Integrate Dropbox with .Net Core 6 web application

Greg - Thank you so much for answering my post and for the great tips you gave for how to get my .NET Core 6 web application to work with Dropbox client. I finally accomplished it by using the SimpleBlogDemo example as you suggested. I added a Dropbox Chooser control and some code to check the expiration date of the online token and regenerate the token if it was expired. Below is the code which worked that created shared urls for displaying Dropbox images in the views. 

_GalleryLayout: 

<script>
var options = {

// Required. Called when a user selects an item in the Chooser.
success: function(files) {
//console.log("Here's the file link: " + files[0].link)
//console.log("Here's the file name: " + files[0].name)
$("#ItemUrl").val(files[0].name);
},

Add/Edit Views - 

<div style="display:none;">
<input id="ItemUrl" name="ItemUrl">
@Html.ActionLink(
linkText: "Connect",
actionName: "Connect",
routeValues: new { referrer = "Add", galleryItemId = 0 },
htmlAttributes: new { ID="btnConnect", Class = "btn btn-lg btn-primary" })
</div>

IF (!TempData.ContainsKey("TokenOk"))
{
<script type="text/javascript">
document.getElementById("btnConnect").click();
</script>
}

Gallery Controller -

public IActionResult Connect(string referrer, string galleryItemId)
{
TempData["Referrer"] = referrer;
TempData["GalleryItemId"] = galleryItemId;
Uri redirect = null;

string? authToken = TempData["AuthToken"]?.ToString();
if (!TempData.ContainsKey("AuthToken") || string.IsNullOrEmpty(authToken))
{
redirect = GetDropboxAuthUrl();

return Redirect(redirect.ToString());
}

if (TempData.ContainsKey("TokenExpires"))
{
string? tokenExpires = TempData["TokenExpires"]?.ToString();
CultureInfo culture = new CultureInfo("en-US");
DateTime? tempDate = Convert.ToDateTime(tokenExpires, culture);
if (!tempDate.HasValue || tempDate.HasValue && DateTime.Now > tempDate)
{
redirect = GetDropboxAuthUrl();

return Redirect(redirect.ToString());
}
}

TempData["TokenOk"] = true;
TempData.Keep("TokenOk");

if (referrer == "Add")
{
return RedirectToAction("AddGalleryItem", "Gallery");
}
else
{
return RedirectToAction("EditGalleryItem", "Gallery", new { galleryItemId = galleryItemId});
}
}

public Uri GetDropboxAuthUrl()
{
var state = Guid.NewGuid().ToString("N");
string[] scopeList = new string[4] { "files.metadata.read", "files.content.read", "sharing.read", "sharing.write" };

var redirect = DropboxOAuth2Helper.GetAuthorizeUri(OAuthResponseType.Code,
AppKey,
RedirectUri,
state: state,
tokenAccessType: TokenAccessType.Online,
scopeList: scopeList,
includeGrantedScopes: IncludeGrantedScopes.None);

return redirect;
}

[Authorize]
public async Task<ActionResult> AuthAsync(string code, string state)
{
try
{
var response = await DropboxOAuth2Helper.ProcessCodeFlowAsync(
code,
AppKey,
AppSecret,
RedirectUri);

TempData["AuthToken"] = response.AccessToken;
TempData["TokenExpires"] = response.ExpiresAt;

string? refUrl = TempData["Referrer"]?.ToString();
string? galleryItemId = TempData["GalleryItemId"]?.ToString();

if (!string.IsNullOrEmpty(refUrl) && refUrl.Contains("Add"))
{
return RedirectToAction("AddGalleryItem", "Gallery");
}
else if (!string.IsNullOrEmpty(refUrl) && refUrl.Contains("Edit"))
{
return RedirectToAction("EditGalleryItem", "Gallery", new { galleryItemId = galleryItemId });
}

return RedirectToAction("Index", "Home");
}
catch (Exception e)
{
var message = e.Message;
return RedirectToAction("Index");
}
}

SaveGalleryItem method

[Authorize]
[HttpPost]
public async Task<IActionResult> SaveGalleryItem(GalleryItemModel model)
{
if (TempData.ContainsKey("Referrer") && TempData["Referrer"].ToString().Contains("Add"))
{
model.GalleryItemSequenceId = model.GetLastGalleryItemSequenceId(_galleryItemService, model.CategoryId) + 1;
if (model.GalleryItemSequenceId == 0)
{
model.GalleryItemSequenceId = 1;
}
}

if (Upload != null)
{
if (User.Identity.IsAuthenticated)
{
var fileName = Upload.FileName;

var file = Path.Combine(_environment.WebRootPath, "assets\\images", Upload.FileName);

using var fileStream = new FileStream(file, FileMode.Create);
await Upload.CopyToAsync(fileStream);
if (model.CategoryId == 4)
{
if (!string.IsNullOrEmpty(model.ImageUrl))
{
model.ImageUrl += ",assets/images/" + fileName;
}
else
{
model.ImageUrl = "assets/images/" + fileName;
}
}
else
{
model.ImageUrl = "assets/images/" + fileName;
}
}
}
else
{
if (!string.IsNullOrEmpty(ItemUrl))
{
model.ImageUrl = ItemUrl;
}
}

if (User.Identity.IsAuthenticated)
{
var authToken = string.Empty;

if (TempData != null && TempData.ContainsKey("AuthToken"))
{
authToken = TempData["AuthToken"]?.ToString();
}

if (!string.IsNullOrEmpty(authToken) && !string.IsNullOrEmpty(ItemUrl) && !ItemUrl.Contains("https"))
{
using (var dbx = new DropboxClient(authToken))
{
var sharedLinkUrl = string.Empty;
var sharedLink = await dbx.Sharing.CreateSharedLinkAsync($"/Gourmet Philatelist/{ItemUrl}");
if (sharedLink != null)
{
sharedLinkUrl = sharedLink.Url;
model.ImageUrl = sharedLinkUrl.Replace("dl=0", "raw=1");
}
}
}
await model.AddUpdateGalleryItem(_galleryItemService, _categoryItemService, model);
}

if (TempData != null && TempData.ContainsKey("TokenOk"))
{
TempData.Remove("TokenOk");
}

return RedirectToAction("ManageGalleryItems", "Gallery");
}