Mnemonic for C# generic types

Question

I often forget if i have to use in or out when defining covarient and contravarient generic types. In java i have the mnemonic PECS (producer extends consumer super) to help me. Do you know a similar mnemonic for c#?

RELB - Read Eric Lippert's Blog http://blogs.msdn.com/b/ericlippert/archive/tags/covariance+and+contravariance/ — JaredPar, Aug 27 '10 at 16:15
I infrequently define genetics, except when attempting to create 12 foot lizardmen, everybody like's lizardmen. — Jimmy Hoffa, Aug 27 '10 at 16:21
@Jimmy Hoffa - yah but anything over 10 feet is just overkill. — Joel Etherton, Aug 27 '10 at 16:31
Isn't it fairly easy to know when to use each, since `in` means the interface is taking something in, and `out` means it is giving something out? I can never remember which is which of co- and contra-variance, but the usage of `in` and `out` is never a problem. — Lasse V. Karlsen, Aug 27 '10 at 17:38

score 3 · Answer 1 · edited Aug 27 '10 at 16:34

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Didn't they do this for us when they called them 'in' and 'out' rather than covariant and contravariant? Just think: am I pushing values 'in', or getting them 'out'? If unsure, try 'out' - it is far more common (and easier to understand).

edited Aug 27 '10 at 16:34

LBushkin

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answered Aug 27 '10 at 16:29

Marc Gravell

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Although this is true, it is not very catchy. – mR_fr0g Sep 01 '10 at 14:10

score 1 · Answer 2 · answered Aug 27 '10 at 17:28

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in types are passed in to functions; out types are returned out from functions.

answered Aug 27 '10 at 17:28

Gabe

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This is not what mR_fr0g has asked for. He wants a mnemonic for in and out when used as generic class variance type modifiers (which is a new funcionality of C# 4.0), not the common, old usage on method parameters. – rsenna Aug 27 '10 at 19:10
rsenna: That's exactly what rsenna asked for. When creating a generic interface, types that are only passed in to functions can get the `in` label and types that are only returned from functions can get the `out` label. Types that are both passed in and returned (or used as `ref` or `out` parameters) are invariant and can't get either label. – Gabe Aug 27 '10 at 19:36

score 0 · Answer 3 · answered Aug 27 '10 at 17:04

When I don't remember, I always refer to IEnumerable<out T> (which means of course I have to remember the signature of that interface...). You can only get instances of T "out" of an IEnumerable<out T>, so it is covariant. If you can only pass instances of T "in" to an interface (or delegate, which is more common), it's contravariant.

Mnemonic for C# generic types

3 Answers3