How do I create an “unfocusable” form in C#?

Question

I\'m looking to create a form in C# that cannot accept focus, i.e. when I click a button on the form, focus is not stolen from the application that currently has the focus.<

Answer 1

To disable activation by mouse:

class NonFocusableForm : Form
{
    protected override void DefWndProc(ref Message m)
    {
        const int WM_MOUSEACTIVATE = 0x21;
        const int MA_NOACTIVATE = 0x0003;

        switch(m.Msg)
        {
            case WM_MOUSEACTIVATE:
                m.Result = (IntPtr)MA_NOACTIVATE;
                return;
        }
        base.DefWndProc(ref m);
    }
}

To show form without activation (the only one way that worked for me in case of borderless form):

    [DllImport("user32.dll")]
    public static extern bool ShowWindow(IntPtr handle, int flags);

    NativeMethods.ShowWindow(form.Handle, 8);

Standard way to do this (seems like it doesn't work for all form styles):

    protected override bool ShowWithoutActivation
    {
        get { return true; }
    }

If there are other ways of activating the form, they can be suppressed in similar manner.

Answer 2

This is the "NoFocusForm" I use:

public class NoFocusForm : Form
{
    /// From MSDN <see cref="https://docs.microsoft.com/en-us/windows/win32/winmsg/extended-window-styles"/>
    /// A top-level window created with this style does not become the 
    /// foreground window when the user clicks it. The system does not 
    /// bring this window to the foreground when the user minimizes or 
    /// closes the foreground window. The window should not be activated 
    /// through programmatic access or via keyboard navigation by accessible 
    /// technology, such as Narrator. To activate the window, use the 
    /// SetActiveWindow or SetForegroundWindow function. The window does not 
    /// appear on the taskbar by default. To force the window to appear on 
    /// the taskbar, use the WS_EX_APPWINDOW style.
    private const int WS_EX_NOACTIVATE = 0x08000000;

    public NoFocusForm()
    { 
        // my other initiate stuff
    }

    /// <summary>
    /// Prevent form from getting focus
    /// </summary>
    protected override CreateParams CreateParams
    {
        get
        {
            var createParams = base.CreateParams;

            createParams.ExStyle |= WS_EX_NOACTIVATE;
            return createParams;
        }
    }
}