How to create a semi transparent form in WPF

You could try this, it will create a Glass Background for your Window (looks like the Vista and Windows7 transparency effect)

Here is some further explanation from Microsoft.

First, you need to set AllowTransperency to True. Then, you can set the background of the window to a transparent (to desired extent) brush:

<Window x:Class="MyWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        WindowStyle="None"
        AllowsTransparency="True"
        Background="{DynamicResource WindowBackground}">
    <Window.Resources>
       <SolidColorBrush x:Key="WindowBackground" 
                        Color="White"
                        Opacity="0.5"/>
    </Window.Resources>
    ...
</Window>

Please note that AllowTransperency can be set to True only if WindowStyle set to None.

Update: If you don't want to set WindowStyle to None and would like to keep standart border and window buttons there is an alternative that will work only on Windows Vista/7 with Windows Aero theme.

The trick is that you can extend the "Glass" area to the whole window using the following code:

public static class WindowUtils
{
    /// <summary>
    /// Extends the glass area into the client area of the window
    /// </summary>
    /// <param name="window">Window to extend the glass on.</param>
    /// <param name="thikness">Thickness of border to extend.</param>
    public static void ExtendGlass(this Window window, Thickness thikness) {
        try {
            int isGlassEnabled = 0;
            Win32.DwmIsCompositionEnabled(ref isGlassEnabled);
            if (Environment.OSVersion.Version.Major > 5 && isGlassEnabled > 0) {
                // Get the window handle
                var helper = new WindowInteropHelper(window);
                var mainWindowSrc = HwndSource.FromHwnd(helper.Handle);

                if (mainWindowSrc != null) {
                    if (mainWindowSrc.CompositionTarget != null) {
                        mainWindowSrc.CompositionTarget.BackgroundColor = Colors.Transparent;
                    }

                    // Get the dpi of the screen
                    System.Drawing.Graphics desktop =
                        System.Drawing.Graphics.FromHwnd(mainWindowSrc.Handle);

                    float dpiX = desktop.DpiX / 96;
                    float dpiY = desktop.DpiY / 96;

                    // Set Margins
                    var margins = new MARGINS {
                        cxLeftWidth = (int)(thikness.Left * dpiX),
                        cxRightWidth = (int)(thikness.Right * dpiX),
                        cyBottomHeight = (int)(thikness.Bottom * dpiY),
                        cyTopHeight = (int)(thikness.Top * dpiY)
                    };

                    window.Background = Brushes.Transparent;

                    Win32.DwmExtendFrameIntoClientArea(mainWindowSrc.Handle, ref margins);
                }
            }
            else {
                window.Background = SystemColors.WindowBrush;
            }
        }
        catch (DllNotFoundException) {

        }
    }
}

public class Win32
{
    [DllImport("dwmapi.dll")]
    public static extern int DwmExtendFrameIntoClientArea(IntPtr hWnd, ref MARGINS pMarInset);

    [DllImport("dwmapi.dll")]
    public static extern int DwmIsCompositionEnabled(ref int en);

    [DllImport("user32.dll")]
    public static extern bool SetCursorPos(int X, int Y);


    [DllImport("User32", EntryPoint = "ClientToScreen", SetLastError = true, ExactSpelling = true, CharSet = CharSet.Auto)]
    public static extern int ClientToScreen(IntPtr hWnd, [In, Out] POINT pt);
}

[StructLayout(LayoutKind.Sequential)]
public struct MARGINS
{
    public int cxLeftWidth;
    public int cxRightWidth;
    public int cyTopHeight;
    public int cyBottomHeight;
}

[StructLayout(LayoutKind.Sequential)]
public class POINT
{
    public int x = 0;
    public int y = 0;
}

To extend the glass to the whole window you need to call the ExtendGlass extension method in a SizeChanged event handler of the window and pass a Thickness that covers whole window:

public MyWindow() {
    InitializeComponent();

    SizeChanged += OnSizeChanged;
}

private void OnSizeChanged(object sender, SizeChangedEventArgs e) {
    double horisontalThickness = Width / 2;
    double verticalThickness = Height / 2;

    var glassThickness = new Thickness(horisontalThickness, verticalThickness, horisontalThickness, verticalThickness);

    this.ExtendGlass(glassThickness);
}