Can't list users with Google Directory API Admin SDK

Question

I'm trying to use the AdminService to manage my domain's users and groups, but I'm stuck with a simple request to get all the users of my domain. There is the code in C#:

public Users GetAllUsers()
{
    var provider = new AssertionFlowClient(
        GoogleAuthenticationServer.Description,
        new X509Certificate2(privateKeyPath, keyPassword, X509KeyStorageFlags.Exportable))
    {
        ServiceAccountId = serviceAccountEmail,
        Scope = AdminService.Scopes.AdminDirectoryUser.GetStringValue()
    };

    var auth = new OAuth2Authenticator<AssertionFlowClient>(provider, AssertionFlowClient.GetState);

    m_serviceGroup = new AdminService(new BaseClientService.Initializer()
    {
        Authenticator = auth,
    });

    var request = m_serviceUser.Users.List();
    request.Domain = m_domainName;
    return request.Fetch();
}

I'm getting an exception when Fetch() that says:

Code: 403    
Message: Not Authorized to access this resource/api 
Error: {Message[Not Authorized to access this resource/api] Location[ - ] Reason[forbidden] Domain[global]}

I've followed the instructions here to have enabled API access, and also authorized my service account in domain control panel:

[Security]->[Advanced Setting]->[Authentication]->[Manage third party OAuth Client access]

with scopes:

https://www.googleapis.com/auth/admin.directory.group 
https://www.googleapis.com/auth/admin.directory.user

Admin SDK Service is also enabled in API control panel.

I tried the code to use the DriveService and successfully listed/created/deleted files without any problem, so the authentication part of the code should be alright. I couldn't figure out what else needs to be configured or if there is any other problems with my code.

Thanks for any help.

Answer 1

As described on the page:

Manage API client access

Developers can register their web applications and other API clients with Google to enable access to data in Google services like Calendar. You can authorize these registered clients to access your user data without your users having to individually give consent or their passwords. Learn more

The service account needs to act on behave of a user, so when initializing the client the ServiceAccountUser needs to be assigned.

    var provider = new AssertionFlowClient(
        GoogleAuthenticationServer.Description,
        new X509Certificate2(privateKeyPath, keyPassword, X509KeyStorageFlags.Exportable))
        {
            ServiceAccountId = serviceAccountEmail,
            Scope = AdminService.Scopes.AdminDirectoryUser.GetStringValue(),
            ServiceAccountUser = domainManangerEmail
        };

Edit: AssertionFlowClient is deprecated, the following should work:

var cert = new X509Certificate2(privateKeyPath, keyPassword, X509KeyStorageFlags.Exportable);
var serverCredential = new ServiceAccountCredential(
    new ServiceAccountCredential.Initializer(serviceAccountEmail)
        {
            Scopes = new []{DirectoryService.Scope.AdminDirectoryUser},
            User = domainManagerAccountEmail
        }.FromCertificate(cert));
var dirService = new DirectoryService(new BaseClientService.Initializer()
        {
            HttpClientInitializer = serverCredential
        });