Why showing error message while opening .xls file

Question

In my asp.net, C# application we are generating and downloading .xls file. But when I\'m trying to open, it\'s giving a message

\"The file you are try

Answer 1

Do you completely create the xls file or do you copy and fill a template xls file ? An incorrect format template file may cause the problem.

Also, what provider do you use to fill your file ? - Microsoft.ACE.OLEDB.12.0 for xlsx/xlsm ? - Microsoft.Jet.OLEDB.4.0 for xls ?

An incorrect provider/extension combination may cause the problem.

According to your comment, here is a part of code where I have done that in the past : ( commented some of the lines as I don't remember why they where useful )

Response.Clear();                           
Response.ContentType = "application/vnd.ms-excel";
//Response.ContentEncoding = Encoding.Default; 
//Response.Charset=""; 
Response.AddHeader("Content-Disposition: ",
    String.Format(@"attachment; filename={0}",myfileName));

//EnableViewState = false; 

Response.Write(myFileContentAsString); 
Response.Flush();
Response.Close();