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    <p>What you want is <em>call site covariance</em>, which is not a feature that C# supports. C# 4 and above support generic variance, but not call site variance.</p> <p>However, that doesn't help you here. You want a dog groomer to be put in a list of animal groomers, but that can't work in C#. A dog groomer cannot be used in any context in which an animal groomer is needed because <strong>a dog groomer can only groom dogs</strong> but an animal groomer can also groom cats. That is, you want the interface to be <em>covariant</em> when it cannot be safely used in a covariant manner. </p> <p>However your <code>IAnimalGroomer&lt;T&gt;</code> interface could be <em>contravariant</em> as it stands: an animal groomer can be used in a context in which a dog groomer is required, because an animal groomer can groom dogs. If you made <code>IAnimalGroomer&lt;T&gt;</code> contravariant by adding <code>in</code> to the declaration of <code>T</code> then you could put an <code>IAnimalGroomer&lt;IAnimal&gt;</code> into an <code>IList&lt;IAnimalGroomer&lt;Dog&gt;&gt;</code>.</p> <p>For a more realistic example, think of <code>IEnumerable&lt;T&gt;</code> vs <code>IComparer&lt;T&gt;</code>. A sequence of dogs may be used as a sequence of animals; <code>IEnumerable&lt;T&gt;</code> is <em>covariant</em>. But a sequence of animals may not be used as a sequence of dogs; there could be a tiger in there. </p> <p>By contrast, a comparer that compares animals may be used as a comparer of dogs; <code>IComparer&lt;T&gt;</code> is <em>contravariant</em>. But a comparer of dogs may not be used to compare animals; someone could try to compare two cats.</p> <p>If that is still not clear then start by reading the FAQ:</p> <p><a href="http://blogs.msdn.com/b/csharpfaq/archive/2010/02/16/covariance-and-contravariance-faq.aspx" rel="noreferrer">http://blogs.msdn.com/b/csharpfaq/archive/2010/02/16/covariance-and-contravariance-faq.aspx</a></p> <p>and then come back and ask more questions if you have them.</p>
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    1. COAre you sure that the OP is asking for call site covariance? From what I gather from his question, he just wants to be able to put a DogGroomer and a CatGroomer in the same List . That is certainly possible in C#, as you mentioned in your answer.
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      1. USBoluc Papuccuoglu
    2. CONo, because DogGroomer and CatGroomer can't be upcast to the same base generic type. You can't upcast IAnimalGroomer<Dog> to IAnimalGroomer<IAnimal>, unles IAnimalGroomer<T> is covariant which it is not. You could put them in a List<object>, which isn't very useful.
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    3. COAfter doing some research, this answer makes sense to me. I understand that c# is trying to prevent runtime errors (of accidentally passing a cat to a dog groomer). However, it comes at the expense of flexibility. I would really like to have a list of AnimalGroomers where I could figure out (at runttime) which one I want to use.
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