public enum GWL
{
    ExStyle = -20
}

public enum WS_EX
{
    Transparent = 0x20,
    Layered = 0x80000
}

public enum LWA
{
    ColorKey = 0x1,
    Alpha = 0x2
}

[DllImport("user32.dll", EntryPoint = "GetWindowLong")]
public static extern int GetWindowLong(IntPtr hWnd, GWL nIndex);

[DllImport("user32.dll", EntryPoint = "SetWindowLong")]
public static extern int SetWindowLong(IntPtr hWnd, GWL nIndex, int dwNewLong);

[DllImport("user32.dll", EntryPoint = "SetLayeredWindowAttributes")]
public static extern bool SetLayeredWindowAttributes(IntPtr hWnd, int crKey, byte alpha, LWA dwFlags);

protected override void OnShown(EventArgs e)
{
    base.OnShown(e);
    int wl = GetWindowLong(this.Handle, GWL.ExStyle);
    wl = wl | 0x80000 | 0x20;
    SetWindowLong(this.Handle, GWL.ExStyle, wl);
    SetLayeredWindowAttributes(this.Handle, 0, 128, LWA.Alpha);
}