Error Call Service asmx

I am creating class to be the structure that will always return all services (called asynchronous)

public class AnswerService { 
public  bool IsSuccess {get; set; }
public string Error {get; set; }
public int Code {get; set; }
public  string Exception {get; set; }
public  Object Result {get; set; }
}

It is assumed that the result should be the data that before I returned the function (List, string, bool, Object etc)

When I call the service and the result variable is a List for example gives this error

Error generating XML document. ---> The System.Collections.Generic.List` type can not be used in this context.

Try with an array and neither, I do not know if you can do this in this asmx schema because I know that internally there is a mapping of the service q does .net when generating the proxy in the client and object is generic.

How could I solve this or what alternative could I use to return both the data and the object of error?

NOTE: In the client when calling service it shut down in another method that uses async and await, reason why it can not use parameters by reference