HOW TO ACCESS DROPBOX FILES

Thank you very much for your comprehensive reply.

For an initial test with your own Dropbox account, you can generate an access token from the OAuth 2 section of your Dropbox app settings.

This is exactly the point!

Following is the code that should do the job, taken from the sample https://github.com/dropbox/dropbox-sdk-dotnet:

using Dropbox.Api.Files;
using Dropbox.Api;
using System.Net;
using System.Text;
using System.Threading.Tasks;

namespace TestDropbox;

public static class Dropbox
{
    // Authentication constants

    // Add an ApiKey (from https://www.dropbox.com/developers/apps) here
    private const string ApiKey = "XXXXXXXXXXXXXXX";

    // This loopback host is for demo purpose. If this port is not
    // available on your machine you need to update this URL with an unused port.
    private const string LoopbackHost = "http://127.0.0.1:52475/";

    // URL to receive OAuth 2 redirect from Dropbox server.
    // You also need to register this redirect URL on https://www.dropbox.com/developers/apps.
    private static readonly Uri RedirectUri = new Uri(LoopbackHost + "authorize");

    // URL to receive access token from JS.
    private static readonly Uri JSRedirectUri = new Uri(LoopbackHost + "token");

    // ------------------------------------------------------------------------

    /// <summary>
    /// Gets the dropbox access token.
    /// </summary>
    public static async Task GetAccessToken()
    {
        var accessToken = Preferences.Get("AccessToken", String.Empty);
        if (string.IsNullOrEmpty(accessToken))
        {
            try
            {
                var state = Guid.NewGuid().ToString("N");
                var authorizeUri = DropboxOAuth2Helper.GetAuthorizeUri(OAuthResponseType.Token, ApiKey, RedirectUri, state: state);
                var http = new HttpListener();
                http.Prefixes.Add(LoopbackHost);
                http.Start();
                await Browser.OpenAsync(authorizeUri);

                // Handle OAuth redirect and send URL fragment to local server using JS.
                await HandleOAuth2Redirect(http);

                // Handle redirect from JS and process OAuth response.
                var redirectUri = await HandleJSRedirect(http);
                var result = DropboxOAuth2Helper.ParseTokenFragment(redirectUri);
                http.Stop();

                if (result?.State != state)
                {
                    // The state in the response doesn't match the state in the request.
                    //return;
                    throw new Exception("The state in the response does not match the state in the request.");
                }

                // save Access token
                Preferences.Set("AccessToken", result.AccessToken);
                Preferences.Set("Uid", result.Uid); // NOT USED
            }
            catch (Exception ex)
            {
                string message = ex.Message;
                if (message == "Invalid OUath 2.0 response, missing access_token and/or uid.")
                {
                    message = "CANCELED BY USER";
                }
                await Shell.Current.DisplayAlertAsync("Exception in GetAccessToken", message, "OK");
            }
        }
    }

    /// <summary>
    /// Handles the redirect from Dropbox server. Because we are using token flow,
    /// the local http server cannot directly receive the URL fragment. We need to
    /// return a HTML page with inline JS which can send URL fragment to local
    /// server as URL parameter.
    /// </summary>
    /// <param name="http">The http listener.</param>
    /// <returns>The <see cref="Task"/></returns>
    private static async Task HandleOAuth2Redirect(HttpListener http)
    {
        var context = await http.GetContextAsync();

        // We only care about request to RedirectUri endpoint.
        while (context.Request.Url?.AbsolutePath != RedirectUri.AbsolutePath)
        {
            context = await http.GetContextAsync();
        }

        context.Response.ContentType = "text/html";

        /*
         * Respond with a page which runs JS and sends URL fragment as query
         * string to TokenRedirectUri.
         */
        using (var file = File.OpenRead("index.html"))
        {
            file.CopyTo(context.Response.OutputStream);
        }

        context.Response.OutputStream.Close();
    }

    /// <summary>
    /// Handle the redirect from JS and process raw redirect URI with fragment to
    /// complete the authorization flow.
    /// </summary>
    /// <param name="http">The http listener.</param>
    /// <returns>Uri</returns>
    private static async Task<Uri?> HandleJSRedirect(HttpListener http)
    {
        var context = await http.GetContextAsync();

        // We only care about request to TokenRedirectUri endpoint.
        while (context.Request.Url?.AbsolutePath != JSRedirectUri.AbsolutePath)
        {
            context = await http.GetContextAsync();
        }
        var query = context.Request.QueryString["url_with_fragment"];
        var redirectUri = new Uri(query!);
        return redirectUri;
     }
...

The file index.html (also from the sample above) is:

<html>
<script type="text/javascript">
    function redirect() {
        // Append fragment as query string so that server can receive it.
        document.location.href = "/token?url_with_fragment=" + encodeURIComponent(document.location.href);
    }
</script>
<body>
    onload="redirect()"
</body>
</html>

This works for the sample, which is a console desktop application, but not in MAUI with the emulator:

• WindowsMachine: gets stuck on the browser page reading 'onload="redirect()"'; closing the page does not solve
• Android emulator: gives error "127.0.0.1 refused to connect"

Thus, I cannot get the access token.

The sample https://github.com/dropbox/dropbox-sdk-dotnet is a console application for desktop. It runs ok - except that the browser remains open after the sign-in process has completed.

I created a MAUI project with code from that sample, but:

• WindowsMachine: it stucks on the redirection browser page
• Android emulator: I have an error "129.0.0.1 refused to connect"

The document https://developers.dropbox.com/oauth-guide is an old document, dated December 07, 2020 and it does not provide any help for implementation in MAUI.

Dropbox API Explorer: it is of no help for developing MAUI applications.

I will check latest one and update to you