My goal is to define a Service class which process method accepts a RequestType argument that is defined by the contained ServiceType and returns an object of a type defined by ServiceType. By doing this I'm able to derive Services that can process a given Request that is compatible with the type of the Service , same for the Response.

I really don't know if it is possible and the following diagram is obviously wrong but depicts the result I'm struggling to achieve.

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1 Answer 1


Many programming languages support generic types (C#), generics (Java) or class templates (C++). All three terms mean the same thing, but I honestly like the C++ terminology best. The term "class template" is good, because it implies that the class is not the actual class, but a template used by the compiler to create real classes. The intention is to solve the exact problem you are running into. I'll use C#, since that is the language I am most fluent with, but many OOP languages support this:

public abstract class Service<TRequest, TResponse>
    public virtual TResponse process(TRequest)
        // common logic goes here

The identifiers inside the angle brackets TRequest and TResponse are just placeholders for real classes that will be specified when initializing a new service object:

Service<GetBlogRequest, GetBlogReponse> service = new Service<GetBlogRequest, GetBlogReponse>();
GetBlogRequestrequest request = new GetBlogRequest(1200);
GetBlogReponseresponse response = service.process(request);

UML diagrams usually depict class templates or generics using the angle bracket notation as well:

|               <<abstract>>               |
| Service<TRequest, TResponse>             |
| +process(request : TRequest) : TResponse |
  • thank you for your suggestion, the problem here is that Service accepts any derived type of TRequest and TResponse, I need to tied them under a common type so that Service can only accept a TRequest correlated with the specific TResponse May 6, 2021 at 19:20
  • TRequest is not a class. It is a placeholder for a real class that is specified later when other code creates a new instance of the service. Some languages allow you to restrict which types can be specified in generics. For instance in C# you would do class Service<TRequest, ...> where TRequest : RequestType. May 6, 2021 at 19:29
  • I wrongly expressed myself. My goal design is not to define a template because each different Service that accepts a different correlated couple of <TRequest, TResponse> must define a different process logic in advance May 6, 2021 at 19:46
  • Ok. So override process in the child class, or make process an abstract method. May 6, 2021 at 19:47
  • The language you're using feels like the Mediator pattern. @GregBurghardt is using a c# example - here is a good github repo that implements a more detailed version of what Greg was doing: github.com/jbogard/MediatR/wiki
    – Patrick
    May 6, 2021 at 19:50

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