MongoDB.Driver.MongoAuthenticationException: Unable to authenticate using sasl protocol mechanism SCRAM-SHA-1

The issue is you are authenticating the mongoadmin user for clientdb. But mongoadmin user is authenticated to admin db. Get the mongoadmin user authenticated and then you can access clientdb.

IMongoClient _client;
        public readonly IMongoDatabase _database;

        public MongoContext_URIBased()
            var mongoUrl = new MongoUrl("mongodb://mongoadmin:[email protected]:2020/admin");
            _client = new MongoClient(mongoUrl);
            _database = _client.GetDatabase("clientdb");