Google API - Authentication setup and .NET client - api

Question

I'm trying to achieve two things from my C# client application using Google API.
List all users of my google directory
Get metadata of all emails for each user
It appears, however, that my service account configuration is giving me issues. This is what I have done.
Created project in console.cloud.google.com
Enabled Admin SDK and Gmail API from API's and Services
Created Service Account with Domain-Wide delegation and obtained credentials.json file.
4. On admin.google.com I went to Security / API Controls and manage Domain-wide delegation. From here I added new API Client from my Service Account client id and assigned the following scopes:
https://www.googleapis.com/auth/gmail.settings.basic
https://www.googleapis.com/auth/gmail.settings.sharing
https://www.googleapis.com/auth/admin.directory.user.readonly
Finally, when I want to try to retrieve users list I get error: Expected OAuth 2 access token, login cookie or other valid authentication credential
static void Main(string[] args)
{
using (var stream =
new FileStream("..\\..\\..\\credentials.json", FileMode.Open, FileAccess.Read))
{
var credential = GoogleCredential.FromFile("..\\..\\..\\credentials.json");
var dirservice = new DirectoryService(new BaseClientService.Initializer()
{
HttpClientInitializer = credential,
ApplicationName = "testbuildingnomiproductname",
});
var listReq = dirservice.Users.List();
Users allUsers = listReq.Execute();
int counter = 0;
foreach (User myUser in allUsers.UsersValue)
{
Console.WriteLine("*" + myUser.PrimaryEmail);
counter++;
}
Console.WriteLine(counter);
Console.ReadKey();
}
}