Show folders in listbox and subfolders in another listbox

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C#

ListBox

hi all

I'm trying to create an Autocad plugin to preview thumbnails in datagridview

I have two listboxes I want to binding them together and show the .bmp in datagridview

1-listbox1 show Directory
2-listbox2 show subfolder from the selected folder (listbox1)
3-and then the .bmp files show in datagridview

I succeed to figure the first question

What I have tried:

String[] dirs = System.IO.Directory.GetDirectories(@"D:\test\Blocks");
            int i;
            for (i = 0; i < dirs.Length; i++)
            {
                listBox1.Items.Add(Path.GetFileName(dirs[i]));
            }

            String[] files = System.IO.Directory.GetFiles(@"D:\test\Blocks");
            for (i = 0; i < files.Length; i++)
            {
                listBox1.Items.Add(Path.GetFileName(files[i]));
            }