Export the file by default on Desktop....?

try it. .

string add;
add="C:\Windows\Desktop";

MyReport.ExportToDisk(ExportFormatType.PortableDocFormat,"add + MyReport");

Try this

var desktopFolder = Environment.GetFolderPath(Environment.SpecialFolder.DesktopDirectory);
var fullFileName = Path.Combine(desktopFolder, "Test.txt");
var fs = new FileStream(fullFileName, FileMode.Create);