//populating:
var files = Directory.GetFiles(YOUR_FOLDER_PATH, "*.txt");
foreach (var file in files)
{
    var fileName = Path.GetFileName(file);
    //assuming ListBox:
    listBox.Items.Add(filename);
}

//opening (from listbox)
var fileName = Path.Combine(YOUR_FOLDER_PATH, listBox.SelectedItem.ToString());
File.ReadAllText(fileName);

var fileNames = new List<string>();
var fileContents = new Dictionary<string, string>();
var filePaths = Directory.EnumerateFiles(folderBrowserDialog1.SelectedPath, "*.txt");
foreach (var filePath in filePaths)
{
    var fileName =new FileInfo(filePath).Name;
    fileNames.Add(fileName);
    fileContents.Add(fileName, File.ReadAllText(filePath));
}