Accessing older GData APIs (Spreadsheet API) using OAuth 2 and a service account

Question

The short question is whether is this possible and if so, how?

Outline

I have a .NET application which currently uses a service account to access information across a Google Apps domain using the Google Drive API. This works fine using the google-api-dotnet-client library and code along the same lines as shown in the samples here - which are currently a very good basic example of what I'm doing.

What I want to do now is extend it so as well as using those APIs provided by the "new" google-api-dotnet-client library, it uses the older "GData" libraries, as provided for via the older google-gdata library, specifically the Spreadsheets API (and perhaps more to come).

The Problem

This is where the difficulty arises. The former library does exactly what I want, as evidenced by the second link in the first paragraph above - and the fact I have it doing it myself. HOWEVER... although the second library has been updated to support OAuth 2.0 in addition to OAuth 1.0 and the other older auth techniques, it does not - as far as I can tell from extensive Googling and trail-and-error - allow the "service account on behalf of all my users" operation which I need.

My question is whether I'm missing something (possibly a hard to find or undocumented something) which would allow me to do what I want. Failing that, is there any way I could force this behaviour and make these two libraries operate side by side?

The ideal solution

Ideally I would love some way of having the Google.GData.Spreadsheets.SpreadsheetsService instance be able to take advantage of the Google.Apis.Authentication.Auth2Authenticator<AssertionFlowClient> instance I'm already using... somehow. Is such witchcraft possible? I'm I missing the obvious?

Failing that, I'm happy to do the whole OAuth2 "assertion flow client" dance again if I have to, in some way that the older library can handle.

Help?

Other Thoughts

I have considered - and rejected for the time being - the option of starting from scratch and writing my own library to make this happen. This is for two reasons:

1. The gdata library already exists, and has been developed by many people likely cleverer than myself. I'm not so arrogant that I believe I can do better.
2. I'm not certain the OAuth2 with service account approach is even supported/allowed on these older APIs.

An alternate approach which I've been hoping to avoid but may have to fall back to depending on the answers here will be to use 2-legged OAuth 1.0 for portions of this. I'd prefer not to, as having parts of the app rely on one old auth method whilst other parts do it the nice new way just feels wrong to me. And there's that much more to go wrong...

---

Updates

I have considered the possibility of subclassing GDataRequestFactory and GDataRequest so I can make my own request factory and have that take the instance of Google.Apis.Authentication.Auth2Authenticator<AssertionFlowClient> (well, an instance of Google.Apis.Authentication.IAuthenticator anyway) which could step in to authenticate the request just before it's called. However... the constructor for GDataRequest is internal, which has stopped me.

It's really looking like this isn't meant to be.

Answer 1

For the sake of other folks coming across this question (now that the solution linked to in the accepted answer uses deprecated code), here's how I solved it:

First, start in "new API" land (use the Google.Apis.Auth nuget package) by setting up a ServiceAccountCredential following Google's Service Account example:

//In the old api, this accessed the main api accounts' sheets, not anymore
//** Important ** share spreadsheets with the Service Account by inviting the "serviceAccountEmail" address to the sheet
string serviceAccountEmail = "[email protected]";

var certificate = new X509Certificate2(@"key.p12", "notasecret", X509KeyStorageFlags.Exportable);

ServiceAccountCredential credential = new ServiceAccountCredential(
   new ServiceAccountCredential.Initializer(serviceAccountEmail)
   {
       Scopes = new[] { "https://spreadsheets.google.com/feeds", "https://docs.google.com/feeds" }
   }.FromCertificate(certificate));

Tell the credential to request an Access Token:

credential.RequestAccessTokenAsync(System.Threading.CancellationToken.None).Wait();

Now it's time to switch back to "old API" land (use the Google.GData.Spreadsheets nuget package). Start by constructing the SpreadsheetsService (similar to Google's example):

SpreadsheetsService service = new SpreadsheetsService("MySpreadsheetIntegration-v1");

To use Service Account authentication, we'll create an instance of the GDataRequestFactory and set a custom Authorization header:

var requestFactory = new GDataRequestFactory("My App User Agent");
requestFactory.CustomHeaders.Add(string.Format("Authorization: Bearer {0}", credential.Token.AccessToken));

Finally, set the SpreadsheetsService's RequestFactory property to this new factory:

service.RequestFactory = requestFactory;

And go ahead and use the SpreadsheetsService as you would had you authenticated using any other technique. (Tip: share spreadsheets with the Service Account by inviting the serviceAccountEmail address to the sheet)