I Cant Return List Type from Wcf - wcf

Question

[DataContract]
public class UserDetails
{
string _userid;
string tckimlik;
string ad;
string tarih;
string aciklama;
[DataMember]
public string userid
{
get { return _userid; }
set { _userid = value; }
}
[DataMember]
public string Tckimlik
{
get { return tckimlik; }
set { tckimlik = value; }
}
[DataMember]
public string Ad
{
get { return ad; }
set { ad = value; }
}
[DataMember]
public string Tarih
{
get { return tarih; }
set { tarih = value; }
}
[DataMember]
public string Aciklama
{
get { return aciklama; }
set { aciklama = value; }
}
}
public interface IService1
{
[OperationContract]
List<UserDetails> GetAllPersons();
}
My Class DataContract and OperationContract given above.
My Method GetAllPersons type of List beloew.
public List<UserDetails> GetAllPersons()
{
List<UserDetails> userL = new List<UserDetails>();
try
{
NpgsqlConnection con = new NpgsqlConnection("");
con.Open();
NpgsqlCommand cmd = new NpgsqlCommand("SELECT * FROM RegistrationTable",con);
NpgsqlDataAdapter sda = new NpgsqlDataAdapter(cmd);
NpgsqlDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
UserDetails us = new UserDetails()
{
userid = reader[0].ToString(),
Tckimlik = reader[1].ToString(),
Ad = reader[2].ToString(),
Tarih = reader[3].ToString(),
Aciklama = reader[4].ToString()
};
userL.Add(us);
}
con.Close();
return userL;
}
catch (Exception)
{
throw;
}
}
The Code above is in WCF Service. I want to call GetAllPersons method in Xamarin. But i have trouble with it. My Xamarin code is below.
void getVisitor()
{
List<ServiceReference1.UserDetails> getir = new List<ServiceReference1.UserDetails>();
getir = service.GetAllPersonsAsync();
}
When i wanted to call method in service, i get an error given below.
Severity Code Description Project File Line Suppression State
Error CS0029 Cannot implicitly convert type 'System.Threading.Tasks.Task<ServiceReference1.GetAllPersonsResponse>' to 'System.Collections.Generic.List<ServiceReference1.UserDetails>' XamarinCRUD C:\Users\Eyyub\source\repos\XamarinCRUD\XamarinCRUD\XamarinCRUD\MainPage.xaml.cs 59 Active
Who can hint me, appreciate it!
Best for all!
I also checked logs and no show. Some says that WCF doesnt (de)serialize List return type. Is it true? if yes, what is the solution?. When i used same service through FormApp, works fine. But Xamarin doesnt..