Open Closed Principle with Inheritance and Dependency Injection

Question

User439975351 posted

Sorry if this is a stupid question but I can't seem to find a definitive answer via the usual Googling/Stack Overflowing ;-)

If I have an interface as follows:

public interface IMyInterface
{
    string Result(int number, Dictionary<int, string> interfaceParameters);
}

Then I inherit like so:

 public class TestClass : IMyInterface
 {
     public string Result(int number, Dictionary<int, string> interfaceParameters)
     {
         var sb = new StringBuilder();

         foreach (var m in interfaceParameters)
         {
             sb.Append(m.Value);
         }

         var result = sb.ToString();

         return string.IsNullOrWhiteSpace(result) ? $"{number}" : result;
     }
}

Then I add this for DI (Asp Core): services.AddScoped<IMyInterface, TestClass>();

Use it in the controller:

  [ApiController]
  [Route("[controller]")]
  public class TestController : ControllerBase
  {
     private readonly IMyInterface _testClass;

     public TestController(IMyInterface testClass)
     {
         _testClass = testClass;
     }

     [HttpGet]
     public IEnumerable<string> Get()
     {
         var interfaceParameters = new Dictionary<int, string>
         {
             { 1, "One" },
             { 2, "Two" }
         };

         var model = new List<string>();

         for (var i = 1; i <= 10; i++)
         {
             model.Add(_testClass.Result(i, interfaceParameters));
         }

         return model;
     }
 }

How would I extend the TestClass/IMyInterface using Open Close principles?

Answer 1

User475983607 posted

Create a new class that implements the interface but has a different implementation.  IMHO, it seems your design is very specific and not open to a different implementation

 public class NewTestClass : IMyInterface
 {
     public string Result(int number, Dictionary<int, string> interfaceParameters)
     {
         //New implementation code
     }
}

http://joelabrahamsson.com/a-simple-example-of-the-openclosed-principle/

Answer 2

User439975351 posted
Thanks mgebhard,

How could I use this with DI if I already have it added here?:

services.AddScoped<IMyInterface, TestClass>();

Answer 3

User475983607 posted

Thanks mgebhard,

How could I use this with DI if I already have it added here?:

services.AddScoped<IMyInterface, TestClass>();

Use a factory pattern if you have multiple implementations of the same interface.  

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection?view=aspnetcore-3.1

https://espressocoder.com/2018/10/08/injecting-a-factory-service-in-asp-net-core/

Answer 4

User439975351 posted
Thanks mgebhard, I will read through these.

Kind regards,

Justin