Get only select values of the Checkbox in GridView

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C#

The first column of a GridView is having CheckBox.
GridView is having paging of page size =5.

IThere are 12 records in a GridView. So Gridview shows 3 pages Now i check only 2 checkboxes in first page and 3 checkboxes in second page and only 1 checkbox in third page.

Now in Button click event i am writing the code

string DID= "";
 foreach (GridViewRow gvrow in gvFiles.Rows)
{
            CheckBox chk = new CheckBox();
            chk = (CheckBox)gvrow.FindControl("chkSelect");
            if (chk.Checked)
            {
               if (DID== "")
DID= gvFiles.DataKeys[gvrow.RowIndex].Values["DocumentID"].ToString();
             else
             DID= DID+ "," + gvFiles.DataKeys[gvrow.RowIndex].Values["DocumentID"].ToString();
            }
}
return DID;

Only the checked rows for the first page of the grid is showing. the checked rows in the other pages of Gridview are not captured.

Can i get the code for the following.. where i can get only the checked rows in all the pages of a GridView.

Thankyou,
yashoda