Click-through in C# Form

Question

I've created a semi-transparent form. I'd like for people to be able to click on the form area, and for form not handle the click. I'd like whatever is underneath the form to receive the click event instead.

Outdoor

possible duplicate of Click through transparency for Visual C# Window Forms? – Frowsty

What will underneath the form? – Inventive

Answer 1

You can do this with SetWindowLong:

int initialStyle = GetWindowLong(this.Handle, -20);
SetWindowLong(this.Handle, -20, initialStyle | 0x80000 | 0x20);

There are a few magic numbers in here:

• -20 – GWL_EXSTYLE

  Retrieves the extended window styles.

• 0x80000 – WS_EX_LAYERED

  Creates a layered window.

• 0x20 – WS_EX_TRANSPARENT

  Specifies that a window created with this style should not be painted until siblings beneath the window (that were created by the same thread) have been painted. The window appears transparent because the bits of underlying sibling windows have already been painted.

There are numerous articles all over the web on how to do this, such as this one.

Frowsty

Answer 2

The SetWindowLong in @Joey's answer can only make the form semi-transparent after showing the form on screen. If you call SetWindowLong in Form1_Load, the form will be opaque upon creation, and then quickly being changed to semi-transparent. This causes the users to see the non-semi-transparent form for a short moment upon launching the program. To prevent this, you can override CreateParams instead :

public partial class Form1 : Form
{
    protected override CreateParams CreateParams
    {
        get
        {
            const int WS_EX_LAYERED = 0x80000;
            const int WS_EX_TRANSPARENT = 0x20;
            CreateParams cp = base.CreateParams;
            cp.ExStyle |= WS_EX_LAYERED;
            cp.ExStyle |= WS_EX_TRANSPARENT;
            return cp;
        }
    }
}

The code above makes the form semi-transparent before showing the form on screen.

The solution above is similar to this post, which makes the form top-most instead.

Suboxide