MainForm.cs

using System;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace DropboxExplorer
{
    public partial class MainForm : Form
    {
        readonly IStorageRepository _storageRepository = new DropboxRepository();

        public MainForm()
        {
            InitializeComponent();

            UpdateControlStates(false);

            SetSizeLabels(0);
        }

        private async void button_GetFolderSize_Click(object sender, EventArgs e)
        {
            try
            {
                UpdateControlStates(false);

                var path = CalculatePath(treeView_DirectoryTree.SelectedNode);

                var folderSize = await _storageRepository.GetFolderSizeAsync(path, true);

                SetSizeLabels(folderSize);
            }
            finally
            {
                UpdateControlStates(true);
            }
        }

        private void SetSizeLabels(ulong sizeInBytes)
        {
            var folderSizeInMB = sizeInBytes / 1024.0 / 1024.0;
            var folderSizeInGB = folderSizeInMB / 1024.0;

            label_SizeInGB.Text = $"{folderSizeInGB:F2} GB";
            label_SizeInMB.Text = $"{folderSizeInMB:F2} MB";
            label_SizeInBytes.Text = $"{sizeInBytes:n0} bytes";
        }

        private void textBox_AuthToken_TextChanged(object sender, EventArgs e)
        {
            var authToken = textBox_AuthToken.Text;

            UpdateControlStates(!string.IsNullOrEmpty(authToken));

            _storageRepository.SetAuthToken(authToken);
        }

        private async void button_ListDirectoryStructure_Click(object sender, EventArgs e)
        {
            try
            {
                UpdateControlStates(false);

                var items = await _storageRepository.GetItemsAsync("", false);

                treeView_DirectoryTree.Nodes.Clear();

                treeView_DirectoryTree.Nodes.AddRange((from x in items
                                                       where x.StorageItemType == StorageItemType.Directory
                                                       select new TreeNode(x.Name)).ToArray());
            }
            finally
            {
                UpdateControlStates(true);
            }
        }

        private void UpdateControlStates(bool enable)
        {
            treeView_DirectoryTree.Enabled = enable;
            button_ListDirectoryStructure.Enabled = enable;
            button_GetFolderSize.Enabled = enable;
        }

        private async void treeView_DirectoryTree_DoubleClick(object sender, EventArgs e)
        {
            try
            {
                UpdateControlStates(false);

                var selectedNode = treeView_DirectoryTree.SelectedNode;

                if(selectedNode != null)
                {
                    var items = await _storageRepository.GetItemsAsync(CalculatePath(selectedNode), false);

                    selectedNode.Nodes.Clear();

                    selectedNode.Nodes.AddRange((from x in items
                                                 where x.StorageItemType == StorageItemType.Directory
                                                 select new TreeNode(x.Name)).ToArray());

                    selectedNode.Expand();
                }
            }
            finally
            {
                UpdateControlStates(true);
            }
        }

        private string CalculatePath(TreeNode node)
        {
            var sb = new StringBuilder();

            sb.Insert(0, $"/{node.Text}");

            var thisNode = node.Parent;

            while(thisNode != null)
            {
                sb.Insert(0, $"/{thisNode.Text}");

                thisNode = thisNode.Parent;
            }

            return sb.ToString();
        }
    }
}