Creating a collection of generic generics - Dictionary covariance

Question

I'm not sure how to better word the question, but I've run into the following problem with trying to create a Dictionary of generic interfaces more than once. Often this has come about when trying to create registry type collections which handle different types:

namespace GenericCollectionTest
{
    [TestFixture]
    public class GenericCollectionTest
    {
        interface IValidator<T>
        {
            bool Validate(T item);
        }

        class TestObject
        {
            public int TestValue { get; set; }
        }

        private Dictionary<Type, IValidator<object>> Validators = new Dictionary<Type, IValidator<object>>();

        class BobsValidator : IValidator<TestObject>
        {
            public bool Validate(TestObject item)
            {
                if (item.TestValue != 1)
                {
                    return false;
                }
            }
        }

        [Test]
        public void Test_That_Validator_Is_Working()
        {
            var Test = new TestObject {TestValue = 1};
            Validators.Add(typeof(BobsValidator), new BobsValidator());

            Assert.That(Validators[typeof(TestObject)].Validate(Test));
        }
    }
}

However, compilation fails because BobsValidator is not assignable to parameter type IValidator. Basically, I don't need type safety outside of the validator, but once I'm inside, I don't the consumer of the interface to have to cast it to the type they want to use.

In Java, I could:

Dictionary<Type, IValidator<?>>

I know I can do something like this (ala IEnumerable):

interface IValidator
{
    bool Validate(object item);
}

interface IValidator<T> : IValidator
{
    bool Validate(T item);
}

abstract class ValidatorBase<T> : IValidator<T>
{
    protected bool Validate(object item)
    {
        return Validate((T)item);
    }

    protected abstract bool Validate(T item);
}

Then make the dictionary take IValidator instead and extend ValidatorBase, but it seems like there must be a better way that I'm not seeing. Or, is this just poor design overall? It seems like I need some kind of structure like this:

WhatTheHecktionary<T, IValidator<T>>

Thanks!

Answer 1

In order to assign BobsValidator to IValidator, your you need to make your interface generic parameter into a covariant, this would allow your IValidator to point to a more specific type like IValidator.

interface IValidator<out T>
{
   bool Validate(T item);
}

However, you will realize you can't compile because your interface is no longer type safe, so the compiler won't allow. So why it is no longer type safe? Imagine this:

using NUnit.Framework;
namespace GenericCollectionTest
{
    [TestFixture]
    public class GenericCollectionTest
    {
        //.NET Compiling Error:
        //"Invalid variance: The type parameter 'T' must be contravariantly valid ..."
        interface IValidator<out T>
        {
            //Error: "Parameter must be type-safe. Invalid variance..."
            bool Validate(T item);
        }

        class MyObject
        {
            public int TestValue { get; set; }
        }

        class YourObject
        {
            public int CheckValue { get; set; }
        }

        class MyValidator : IValidator<MyObject>
        {
            public bool Validate(MyObject item)
            {
                return (item).TestValue == 1;
            }
        }

        class YoursValdator : IValidator<YourObject>
        {
            public bool Validate(YourObject item)
            {
                return (item).CheckValue == 1;
            }
        }

        [Test]
        public void Test_That_Validator_Is_Working()
        {
            //.NET compiler tries to prevent the following scenario:

            IValidator<object> someObjectValidator = new MyValidator();
            someObjectValidator.Validate(new YourObject()); // Can't use MyValidator to validate Yourobject

            someObjectValidator = new YoursValdator();
            someObjectValidator.Validate(new MyObject()); // Can't use YoursValidator to validate MyObject

        }
    }
}

To work around this, I suggest you try using non-generic as a base class so that you can store your validator in a dictionary. See if the following works for your case:

using System;
using System.Collections.Generic;
using NUnit.Framework;
namespace GenericCollectionTest
{
    [TestFixture]
    public class GenericCollectionTest
    {

        interface IValiadtor
        {
            bool Validate(object item);
        }

        abstract class ValidatorBase<T> : IValidator<T>
        {
            public bool Validate(object item)
            {
                return Validate((T)item);
            }

            public abstract bool Validate(T item);
        }

        interface IValidator<T> : IValiadtor
        {
            //Error: "Parameter must be type-safe. Invalid variance..."
            bool Validate(T item);
        }

        class MyObject
        {
            public int TestValue { get; set; }
        }

        class YourObject
        {
            public int CheckValue { get; set; }
        }

        class MyValidator : ValidatorBase<MyObject>
        {
            public override bool Validate(MyObject item)
            {
                return (item).TestValue == 1;
            }
        }

        class YoursValdator : ValidatorBase<YourObject>
        {
            public override bool Validate(YourObject item)
            {
                return (item).CheckValue == 1;
            }
        }

        [Test]
        public void Test_That_Validator_Is_Working()
        {
            Dictionary<Type, IValiadtor> Validators = new Dictionary<Type, IValiadtor>();
            Validators.Add(typeof(MyObject), new MyValidator() );
            Validators.Add(typeof(YourObject), new YoursValdator());

            var someObject = new MyObject();
            someObject.TestValue = 1;
            Assert.That(Validators[someObject.GetType()].Validate(someObject));


        }
    }
}