C#•14mo ago

❔ Covariance and Contravariance C#

So I stumbled on this topic and I dont seem to grasp it properly, though I saw the following example somewhere:

class Base
{
  void func() { ... }
}
class Derived : Base
{
  void fuc() { ... }
}

Base a = new Base(); // obv fine
Base b = new Derived(); // obv fine

class Base
{
  void func() { ... }
}
class Derived : Base
{
  void fuc() { ... }
}

Base a = new Base(); // obv fine
Base b = new Derived(); // obv fine

Both now can call func(), but b cannot call fuc() too as its treated as a Base (reference) type. Now in C++, you'd just cast b to Derived and it'd work and be perfectly valid code. The sources I looked at, mentioned that this is why covariance/contravariance exists?

75 Replies

Thinker•14mo ago

Co/contravariance (in C#) is specifically in relation to interfaces and delegates

jorenOP•14mo ago

to solve the problem above a cast would be perfectly fine?

Thinker•14mo ago

yeah ~~although Derived above doesn't actually derive from Base~~

jorenOP•14mo ago

Okay, could you write me an example where co/contra variance comes into play

Thinker•14mo ago

wait no nevermind, you can call Func on both Base and Derived, but you can only call Fuc on Derived

jorenOP•14mo ago

and solves a problem that you'd have with interfaces/delegates yes func you can, fuc neither in this case as b is treated as a base type, not derived despite it being a Derived type under the hood until its casted it wouldnt work at least that how it is in C++

Thinker•14mo ago

C# and C++ are very different

jorenOP•14mo ago

Base a = new Base(); // obv fine
Base b = new Derived(); // obv fine

a.func(); // fine
b.func(); // fine
a.fuc(); // will never work, type is Base
b.fuc(); // could work, but not while b is treated as Base type
((Derived)b).fuc(); // works, might be wrong syntax

Base a = new Base(); // obv fine
Base b = new Derived(); // obv fine

a.func(); // fine
b.func(); // fine
a.fuc(); // will never work, type is Base
b.fuc(); // could work, but not while b is treated as Base type
((Derived)b).fuc(); // works, might be wrong syntax

I am well aware that they are different, otherwise I wouldnt be asking trivial questions ;)

Thinker•14mo ago

~~well b is of type Derived, so calling Fuc on it will always work~~ wait no I can't read yes that code is correct

jorenOP•14mo ago

nah Severity Code Description Project File Line Suppression State Error CS1061 'Base' does not contain a definition for 'fuc' and no accessible extension method 'fuc' accepting a first argument of type 'Base' could be found (are you missing a using directive or an assembly reference?) 1.1.2.8 - Extension Methods D:\Documents\coding\learncsharp\1.1.2.8 - Extension Methods\Program.cs 51 Active ure incorrect

Thinker•14mo ago

yeah it's a compile error

jorenOP•14mo ago

b is treated as a Base type, despite it being a Derived type Obviously, as I said: b.fuc(); // could work, but not while b is treated as Base type wont work, its illegal and it doesnt make sense

Thinker•14mo ago

it's not "despite it being a derived type", the variable is Base yeah

jorenOP•14mo ago

its not a variable, its an object and its underlying type is Derived but its treated as a base type

Thinker•14mo ago

b is a variable of type Base

jorenOP•14mo ago

with all due respect I dont think you are able to help me

Thinker•14mo ago

Therefore you can only call methods available on Base from b, unless you cast it

jorenOP•14mo ago

I said that at the start

Thinker•14mo ago

So what is your actual question?

jorenOP•14mo ago

I am already aware of this, I am wondering where covariance/contravariance gets into play what problem it solved, by showcasing it

Thinker•14mo ago

Okay so you know about generics right

jorenOP•14mo ago

yes

Thinker•14mo ago

Variance allows this to work

IEnumerable<string> strings = new List<string>();
IEnumerable<object> objects = strings;

IEnumerable<string> strings = new List<string>();
IEnumerable<object> objects = strings;

jorenOP•14mo ago

yes because every type originates from object, anything fits in that type.

