Control.GetUniqueIDRelativeTo Method (Control)
Returns the prefixed portion of the UniqueID property of the specified control.
Assembly: System.Web (in System.Web.dll)
The UniqueID property of a control is generated by adding a prefix to the control's ID property. The prefix is composed of the UniqueID property of the control's naming container concatenated with a separator character. If the naming container is the page, there is no prefix. This method returns the prefix string. If there is no prefix string, it returns the UniqueID value of the control.
Available since 4.0