find user control in master page when user control added dynamicaly

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C#

ASP.NET

C#4.0

user-controls

hi i have a master page and some user control .i wanna load user controls in master page dynamicaly.i had to know which user control is located in form when a post back occur .i use following code but it is not working.my problem is how can i recognize which usercontrol loaded before postback.

if (Page.FindControl("ContentPlaceHolder").FindControl("UC1") == null)
    Control ctl = LoadControl("MasterDetail.ascx");
                     ctl.ID = "UC2";
                     flag = 1;
                     this.ContentPlaceHolder.Controls.Add(ctl);