Thinker•14mo ago

well yeah but that's not the entire story You could have an interface like this

interface IFoo<T>
{
    void DoStuff(T value);
}

interface IFoo<T>
{
    void DoStuff(T value);
}

and you could use it like

interface Foo<T> : IFoo<T>
{
    public void DoStuff(T value) => Console.WriteLine(value);
}

class A {}
class B : A {}

IFoo<A> a = new Foo<A>();

interface Foo<T> : IFoo<T>
{
    public void DoStuff(T value) => Console.WriteLine(value);
}

class A {}
class B : A {}

IFoo<A> a = new Foo<A>();

but this wouldn't work:

IFoo<B> b = a;

IFoo<B> b = a;

What variance does is allow you to do is to treat IFoo<A> in this case as an IFoo<B>, because IFoo<T> only takes a T as a parameter, therefore it could accept any type which derives from T. Marking IFoo<T> as contravariant would allow IFoo<A> to be assigned to IFoo<B>

interface IFoo<in T>
{
    // ...
}

IFoo<A> a = new Foo<T>();
IFoo<B> b = a;

interface IFoo<in T>
{
    // ...
}

IFoo<A> a = new Foo<T>();
IFoo<B> b = a;

Covariance is the same, except instead of for parameter types it's if the interface only returns T aaaand I have it all backwards, what is described above is contravariance

jorenOP•14mo ago

interface IFoo<T>
{
    void DoStuff(T value);
}
interface Foo<T> : IFoo<T>
{
    public void DoStuff(T value) => Console.WriteLine(value);
}

class A {}
class B : A {}

IFoo<B> b = new Foo<B>();
IFoo<A> a = b;

interface IFoo<T>
{
    void DoStuff(T value);
}
interface Foo<T> : IFoo<T>
{
    public void DoStuff(T value) => Console.WriteLine(value);
}

class A {}
class B : A {}

IFoo<B> b = new Foo<B>();
IFoo<A> a = b;

would this work? as b derives from a, I assume it wouldnt make a difference? as T does not match regardless

Thinker•14mo ago

no, because IFoo<T> isn't contravariant, as mentioned

jorenOP•14mo ago

Yeah, so unless its contravariant T has to match at least implicitly so a = b or b = a wouldnt make a difference

MODiX•14mo ago

Thinker

REPL Result: Failure

interface IFoo<T>
{
    void DoStuff(T value);
}

class Foo<T> : IFoo<T>
{
    public void DoStuff(T value) => Console.WriteLine(value);
}

class A {}
class B : A {}

IFoo<B> b = new Foo<B>();
IFoo<A> a = b;

interface IFoo<T>
{
    void DoStuff(T value);
}

class Foo<T> : IFoo<T>
{
    public void DoStuff(T value) => Console.WriteLine(value);
}

class A {}
class B : A {}

IFoo<B> b = new Foo<B>();
IFoo<A> a = b;

Exception: CompilationErrorException

- Cannot implicitly convert type 'IFoo<B>' to 'IFoo<A>'. An explicit conversion exists (are you missing a cast?)

- Cannot implicitly convert type 'IFoo<B>' to 'IFoo<A>'. An explicit conversion exists (are you missing a cast?)

Compile: 678.252ms | Execution: 0.000ms | React with ❌ to remove this embed.

MODiX•14mo ago

Thinker

REPL Result: Success

interface IFoo<in T>
{
    void DoStuff(T value);
}

class Foo<T> : IFoo<T>
{
    public void DoStuff(T value) => Console.WriteLine(value);
}

class A {}
class B : A {}

IFoo<A> a = new Foo<A>();
IFoo<B> b = a;

interface IFoo<in T>
{
    void DoStuff(T value);
}

class Foo<T> : IFoo<T>
{
    public void DoStuff(T value) => Console.WriteLine(value);
}

class A {}
class B : A {}

IFoo<A> a = new Foo<A>();
IFoo<B> b = a;

Compile: 665.086ms | Execution: 91.048ms | React with ❌ to remove this embed.

jorenOP•14mo ago

okay, so essentially, if I were to have an interface that takes T for instance, and I want two instances of that interface with different types for T to be able to hold eachother it has to be contravariant

Thinker•14mo ago

yes but it only down down into the inheritance chain

jorenOP•14mo ago

Yeah, because inheritance allows it the other way around?

