将复选框添加到Visual Studio中的GridView

I am trying to add a column of checkboxes to my grid view so that I can export only selected rows to Excel(current it exports the whole grid). I am using ASP.Net and C# and I am new to both languages. When I add a the checkbox column it displays with a checkbox in each row.It works until I link the master page to the aspx page and then the column appears but only one check box appears. The checkbox also does not appear in any row but half way down the page. Does anyone know how I can fix this or another way to select multiple rows from a data grid to export(the checkbox method is preferable)? Any help greatly appreciated.I am trying to add a column of checkboxes to my