WPF: Make window unresizeable, but keep the frame?

You can hook the wndproc and intercept the WM_WINDOWPOSCHANGING message. Not strictly WPF, but probably your best bet.

If you want to hide the resize cursors, then your best bet is to intercept WM_NCHITTEST. Call the DefWindowProc (to get the default behavior), and test the return value; if it's HTBOTTOM, HTBOTTOMLEFT, HTBOTTOMRIGHT, HTTOP, HTTOPLEFT, or HTTOPRIGHT, change the return value to HTBORDER.

Based on Erics answer.

public partial class MainWindow : Window
{
    [DllImport("DwmApi.dll")]
    public static extern int DwmExtendFrameIntoClientArea(
        IntPtr hwnd,
        ref MARGINS pMarInset);

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    public static extern IntPtr DefWindowProc(
        IntPtr hWnd,
        int msg,
        IntPtr wParam,
        IntPtr lParam);

    private const int WM_NCHITTEST = 0x0084;
    private const int HTBORDER = 18;
    private const int HTBOTTOM = 15;
    private const int HTBOTTOMLEFT = 16;
    private const int HTBOTTOMRIGHT = 17;
    private const int HTLEFT = 10;
    private const int HTRIGHT = 11;
    private const int HTTOP = 12;
    private const int HTTOPLEFT = 13;
    private const int HTTOPRIGHT = 14;

    public MainWindow()
    {
        InitializeComponent();

        this.Loaded += new RoutedEventHandler(MainWindow_Loaded);
    }

    void MainWindow_Loaded(object sender, RoutedEventArgs e)
    {
        try
        {
            // Obtain the window handle for WPF application
            IntPtr mainWindowPtr = new WindowInteropHelper(this).Handle;
            HwndSource mainWindowSrc = HwndSource.FromHwnd(mainWindowPtr);
            mainWindowSrc.CompositionTarget.BackgroundColor = Color.FromArgb(0, 0, 0, 0);
            mainWindowSrc.AddHook(WndProc);

            // Set Margins
            MARGINS margins = new MARGINS();
            margins.cxLeftWidth = 10;
            margins.cxRightWidth = 10;
            margins.cyBottomHeight = 10;
            margins.cyTopHeight = 10;

            int hr = DwmExtendFrameIntoClientArea(mainWindowSrc.Handle, ref margins);
            //
            if (hr < 0)
            {
                //DwmExtendFrameIntoClientArea Failed
            }
        }
        // If not Vista, paint background white.
        catch (DllNotFoundException)
        {
            Application.Current.MainWindow.Background = Brushes.White;
        }
    }

    private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
    {
        // Override the window hit test
        // and if the cursor is over a resize border,
        // return a standard border result instead.
        if (msg == WM_NCHITTEST)
        {
            handled = true;
            var htLocation = DefWindowProc(hwnd, msg, wParam, lParam).ToInt32();
            switch (htLocation)
            {
                case HTBOTTOM:
                case HTBOTTOMLEFT:
                case HTBOTTOMRIGHT:
                case HTLEFT:
                case HTRIGHT:
                case HTTOP:
                case HTTOPLEFT:
                case HTTOPRIGHT:
                    htLocation = HTBORDER;
                    break;
            }

            return new IntPtr(htLocation);
        }

        return IntPtr.Zero;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        this.Close();
    }
}

[StructLayout(LayoutKind.Sequential)]
public struct MARGINS
{
    public int cxLeftWidth;      // width of left border that retains its size
    public int cxRightWidth;     // width of right border that retains its size
    public int cyTopHeight;      // height of top border that retains its size
    public int cyBottomHeight;   // height of bottom border that retains its size
};

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="150" Width="200" 
    Background="Transparent"
    WindowStyle="None"
    ResizeMode="CanResize"
>
    <Grid Background="White" Margin="10,10,10,10">
        <Button Content="Go Away" Click="Button_Click" Height="20" Width="100" />
    </Grid>
</Window>