How to Programmatically show a WPF/C# Windows.Control.ToolTip?

What you need to do is make sure the ToolTip on the control is of type ToolTip. Then you can set the IsOpen property to true like so:

ToolTip tooltip = new ToolTip{ Content = "My Tooltip" };
NameTextBox.ToolTip = tooltip;
tooltip.IsOpen = true;