Overriding / hiding an explicity implemented interface method

Question

I'm having trouble overriding a method that explicitly implements an interface.

I have two classes. A base one called OurViewModel, and an inherited one called MyViewModel. They share a method called Validate, and until recently I was able to hide the base version of the method, like so:

public class OurViewModel
{
  public bool Validate(ModelStateDictionary modelState){ return true; }
}


public class MyViewModel : OurViewModel
{
  public new bool Validate(ModelStateDictionary modelState) {return false;}
}

This all changed a couple of days ago. A new interface has appeared on the scene--

public interface IValidatableObject
{
  IEnumerable<ValidationResult> Validate(ValidationContext validationContext);
}

Subsequently, OurViewModel has also been changed. I did not request this, but it happened and I have to live with it. The class now looks like this:

public class OurViewModel : IValidatableObject
{
  IEnumerable<ValidationResult> IValidatableObject.Validate(ValidationContext validationContext) {..}
}

I'm having difficult time figuring out how to override or hide this rewritten Validate method in MyViewModel. If I try placing the new keyword the method signature (as I did originally) I get a compilation error. I also can't declare the Validate method as virtual in OurViewModel, as it's explicitly implementing an interface.

What to do? If I simply re-implement Validate in MyViewModel using the signature from IValidatableObject, will that hide the implementation in OurViewModel, or am I asking for trouble in some way because of inheritance rules?

again you made mistake in the question.. if the base member is not virtual you cant make it new in derived class. — Dhananjay, Dec 18 '12 at 16:30
@KonstantinVasilcov , so you meant if base member is non virtual you can still make it new ? — Dhananjay, Dec 19 '12 at 15:06
@Dhananjay definitely. Read [new Modifier (C# Reference)](http://msdn.microsoft.com/en-us/library/435f1dw2.aspx) — horgh, Dec 19 '12 at 23:40

horgh · Accepted Answer · 2012-12-18T05:56:40.287

I think you need to implement this interface implicitly in the derived class as well.

public class MyViewModel : OurViewModel, IValidatableObject
{
    IEnumerable<ValidationResult> IValidatableObject.Validate(ValidationContext validationContext)
    {
        //...
    }
}

And then

OurViewModel v = new OurViewModel();
MyViewModel m = new MyViewModel();

IValidatableObject ivo = v;
ivo.Validate(null);

ivo = m;
ivo.Validate(null);

Besides if an interface is implemented explicitly, you can only access the implementation through a reference to the interface. Remember, if you try to do

OurViewModel v = new OurViewModel();
v.Validate(null);

It will call the original Validate method of the class, not the interface implementation. I think the old methods should be removed to avoid possible mistakes.

score 0 · Answer 2 · answered Dec 18 '12 at 16:52

Do you really need to implement IValidatableObject explicitly in Base class ?

The solution would be - implement IValidatableObject implicitly in base class , mark it as virtual.

:-

Then implement IValidatableObject in derived class , implicitely, and mark it as virtual new. Dont forget to use interface name on derived class definition.

This is little different approach as compated to answer by @Konstantin Vasilcov. Using this approach you can have more derived classes of MyViewModel who can override the behavior.

Overriding / hiding an explicity implemented interface method

2 Answers2