https://docs.microsoft.com/en-us/dotnet/api/system.web.ui.control.findcontrol?view=netframework-4.7
Searches the current naming container for a server control with the specified id parameter.
public virtual System.Web.UI.Control FindControl (string id);
The specified control, or null if the specified control does not exist.
Use FindControl to access a control from a function in a code-behind page, to access a control that is inside another container, or in other circumstances where the target control is not directly accessible to the caller.
This method will find a control only if the control is directly contained by the specified container; that is, the method does not search throughout a hierarchy of controls within controls.
For information about how to find a control when you do not know its immediate最接近的 container, see How to: Access Server Controls by ID.
Docs-->.NET-->API reference-->System.?Web.?UI-->Control-->Methods-->Find?Control
原文：http://www.cnblogs.com/chucklu/p/6932971.html