IDictionary contravariance?

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Firstly, covariance and contravariance in C# only apply to interfaces and delegates.

So your question is really about IDictionary<TKey,TValue>.

With that out of the way, it’s simplest to just remember that an interface can only be co/contra-variant if all values of a type parameter are either only passed in, or only passed out.

For example (contravariance):

interface IReceiver<in T> // note 'in' modifier
{
    void Add(T item);
    void Remove(T item);
}

And (covariance):

interface IGiver<out T> // note 'out' modifier
{
    T Get(int index);
    T RemoveAt(int index);
}

In the case of IDictionary<TKey,TValue>, both type parameters are used in both an in and out capacity, meaning that the interface cannot be covariant or contravariant. It is invariant.

However, the class Dictionary<TKey,TValue> does implement IEnumerable<T> which is covariant.

A great reference for this is:

https://learn.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance

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