPtr SetWindowLongPtr64(HandleRef hWnd, int nIndex, IntPtr dwNewLong);

public IntPtr myHWND;
public const int GWL_STYLE = -16;

public static class WS
{
    public static readonly long
    WS_BORDER = 0x00800000L,
    WS_CAPTION = 0x00C00000L,
    WS_CHILD = 0x40000000L,
    WS_CHILDWINDOW = 0x40000000L,
    WS_CLIPCHILDREN = 0x02000000L,
    WS_CLIPSIBLINGS = 0x04000000L,
    WS_DISABLED = 0x08000000L,
    WS_DLGFRAME = 0x00400000L,
    WS_GROUP = 0x00020000L,
    WS_HSCROLL = 0x00100000L,
    WS_ICONIC = 0x20000000L,
    WS_MAXIMIZE = 0x01000000L,
    WS_MAXIMIZEBOX = 0x00010000L,
    WS_MINIMIZE = 0x20000000L,
    WS_MINIMIZEBOX = 0x00020000L,
    WS_OVERLAPPED = 0x00000000L,
    WS_OVERLAPPEDWINDOW = WS_OVERLAPPED | WS_CAPTION | WS_SYSMENU | WS_THICKFRAME | WS_MINIMIZEBOX | WS_MAXIMIZEBOX,
    WS_POPUP = 0x80000000L,
    WS_POPUPWINDOW = WS_POPUP | WS_BORDER | WS_SYSMENU,
    WS_SIZEBOX = 0x00040000L,
    WS_SYSMENU = 0x00080000L,
    WS_TABSTOP = 0x00010000L,
    WS_THICKFRAME = 0x00040000L,
    WS_TILED = 0x00000000L,
    WS_TILEDWINDOW = WS_OVERLAPPED | WS_CAPTION | WS_SYSMENU | WS_THICKFRAME | WS_MINIMIZEBOX | WS_MAXIMIZEBOX,
    WS_VISIBLE = 0x10000000L,
    WS_VSCROLL = 0x00200000L;
}

public IntPtr SetWindowLongPtr(HandleRef hWnd, int nIndex, IntPtr dwNewLong)
{
    if (IntPtr.Size == 8)
    {
        return SetWindowLongPtr64(hWnd, nIndex, dwNewLong);
    }
    else
    {
        return new IntPtr(SetWindowLong32(hWnd, nIndex, dwNewLong.ToInt32()));
    }
}

myHWND = new WindowInteropHelper(this).Handle;
IntPtr myStyle = new IntPtr(WS.WS_CAPTION | WS.WS_CLIPCHILDREN | WS.WS_MINIMIZEBOX | WS.WS_MAXIMIZEBOX | WS.WS_SYSMENU | WS.WS_SIZEBOX);
SetWindowLongPtr(new HandleRef(null, myHWND), GWL_STYLE, myStyle);

this.WindowState = WindowState.Maximized;
//SystemCommands.MaximizeWindow(this);