Google Contacts API failing to retrieve contact information via a service account

Question

I have been unsuccessful using the Google Contacts API to retrieve contact information from a service account (not using a live user session). No matter what I try, I either get an exception (such as: Message = "Error:\"invalid_request\", Description:\"Invalid impersonation prn email address.\", Uri:\"\""), or no error but 0 contacts returned.

I have double-checked the settings in the Google Apps Admin console and Manage API Client Access pages (including getting verification of those settings from Google App tech support), but regardless, I cannot get contacts to load. I also have tried all of the examples that I could find on "Stack Overflow," but none of those seem to correct the issue.

These attempts have been in C# / ASP.net, but I am conversant enough in other languages that I should be able to adapt any example that somebody might have that works.

I'm hoping somebody has been successful at using Google Contacts API from a service account and would be willing to share the code that they have done to accomplish that.

Thank you!!!

Here is an example of an attempt I made that always results in the exception "invalid_request\": "Invalid impersonation prn email address.\"

    private void TestLoadContacts()
    {
        try
        {
            string strClientID = "STRINGGENERATEDFROMGOOGLEAPPSINTERFACE.apps.googleusercontent.com";
            var credential = GenerateCred(new[] { "https://www.google.com/m8/feeds" }, strClientID);

            // Get the token for this scope and user
            if (credential.RequestAccessTokenAsync(new CancellationToken()).Result)
            {
                // use the token to initalize the request
                var rs = new RequestSettings("Google Sync")
                {
                    OAuth2Parameters = new OAuth2Parameters()
                    {
                        AccessToken = credential.Token.AccessToken
                    }
                };

                var request = new ContactsRequest(rs);
                Feed<Contact> f = request.GetContacts();
                foreach (var c in f.Entries)
                {
                    //process each contact
                }
            }
        }
        catch (Exception ex)
        {
            string strError = ex.Message;
        }
    }

    private static ServiceAccountCredential GenerateCred(IEnumerable<string> scopes, string delegationUser)
    {
        string strServiceAccountEmail = "[email protected]";
        X509Certificate2 certificate = new X509Certificate2(@"C:\MyP12s\MyGoogleAccount-9da1a08f4eef.p12",
            "notasecret", X509KeyStorageFlags.Exportable);

        var credential = new ServiceAccountCredential(
            new ServiceAccountCredential.Initializer(strServiceAccountEmail)
            {
                Scopes = scopes,
                User = delegationUser
            }.FromCertificate(certificate));
        return credential;
    }

Answer 1

Retrieving contacts should be easily done (as cited at the Google Contacts API documentation)

Is the question more for using Service Accounts itself? If so, you'll need to enable domain-wide delegation to the service account (more information can be found here; which is also linked on the reference above). Basically you'll need to add the specific scopes you'll use to the authorized service account.

Hope this helps!