Setting a Button to Focusable = false

I think it's going to be final answer:

The question is not correct, because it makes no sense.
Being in a focus is not a control's exclusive property, this is essentially a property if a set of controls.

First of all, essentially, the focus is the target of dispatching of the keyboard event; in what control the events go. Additionally, there should be a visual clue for that state (implementation of this visual clue in a control can easily be failed, in case of sloppy programming).

Generic de-focusing of the control is not possible or impossible, it is unspecified action. It is not possible to "remove" focus, it is only possible to move focus somewhere else. Also, a request to remove a focus from a particular control make no practical sense.

When you want your eyes to remove focus from an object you look at, you normally realize it like focusing on something else or else you can close your eyes.

Now, here is more on background: there are also windows or forms. They are not focused, they are activated or deactivated. Among windows, "activity" status plays the role of the focus within a window. When a windows is activated, one of its controls gets focus. When it is deactivated, nothing on it is in focus, but on next activation, the same very control will get in focus. Within the window, the focus can be moved from control to control. So we can talk about "logical" and "physical" focus of a control (on WPF, whole hierarchy of logical focus is supported, on WinForms we can talk only about one control). "Logical" focus does not depend on the activation status of the form, but "physical" does -- a control is in focus (can physically accept keyboard events) only when it is in a "logical" focus and the window is activated.

class ButtonNoFocus : Button
{
    protected override void WndProc(ref Message m)
    {
        const int WM_MOUSEACTIVATE = 0x0021;
        const int MA_NOACTIVATEANDEAT = 4;

        if (m.Msg == WM_MOUSEACTIVATE)
        {
            m.Result = new IntPtr(MA_NOACTIVATEANDEAT);
            DoMouseDown(new MouseEventArgs(MouseButtons.Left, 0, 0, 0, 0));
        }
        else
            base.WndProc(ref m);
    }

    public new event MouseEventHandler MouseDown;

    private void DoMouseDown(MouseEventArgs mouseEventArgs)
    {
        var e = MouseDown;
        if (e != null)
            MouseDown(this, mouseEventArgs);
    }
}

This will prevent focus and raise a fake MouseDown event when clicked. Note: Click event will not fire as the control must take focus for the Click to happen; however, if want to capture the mouse and manually track this should work.

UPD: It is better to return MA_NOACTIVATE (3) as it will not bring focus to the control, but will still raise the usual MouseDown event.

This is not going to be easy. At least I am not aware of an easy way. I think you will have to extend th eexisting button control for that.

You can do this :

Button1.Focusable=False

Try ActiveControl property of form

this.activecontrol = textbox1;