Generate Long-live access token for Oauth

There are a number of scenarios that can cause the refresh to fail like this, such as:

• missing/incorrect/malformed refresh token
• revoked refresh token (e.g., when the user unlinks the app via https://www.dropbox.com/account/connected_apps , etc.)
• missing/incorrect/malformed app key
• incorrect/malformed app secret
• missing app secret (if the refresh token was retrieved using the non-PKCE flow)

Check and make sure you're supplying the correct values when building the DropboxClient. If you're supplying any incorrect value(s), update your app to supply the correct value(s). If the refresh token has been revoked, you'll need to have the user re-authorize the app to access the account to receive a new valid refresh token.

The response body on that 400 error response may contain a more specific error message, though the .NET SDK unfortunately does not currently expose that. There's an open request to update it to do so. You can perform the call manually to check for an error message in the response body however, e.g., using curl like this:

curl https://api.dropboxapi.com/oauth2/token \
    -d refresh_token=REFRESHTOKENHERE \
    -d grant_type=refresh_token \
    -d client_id=APPKEYHERE

By the way, you do not need to check for 'expired_access_token' yourself like that. As long as you provide correct credentials to the DropboxClient, it will catch and handle that error for you automatically.

I recommend running the sample code first to see how it works first.