Thinker•14mo ago

i.e. (as I tried to eval before but it failed) you wouldn't be able to assign IFoo<B> to IFoo<A> even if IFoo<T> is contravariant An example of a covariant interface (which is the other way around) is IEnumerable<T>

MODiX•14mo ago

Thinker

REPL Result: Success

class A {}
class B : A {}

IEnumerable<B> b = new List<B>();
IEnumerable<A> a = b;

class A {}
class B : A {}

IEnumerable<B> b = new List<B>();
IEnumerable<A> a = b;

Compile: 492.140ms | Execution: 32.117ms | React with ❌ to remove this embed.

jorenOP•14mo ago

ah ye makes sense

Thinker•14mo ago

And if you want an example of something which is contravariant in practice then you have Action<T>

jorenOP•14mo ago

yeah was about to ask for a use case because I honestly havent found a reason to use it I would avoid it naturally I assume as this is handled differently in C++ hell, I would never try contravariance in practice as it'd be UB or just dumb to begin with

MODiX•14mo ago

Thinker

REPL Result: Success

class A {}
class B : A {}

Action<A> f = x => Console.WriteLine(x);
f(new B());

class A {}
class B : A {}

Action<A> f = x => Console.WriteLine(x);
f(new B());

Console Output

Submission#0+B

Submission#0+B

Compile: 652.231ms | Execution: 120.256ms | React with ❌ to remove this embed.

jorenOP•14mo ago

Action is like a delegate?

Thinker•14mo ago

yeah

delegate void Action<in T>(T value);

delegate void Action<in T>(T value);

In my experience, the only time you really think about variance is when it causes errors Otherwise it's just kinda convenient sometimes, especially when working with IEnumerable<T> and related interfaces

jorenOP•14mo ago

x => Console.WriteLine(x) looks like a lambda correct? takes x and executes the console writeline?

Thinker•14mo ago

yep

jorenOP•14mo ago

or well, returns somewhat?

Thinker•14mo ago

Well, Action returns void, so it's actually just a statement.

jorenOP•14mo ago

suppose its not a return, but I assume that'd be possible too in a C# lambda ah

MODiX•14mo ago

Thinker

REPL Result: Success

var objects = new object[] { 1, true };
var strings = new string[] { "a", "b" };

objects.Concat(strings)

var objects = new object[] { 1, true };
var strings = new string[] { "a", "b" };

objects.Concat(strings)

Result: Concat2Iterator<object>

[
  1,
  true,
  "a",
  "b"
]

[
  1,
  true,
  "a",
  "b"
]

Compile: 584.891ms | Execution: 37.253ms | React with ❌ to remove this embed.

Thinker•14mo ago

This is a (somewhat) more practical example of when variance is actually useful, Concat takes two IEnumerable<T>s and concatenates them. If IEnumerable<T> wasn't covariant then you wouldn't be able to concatenate an IEnumerable<string> with an IEnumerable<object>.

jorenOP•14mo ago

makes sense, though you wouldnt have to mark it with anything? or with in, inside Concat? actually you would mark it thats the whole point I suppose

Thinker•14mo ago

No, IEnumerable<out T> is declared as covariant itself

jorenOP•14mo ago

as normally it wouldnt match wait does:

void func(object a) {}
func(1);

void func(object a) {}
func(1);

compile?

Thinker•14mo ago

yeah

jorenOP•14mo ago

its implicitly convertible?

MODiX•14mo ago

Thinker

REPL Result: Success

void Func(object a) {}
Func(1);

void Func(object a) {}
Func(1);

Compile: 462.239ms | Execution: 37.387ms | React with ❌ to remove this embed.

