Fix: Getting refresh access token for Dropbox .net sdk, without intervention

 To obtain a refresh access token for the Dropbox .NET SDK without user intervention (also known as a "refresh token flow"), you need to use the OAuth 2.0 API provided by Dropbox. Here are the general steps to achieve this in C#:

1. **Set Up Your App on Dropbox**:

   First, you need to create an app on the Dropbox App Console. This will give you the `App Key` and `App Secret` required for the OAuth flow.

2. **Include the Dropbox .NET SDK**:

   Make sure you have the Dropbox .NET SDK installed in your C# project. You can add it using NuGet:

   ```bash

   Install-Package Dropbox.Api

   ```

3. **Implement OAuth 2.0 Flow**:

   You'll need to implement the OAuth 2.0 flow to obtain an initial access token. You can use the `DropboxOAuth2Helper` class provided by the Dropbox SDK for this purpose.

   ```csharp

   using Dropbox.Api;

   using Dropbox.Api.Users;

   using Dropbox.Api.Common;

   var appKey = "YOUR_APP_KEY";

   var appSecret = "YOUR_APP_SECRET";

   var redirectUri = "YOUR_REDIRECT_URI";

   var authUri = DropboxOAuth2Helper.GetAuthorizeUri(OAuthResponseType.Code, appKey, redirectUri);

   // Redirect the user to authUri so they can authorize your app.

   // After user authorization, obtain the access token using the authorization code

   string code = "AUTHORIZATION_CODE"; // This is received after user authorizes the app

   OAuth2Response response = DropboxOAuth2Helper.ProcessCodeFlowAsync(code, appKey, appSecret, redirectUri).Result;

   ```

4. **Store and Manage the Access Token**:

   You'll receive an access token. Store it securely because you'll need it for future API calls.

5. **Use the Refresh Token**:

   To refresh the access token without user intervention, you need to use the refresh token. The Dropbox SDK does not provide a direct method for this, so you'll need to use an HTTP client or library to make a POST request to the Dropbox API. You can use the `curl` command or libraries like `HttpClient` to send a POST request to the Dropbox API for token refresh.

   The endpoint to use is `https://api.dropbox.com/oauth2/token`, and you'll need to provide the `refresh_token`, `client_id`, `client_secret`, and `grant_type=refresh_token` in the request.

   Here's a sample request in C# using `HttpClient`:

   ```csharp

   using System.Net.Http;

   using System.Threading.Tasks;

   var client = new HttpClient();

   var refreshToken = "YOUR_REFRESH_TOKEN";

   var values = new Dictionary<string, string>

   {

       { "refresh_token", refreshToken },

       { "client_id", appKey },

       { "client_secret", appSecret },

       { "grant_type", "refresh_token" }

   };

   var content = new FormUrlEncodedContent(values);

   var response = await client.PostAsync("https://api.dropbox.com/oauth2/token", content);

   if (response.IsSuccessStatusCode)

   {

       string responseBody = await response.Content.ReadAsStringAsync();

       // Parse the response to obtain the new access token.

   }

   ```

Remember to securely store your app's credentials and access tokens, and protect them from unauthorized access.

Please note that the Dropbox SDK may evolve, so refer to the official Dropbox documentation and the SDK's documentation for the most up-to-date information on OAuth flows and token management.