using(var fbd = new FolderBrowserDialog())
{
    DialogResult result = fbd.ShowDialog();

    if (result == DialogResult.OK && !string.IsNullOrWhiteSpace(fbd.SelectedPath))
    {
        string[] files = Directory.GetFiles(fbd.SelectedPath);

        System.Windows.Forms.MessageBox.Show("Files found: " + files.Length.ToString(), "Message");
    }
}

OpenFileDialog dialog = new OpenFileDialog();
if (DialogResult.OK == dialog.ShowDialog()) 
{
    string path = dialog.FileName;
}

// https://github.com/ookii-dialogs/ookii-dialogs-wpf
string path;
var fbd = new VistaFolderBrowserDialog();
fbd.ShowNewFolderButton = true;
fbd.Description = "Select Source Folder";
fbd.ShowDialog();
path = fbd.SelectedPath;
//Fix Length Browse Folder Disk
return path.Length > 3 ? $@"{path}\" : $@"{path}";

CommonOpenFileDialog dialog = new CommonOpenFileDialog();
dialog.InitialDirectory = "C:\\Users";
dialog.IsFolderPicker = true;
if (dialog.ShowDialog() == CommonFileDialogResult.Ok)
{
    MessageBox.Show("You selected: " + dialog.FileName);
}