OAuth2 issues with Dropbox and Xamarin.Auth

I found the solution, but to answer your question first, the text is output from a sniffer trace showing the HTTP Get request.

And you are right, in that the second response_type and client_id are a duplication.  It confused me and at first I thought it was the HTTP body.  One thing I didn't understand is that GetAuthorizeUri returns the URL plus the params.  Xamarin.Auth requires just the URL, so I can just send "https://www.dropbox.com/oauth2/authorize", and I won't call GetAuthorizeUri at all, since OAuth2Authenticator takes care of building the request.

The reason there were two response_types where one was code type rather than token type was because I supplied the AccessTokenURL to OAuth2Authenticator, which is not needed in the case of token flow, as I understand.  Since I started with a dropbox API example for .NET and then plugged in each into the OAuth2Authenticator call, I mistakenly sent an AccessTokenUrl.  I found out from looking at the Xamarin code, if AccessTokenUrl==NULL will assume the implict flow (token flow).  Interestingly, and as a note to help others, I tried setting the AccessTokenUrl to null, which caused an exception.  Instead I found this version worked and now I'm authenticating.

authenticator = new Xamarin.Auth.OAuth2Authenticator(
clientId: ApiKey,
scope: "",
authorizeUrl: new Uri("https://www.dropbox.com/oauth2/authorize"),
redirectUrl: new Uri(RedirectUri),
isUsingNativeUI: false) ;

Hope this information helps somebody else trying to get Xamarin.Auth working with Dropbox.