Thinker•14mo ago

yeah you can implicitly convert (almost) any type to object (the only ones you can't are ref structs and pointers, but those are very special anyway)

jorenOP•14mo ago

i dont really understand how applying covariance to the enumerable makes it work with the Concat though

Thinker•14mo ago

Okay I'll clarify it a bit more

jorenOP•14mo ago

I guess it works fine since concat takes T, Y? and not T, T as parameter types so you can pass two different enumerable types that makes sense, but how does the concat happen and why does it allow it

Thinker•14mo ago

You can pass in two enumerable types where one has elements which are derived from the other

jorenOP•14mo ago

so one of them has to be the base of the other the result yields an enumerable of the derived type?

Thinker•14mo ago

yep

jorenOP•14mo ago

🤔 interesting okay, either way, so we pass the enumerables and it passes these contraints, then what. It has to concat them, but the enumerables dont match in type

MODiX•14mo ago

Thinker

REPL Result: Success

IEnumerable<T> Concat<T>(IEnumerable<T> a, IEnumerable<T> b)
{
    foreach (var x in a) yield return x;
    foreach (var x in b) yield return x;
}

Concat<object>(new object[] { 1, true }, new string[] { "a", "b" })

IEnumerable<T> Concat<T>(IEnumerable<T> a, IEnumerable<T> b)
{
    foreach (var x in a) yield return x;
    foreach (var x in b) yield return x;
}

Concat<object>(new object[] { 1, true }, new string[] { "a", "b" })

Result: <Concat>d__1<object>

[
  1,
  true,
  "a",
  "b"
]

[
  1,
  true,
  "a",
  "b"
]

Compile: 575.889ms | Execution: 125.629ms | React with ❌ to remove this embed.

Thinker•14mo ago

this is essentially that They do match in type, because IEnumerable<string> is assignable to IEnumerable<object>

jorenOP•14mo ago

[
  1,
  true,
  "a",
  "b"
]

[
  1,
  true,
  "a",
  "b"
]

oh wait no its object, really ? oh boy I dont like that at all i mean its the only option but I personally wouldnt allow this, either make them convert the types or compile error this would mean the contraint in Concat is basically: "As long as it derives from object"? I mean cant you basically concat anything at that point?

Thinker•14mo ago

no not as long as it derives from object, as long as a derives from b

MODiX•14mo ago

Thinker

REPL Result: Success

class A {}
class B : A {}

new[] { new A(), new A() }.Concat(new[] { new B(), new B() })

class A {}
class B : A {}

new[] { new A(), new A() }.Concat(new[] { new B(), new B() })

Result: Concat2Iterator<A>

[
  {},
  {},
  {},
  {}
]

[
  {},
  {},
  {},
  {}
]

Compile: 504.428ms | Execution: 30.216ms | React with ❌ to remove this embed.

Thinker•14mo ago

okay the repl doesn't show them but they are two a and two b objects

jorenOP•14mo ago

it does, I see it in the code above

MODiX•14mo ago

joren

REPL Result: Success

class A {}
class B : A {}

new[] { new B(), new B() }.Concat(new[] { new A(), new A() })

class A {}
class B : A {}

new[] { new B(), new B() }.Concat(new[] { new A(), new A() })

Result: Concat2Iterator<A>

[
  {},
  {},
  {},
  {}
]

[
  {},
  {},
  {},
  {}
]

Compile: 580.053ms | Execution: 47.598ms | React with ❌ to remove this embed.

jorenOP•14mo ago

that works too it seems but

[
  {},
  {},
  {},
  {}
]

[
  {},
  {},
  {},
  {}
]

in both examples would be type B?

Thinker•14mo ago

Result: Concat2Iterator<A> It's an enumerable of As because both enumerables passed to Concat are assignable to IEnumerable<A>

jorenOP•14mo ago

Okay, I see - I've gotten a better grasp now. Thank you so much, I'll go do some testing and come back with any questions that ill most likely get! tyvm

Thinker•14mo ago

Accord•14mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Covariance and Contravariance C#