c# - How to implement a non generic interface in a generic class with constraints

Question

Here is my code:

public interface ICar
{
    CarModel Property { get; set; }
    CarModel Method();
}

public Car<T> : ICar where T : CarModel 
{
    T Property { get; set; }
    T Method()
    {
        //Do Stuff
    }
}

I have implemented the interface in my Car<T> class with generic constraints but it doesn't compile with the following error:

Compilation error (line 18, col 15): 'Car<T>' does not implement interface member 'ICar.Property'. 'Car<T>.Property' cannot implement 'ICar.Property' because it does not have the matching return type of 'CarModel'.
Compilation error (line 18, col 15): 'Car<T>' does not implement interface member 'ICar.Method()'. 'Car<T>.Method()' cannot implement 'ICar.Method()' because it does not have the matching return type of 'CarModel'.

I also need the interface to be non generic, here is a .Net fiddle: https://dotnetfiddle.net/m1jDnB

The only work around I have for this is to wrap the property or method with something that implements the interface as it wants it BUT I dont want to do this. ie:

public Car<T> : ICar where T : CarModel 
{
    T Property { get; set; }
    T Method()
    {
        //Do Stuff
    }

    CarModel ICar.Property 
    { 
      get {return Property; }
      set {Property = (CarModel)value; }
    }

    CarModel ICar.Method()
    {
        return (CarModel)Method();
    }
}

Is there a better way?

What was the build error, looks like you implentation method does not return a value. — markmnl
– markmnl, Commented Sep 12, 2016 at 2:52
Is Car<MyCarModel> allowed in your design? (MyCarModel inherits from CarModel) — Cheng Chen
– Cheng Chen, Commented Sep 12, 2016 at 6:04
@markmnl, sorry my fault in the write up made changes as above — Sam Henry
– Sam Henry, Commented Sep 13, 2016 at 23:47
@DannyChen Check out the .Net fiddle it shows what you are asking and it still errors — Sam Henry
– Sam Henry, Commented Sep 14, 2016 at 0:04
The thing you don't want to do is the best way to accomplish what you say you want to accomplish. (except that the setter should have Property = (T)value; instead of Property = (CarModel)value. However, that does increase the chances of a run-time error such as the one shown in @recursive's answer — Grax32
– Grax32, Commented Sep 14, 2016 at 20:56

recursive · Accepted Answer · 2016-09-14 00:19:51Z

1

This can't be done. If the compiler did allow you to do it, the result wouldn't be type safe. If the types were allowed to compile the way you wanted, then this code would be permitted by the compiler.

public class CarModel {
    public string Name { get; set; }
}

public class Toyota : CarModel {
    public string SpecialToyotaNumber { get; set; }
}

public class Honda : CarModel { }

public interface ICar {
    CarModel Property { get; set; }
    CarModel Method();
}

public class Car<T> : ICar where T : CarModel {
    public T Property { get; set; }

    public T Method() {
        return (T)new CarModel();
    }
}

public class Main {
    public void Run() {
        ICar car = new Car<Toyota>();
        car.Property = new Honda(); // the concrete property is declared Toyota
    }
}

answered Sep 14, 2016 at 0:19

recursive

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