Google.GData.Client.GDataRequestException – Authentication suddenly fails in old code

Google has retired their older authentication API. OAuth 2.0 should be used instead.

I spent too much time to figure out how to use newer Auth API with older GData API grabbing bits and pieces of information here and there from the Internet.
I decided to share all the the details with screenshots to save your time.

1. Go to https://console.developers.google.com/project

2. Hit Create Project button

3. Create project. Type in some name.

4. Go to API & Auth > Credentials and hit Create new Client ID button. It will create JSON key for you automatically – ignore that.

5. Hit Generate new P12 key

6. File download will start automatically. Remember the password, you will need it to open the file you just downloaded.

7. Rename downloaded file to Key.p12 and add it to your solution. Make sure you set Build Action and Copy to Output Directory accordingly.

8. Install Google API Auth using Nuget. Type the following in the Package Manager Console

   Install-Package Google.Apis.Auth
   

9. Copy service account email address that was generated in Step #4.

10. Grant appropriate permission to this user in your Google Spreadsheet.

11. Use the following code to query the spreadsheet. Replace email and Google spreadsheet URL address in the code below.

    const string ServiceAccountEmail = "452351479-q41ce1720qd9l94s8847mhc0toao1fed@developer.gserviceaccount.com";
    
    var certificate = new X509Certificate2("Key.p12", "notasecret", X509KeyStorageFlags.Exportable);
    
    var serviceAccountCredentialInitializer = 
        new ServiceAccountCredential.Initializer(ServiceAccountEmail)
        {
            Scopes = new[] { "https://spreadsheets.google.com/feeds" }
        }.FromCertificate(certificate);
    
    var credential = new ServiceAccountCredential(serviceAccountCredentialInitializer);
    
    if (!credential.RequestAccessTokenAsync(System.Threading.CancellationToken.None).Result)
        throw new InvalidOperationException("Access token request failed.");
    
    var requestFactory = new GDataRequestFactory(null);
    requestFactory.CustomHeaders.Add("Authorization: Bearer " + credential.Token.AccessToken);
    
    var service = new SpreadsheetsService(null) { RequestFactory = requestFactory };
    
    var query = new ListQuery("https://spreadsheets.google.com/feeds/list/0ApZkobM61WIrdGRYshh345523VNsLWc/1/private/full");
    var feed = service.Query(query);
    
    var rows = feed.Entries
        .Cast<ListEntry>()
        .Select(arg =>
            new
            {
                Field0 = arg.Elements[0].Value,
                Field1 = arg.Elements[1].Value
            })
        .ToList();