While hovering over a disabled element the tooltip doesn't apper but, we can do this as follows:
1. Use the ShowOnDisabled attached property of the ToolTipService class! From XAML, this would look like the following on a Button:
<Button ToolTipService.ShowOnDisabled=”True”>
</Button>
2. Or from C# code, you can call the static method corresponding to the attached property:
ToolTipService.SetShowOnDisabled(myButton, true);