Save in an Array [] file names of a folder in C #

Use the method GetFiles() of class Directory .

This will return an array with all the files that match the search criteria, for example:

ruta = @"c:\users\juan\documents";
string[] archivos = Directory.GetFiles(ruta, "*.jpg");

It will return all files with .jpg extension that are in my document folder.

You can not use OpenFileDialog to get a route, for that the FolderBrowserDialog

string[] arrayNombres;

using(var folderBrowserDialog1 = new FolderBrowserDialog())
{
    DialogResult result = folderBrowserDialog1.ShowDialog();
    if( result == DialogResult.OK )
    {
        string folderName = folderBrowserDialog1.SelectedPath;

        arrayNombres = Directory.GetFiles(folderName, "*.*");
    }
}

Using Directory.GetFiles() you get the files in the folder

How to: Choose folders with the FolderBrowserDialog component of Windows Forms