Covariance and Contravariance [duplicate]

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Covariance and contravariance real world example

still confused about covariance and contravariance & in/out

Can you give me a description or example of both covariance and cont开发者_如何学Goravaiance, and why they now decided to add it to .NET 4?

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If B inherits from A, ie is a subtype of A, then C function is covariant if C(B) is a subtype of C(A), contravariant if C(A) is a subtype of C(B).

The question is wether or not you want consumers to deal with supertypes or subtypes.

A common example is a class library dealing in fruit classes.

It is OK for me to give you an apple where you expect a fruit. It is not OK for me to give you just any fruit where you expect an apple.

If you were making a fruit library. You should be expecting instances fruit from me and I should be expecting instances of fruit from you. Neither of us should have expectations about a subtype that could be false.

Therefore, the answer to the question above depends on wether you're getting or sending. Wether the parameter is read or write. A fruit list should be accepting bananas or apples, but only send "fruits"

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Since there is no answer given I just summarize the comments

stackoverflow.com/questions/245607/ – MainMa

stackoverflow.com/questions/2662369/ stackoverflow.com/questions/1163465/ stackoverflow.com/questions/3445631/ – In Sane 56 mins ago