Reports

Though it's an older thread, wanted to share my findings. For me it was the combination of Kevin Doyon's answer, and using the MouseEnter event instead of MouseHover.

protected override void OnMouseEnter(EventArgs eventargs)
{
    toolTipError.SetToolTip(this, "Hello");
    toolTipError.Active = true;
}

protected override void OnMouseLeave(EventArgs eventargs)
{
    toolTipError.SetToolTip(this, null);
    toolTipError.Active = false;
}

I also preferred using method SetToolTip over Show. With SetToolTip the tooltip is shown at the expected position, with Show I needed to provide an offset (position determined using PointToClient(Cursor.Position)).

79508461