How to make excel sheet headers as a bold using c# ?

Hi all ,

I am using Stram writer to create a excel sheet. i am creating a excel file and attaching that to mail attchment in crm2011.

All my code is working fine.. 

what my question is while creating only i wnat to make headers as a bold.

Below is my code. Please let me know any idea to do this.

using (MemoryStream memory = new MemoryStream())
                {
                    using (StreamWriter streamwriter = new StreamWriter(memory))
                    {
                        string str = string.Empty;
                       str = "Displayname,LogicalName,OldValue,ModifiedValue";//these are the header names
                         

                        foreach (var att in attributes)
                        {

string ModifiedValue = ""; string displayname = ""; string LogicalName = ""; string OldValue = "";

streamwriter.WriteLine(displayname + "," + LogicalName + "," + OldValue + "," + ModifiedValue);

}

}

streamwriter.Flush();
                        streamwriter.Dispose();
                        SendEmailWithAttachment(service, memory, PrimaryEntity);

}

In the attachment i m using like this

 mailattach[HelperClass.EMAILATTACHMENT.MIMETYPE] = "application/octet-stream";
            mailattach[HelperClass.EMAILATTACHMENT.FILENAME] = "Modfied Fields for " + Entityname + ".csv";

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All are working fine ... but i am not getting how to make headers as in bold.