Your "path" variable contains the folder name, not a filename. You can append your filename and write the file with a StreamWriter, e.g.
Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), "Testfile.txt");
using (StreamWriter writer = new StreamWriter(filename))
{
writer.Write("This is created by Animesh Datta.....");
}
Also note that the Desktop can be read-only due to the security settings of your customer.