Download Excel file which is created by Open XML SDK 2.0

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C#

ASP.NET

Hi Expert I created excel file using Open XML SDK 2.0 and open, it is successfully open.
but when user download this file and open, it cannot open. Error message comes file "Excel found unreadable content myExcel.xlsx do you want to open" when i clicked on yes show message ="the file is corrupt and cannot be opened."
please tell me how to make downloading functionality for this type of file

My downloading code:

FileInfo _file = new FileInfo(file);
            if (_file.Exists)
            {
                Response.Clear();
                Response.ClearHeaders();
                Response.ClearContent();
                Response.AddHeader("content-disposition", "attachment; filename=" + _file.Name);
                Response.AddHeader("Content-Type", "application/Excel");
                Response.ContentType = "application/vnd.xlsx";
                Response.AddHeader("Content-Length", file.Length.ToString());
                Response.WriteFile(_file.FullName);
                Response.End();
}

thanx in advanced.....