CSharp-变体

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// Variant type parameters could be declared in interfaces or delegates only!

interface ICovariant<out T>
{
}

class Covariant<T> : ICovariant<T>
{
}

interface IContravariant<in T>
{
}

class Contravariant<T> : IContravariant<T>
{
}

interface IInvariant<T>
{
}

class Invariant<T> : IInvariant<T>
{
}

class Program
{
    private static void Covariant( /* out */)
    {
        ICovariant<object> obj = new Covariant<object>();
        ICovariant<string> str = new Covariant<string>();

        // You can assign "Derived" to "Base"
        obj = str;
        str = obj; // error
    }

    private static void Contravariant( /* in */)
    {
        IContravariant<object> obj = new Contravariant<object>();
        IContravariant<string> str = new Contravariant<string>();

        // You can assign "Base" to "Derived"
        str = obj;
        obj = str; // error
    }

    private static void Invariant( /* none */)
    {
        IInvariant<object> obj = new Invariant<object>();
        IInvariant<string> str = new Invariant<string>();

        // You can't do any assign 
        obj = str; // error
        str = obj; // error
    }
}