dynamic grid view loses binding when post back occurs ?

See more:

C#

ASP.NET

I have a dynamic grid View with template fields and bound fields as columns

for the first page load all binding looks fine.values are populated in all columns

when I trigger an event such as selected index changed in Drop downlist present in the column header(inside template field ) the binding are lost and the event is never catched.

how to retain controls when post back occurs and how to resolve this problem ?
this is my code

private void Subjectbind(string ExamYear, string ExamType)
{
 
if (ddlExamYear.SelectedIndex == 0)
ExamYear = ddlExamYear.SelectedValue;
if (ddlExamType.SelectedIndex == 0)
ExamType = ddlExamType.SelectedValue;
 


 
List lstCourses =
objMDLClass.GetAllExamTypeCourses(Convert.ToInt32(ExamType),
ExamYear);
objManage = new StudentManage((Users)Session["user"]);
 

foreach (SpGetExamTypeCourses_Result objCourse in lstCourses)
{
 
GridTemplateColumn col = new GridTemplateColumn();
col.UniqueName = objCourse.CourseId.ToString();
col.HeaderText = objCourse.CourseName;
col.ItemTemplate = new GridCustomItemTemplate(objManage.GetExamAnalysisReport(
Convert.ToInt32(objCourse.CourseId), Convert.ToInt32(ExamType),
ExamYear));
 

 
gvSubwisePerformance.MasterTableView.Columns.Add(col);
 
}
gvSubwisePerformance.Rebind();
}