insert string in string[] array in C#

You can't add items to an array because array has fixed length, alternatively you can go for List<string>

To turn List To Array, You could just call list.ToArray().

You have a few options here. First a clarification of the method you are using. Directory.GetDirectories(string) returns an array of string containing the paths of folders contained within the folder at the path passed to the method. This array does not contain file paths or file names. You can use an instance of DirectoryInfo such as:

DirectoryInfo directory = new DirectoryInfo("my path to directory");

DirectoryInfo[] subDirectories = directory.GetDirectories();

This will return your sub-directories contained within the directory at the path you used in the DirectoryInfo constructor.

As for arrays in general; you cannot add an element to an array as an array has a fixed length, however there is a method within the static Array class Array.Resize(ref yourArray, int newSize) that you may use to re-size then insert the new values at necessary indexes (this does create a new array from the backend and assign it the specified length and values from the previous array but in this case you don't need to keep track of multiple array variables within the user code, helping to keep it slightly easier to remember in my opinion) but if you are going to be doing this regularly you may be better off with using another Type such as List.

If you are wanting the file names such as myTextFile.txt and not the full paths you can use a little bit of Linq such as:

DirectoryInfo directory = new DirectoryInfo("my path to directory");

List<string> myFileNames = directory.GetFiles().Select(c => c.Name).ToList();

This will return a List of string, the names of the files within the directory at the path you used within your DirectoryInfo constructor.

You can do the same thing with getting directory names by swapping out the directory.GetFiles() for directory.GetDirectories() then appending the Select() and ToList().

GetDirectories already returns an array of strings. So since you don't really need to know how big the array is, you should just be able to do

string[] myVariable = dir.GetDirectories();

Then myVariable will be the same length as the number of subdirectories in the directory. Note though, that since arrays are fixed-length, you won't be able to add more to this array.

On a all-in-one solution this can be summarized as follows Note: As referred above Arrays are of fixed length hence their length can't be dynamically included

// Include IO, Linq namespace
using System.IO;
using System.Linq;

// To search files in a directory and collect in a collection
var directoryPath = "";
var fileNameList = Directory.GetFiles(directoryPath,"*.*", SearchOption.AllDirectories).ToList().Select(e=>Path.GetFileName(e));

// Casting the List to Array using ToArray()
var fileNameArray = fileNameList.ToArray();