The order in which this function returns the file names is dependent on the file system type. With NTFS and CDFS file systems, the names are returned in alphabetical order. With FAT file systems, the names are returned in the order the files were written to the disk, which may or may not be in alphabetical order. 

DirectoryInfo di = new DirectoryInfo(@"d:/test/");

var files = di.GetFiles().OrderBy(f => f.Name);

var files = di.GetFiles().OrderBy(f => f.LastWriteTime);