In CSharp, on Windows 11, I have a form with a solid background and a transparent PictureBox. The form loads and PictureBox is actually transparent, but when I move the mouse over the transparent area the mouse handling gets sent to objects behind the form.

Since I want the form to handle mouse over events for transparent areas I added an override for WndProc which intercepts the NCHITTEST message and returns the HTCLIENT value of 1. I think this is correct based on Microsoft WM_NCHITTEST message Document.

With this updated WndProc I can see the messages from Form_MouseMove; but none from PictureBox_MouseMove. And objects visible through the transparent element behind the form are handling the mouse.

I suspect it is possible to intercept these the mouse control messages and force my form to handle them but I can’t seem to figure out how.

using System;
using System.Drawing;
using System.Windows.Forms;

namespace TransparentFormExample
{
    public class TransparentForm : Form
    {
        private PictureBox pictureBox;

        public TransparentForm()
        {
            // Set form properties
            this.TransparencyKey = Color.LimeGreen;
            this.Size = new Size(500, 800);
            this.TopMost = true;

            // Initialize and set PictureBox properties
            pictureBox = new PictureBox
            {
                BackColor = Color.LimeGreen,
                Size = new Size(200, 200),
                Location = new Point(150, 300) // Centered in the form
            };

            // Add PictureBox to the form
            this.Controls.Add(pictureBox);

            // Add event handlers
            this.MouseMove += new MouseEventHandler(Form_MouseMove);
            pictureBox.MouseMove += new MouseEventHandler(PictureBox_MouseMove);
        }

        // Override WndProc to handle WM_NCHITTEST message
        protected override void WndProc(ref Message m)
        {
            const int WM_NCHITTEST = 0x84;
            const int HTCLIENT = 0x1;

            if (m.Msg == WM_NCHITTEST)
            {
                m.Result = (IntPtr)HTCLIENT;
                return;
            }

            base.WndProc(ref m);
        }

        private void Form_MouseMove(object sender, MouseEventArgs e)
        {
            Console.WriteLine("Mouse over form");
        }

        private void PictureBox_MouseMove(object sender, MouseEventArgs e)
        {
            // extra spaces to make it easier to spot if it happens
            Console.WriteLine("             Mouse over PictureBox");
        }


        public void Start () 
        {

            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault( false );
            
            Application.Run( this ); 
        }
    }
}

I’m starting the form using the following PowerShell, since I do not have the ability to create a DLL, compile .cs, and am not allowed to run .exe files.

write-host "load main.cs"

add-type -path "TransparentForm.cs" -ReferencedAssemblies @(
    "System.Windows.Forms"
    "System.Drawing"
    "System.Drawing.Primitives"
    )
$ProgramInstance = [TransparentFormExample.TransparentForm]::New()
[System.Windows.Forms.Application]::Run( $ProgramInstance )