how to merge DirectoryInfo[] and FileInfo[] array?

DirectoryInfo and FileInfo both inherit directly from the abstract FileSystemInfo class, which is also in the System.IO namespace.

So, if you create an array of FileSystemInfo, then it will be able to hold objects of either type:

// dia and fia are arrays of DirectoryInfo and FileInfo respectively

FileSystemInfo[] fsia = new FileSystemInfo[dia.Length + fia.Length];

for(int i = 0; i < dia.Length; i++) fsia[i] = dia[i];

for(int i = dia.Length; i < fsia.Length; i++) fsia[i] = fia[i - dia.Length];