Var keyword type inference ambiguity when both interface and implementation are present

Question

Take this example:

interface IEntity {
    string Name { get; set; }
}

class Product : IEntity {
    public string Name { get; set; }
    public int Count { get; set; } // added member
}

class Client {
    void Process() {
        var product = new Product();
        int count = product.Count; // this is valid            

    }
}

In the example above, what is the type of product? Is it IEntity or Product? It appears that product is of type concrete implementation (Product). If that is the case, shouldn't var be used only in special circumstances. But I see that tools like resharper recommend using var by default. Shouldn't one program to an interface?

Answer 1

You are not actually "programming to interfaces" if you are still instantiating the concrete class within the method, as the dependency to the concrete Product class still remains. In order to properly program-to-interfaces you must remove the new instantiation, for example by using a factory or IoC.

Answer 2

What if you had a Product like ...

class Product : IFirst, ISecond, IThrid

The only rational thing the complier can do is what it does. I don't limit my use of var, I use it everywhere. I think it makes code far more readable. In this case, I agree with ReSharper across the board.

Answer 3

If you want Product to be of type IEntity, try this:

var product = new Product() as IEntity;

That said, yes you should program to an interface, but in your case you're instantiating the concrete type directly. If you've already created a dependency to the concrete type, just use the concrete type. If not, use a factory or injection to get an instance of the interface. var will work with those rather well. For example:

public class MyExtremelySimpleFactoryExampleClass
{
  public IEntity Instantiate()
  {
    return new Product();
  }
}

// elsewhere in your code...
var item = myFactory.Instantiate(); // item is of type IEntity

Finally, no, I don't think var should be used only in "special circumstances". I find it quite useful and use it almost always.