Windows Forms: Pass clicks through a partially transparent always-on-top window

Question

I am designing a window that is always on screen and around 20% opaque. It is designed to be a sort of status window, so it is always on top, but I want people to be able to click through the window to any other application below. Here's the opaque window sitting on top of this SO post as I type right now:

See that grey bar? It would prevent me from typing in the tags box at the moment.

Manure

Not possible with winforms. – Liner

Do you have any evidence to back up that answer? I find that hard to believe... – Manure

Believe it. My "evidence" is over a decade of experience using win forms. – Liner

@Liner I think the posted answer is what the OP is looking for. Am I missing some part of the question? It's working on Windows 7. – Headstrong

I stand corrected. That's pretty cool! – Liner

Answer 1

You can make a window, click-through by adding WS_EX_LAYERED and WS_EX_TRANSPARENT styles to its extended styles. Also to make it always on top set its TopMost to true and to make it semi-transparent use suitable Opacity value:

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        this.Opacity = 0.5;
        this.TopMost = true;
    }
    [DllImport("user32.dll", SetLastError = true)]
    static extern int GetWindowLong(IntPtr hWnd, int nIndex);
    [DllImport("user32.dll")]
    static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);
    const int GWL_EXSTYLE = -20;
    const int WS_EX_LAYERED = 0x80000;
    const int WS_EX_TRANSPARENT = 0x20;
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
        var style = GetWindowLong(this.Handle, GWL_EXSTYLE);
        SetWindowLong(this.Handle,GWL_EXSTYLE , style | WS_EX_LAYERED | WS_EX_TRANSPARENT);
    }
}

Sample Result

Headstrong

You could just override CreateParams and set the ExStyle instead using P/Invoke. – Clo

I tried this solution but when I click through the window happens nothing – Antimasque

@BruceStackOverFlow Click will pass to the background window. – Headstrong

I use ubuntu with mono seems that this not working... #61000580 – Antimasque

@BruceStackOverFlow It's WinForms (.NET) on Windows not mono on Linux. – Headstrong

Ya but mono support completely windows form 2.0, this work in part, for example, like as the posted gif but the click non work.... – Antimasque

@BruceStackOverFlow The part which is not working is in fact the Windows API part :) – Headstrong