Tag Archives: AutoMapper

Using delegates for loose coupling and easy unit testing

In a typical ASP.NET MVC or WebApi project, controller actions often do nothing more than make a service call, map the returned domain object to a view model, and either render a view with the model or simply return it for the framework to serialize as JSON. Using AutoMapper, a simplified WebApi controller action may look something like

public EmployeeViewModel GetEmployee(string employeeName)
  var employee = _employeeService.GetByName(employeeName);
  var viewModel = Mapper.Map<EmployeeViewModel>(employee);
  return viewModel;

This is a pretty common but difficult to test pattern because of the static Mapper dependency. You can’t actually unit test the controller action without having a properly (or at least minimally) configured static Mapper. New versions of AutoMapper provide different ways to solve this problem, like the injectable IMapper interface, but I want to demonstrate another method to reduce coupling and ease unit testing.

In this entry, I’ll demonstrate how to get rid of the static Mapper dependency without using the IMapper interface, which exposes several overloads to map one data type to another, or creating your own interface that serves a similar purpose. Instead, we’ll inject a delegate whose signature is very expressive.

public delegate TDest MapperFunc<in TSrc, out TDest>(TSrc source);

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