What's the actual type of lambda in C#?

Asked 6/1, 2015 at 9:38 Answered 6/1, 2015 at 9:43

I read that C# lambdas can be imlicitly converted to Action or Func , but lambda cannot be executed directly Define a lambda function and execute it immediately For example :

int n = (()=>5)(); //doesn't work
int n = ((Func<int>)(()=>5))(); //works

So what is the actual type of lambda and why it cannot be directly called? Is it because C# type system is "weaker" than Haskell or Scala one?

Faustena answered 6/1, 2015 at 9:38 Comment(6)

The answer in the linked question is valid for your question. C# compiler needs to know context, which is either assignment or casting. In the first sample you do neither – Doorkeeper 6/1, 2015 at 9:43

The type system is not related to this. Dim n = Function() 5 works just fine in VB.NET, and it's similar in F#, and they have the same underlying type system. This is just an issue with C#. – Spitsbergen 6/1, 2015 at 23:17

@MauricioScheffer no F# and C# doesn't have the same "type system" . Why? Because 5.0+5 is incorrect in F#, but is still correct in C#. – Faustena 6/1, 2015 at 23:32

@Faustena that's a feature of the language, not of the type system. – Spitsbergen 7/1, 2015 at 0:13

@MauricioScheffer and this "feature" is called "type system" - how types "work" together. – Faustena 7/1, 2015 at 0:48

@Faustena language != type system , but whatever, honestly I'm not interesting in discussing this. – Spitsbergen 7/1, 2015 at 0:54

A lambda expression doesn't have a type. It cannot, because any type in the .NET world that it could have, would also hard-code the type of the lambda's parameters and result. Now consider:

x => x + 1

What type could x have? What type will the result be? There isn't any single answer, and the lambda expression can indeed be converted to Func<int, int>, Func<double, double>, and many other delegate types with different parameters. Giving a lambda expression a type would disallow such expressions. C# did want to allow such expressions, so was designed not to give such expressions any type.

Cudweed answered 6/1, 2015 at 9:43 Comment(8)

well explained, if a code example be given, i am instersted – Finfoot 6/1, 2015 at 9:48

I'm not sure what sort of code example you have in mind, all I can think to add is examples of delegate types the expression could be converted to, which I've now included in the answer. – Cudweed 6/1, 2015 at 9:51

It makes sense. I have just tested this in F# and Haskell. In F# in works only for "int" - the reason is that F# doesn't use implicit conversions. In the case of C# - compiler really can't "know" the type, so you are right. – Faustena 6/1, 2015 at 9:59

@Faustena What about x => x + x? This would not use any conversion from int to double. – Cudweed 6/1, 2015 at 10:9

I thought it can create in the compile-time different functions for example : (x=>x+x)(5) : Func<int,int> because we called this function with an int argument .By the way, I accepted your answer because it really answers my question – Faustena 6/1, 2015 at 10:13

@Faustena Even if that did implicitly convert the lambda expression to Func<int,int> in an extended C#, wouldn't the lambda expression itself still not have any type? – Cudweed 6/1, 2015 at 10:19

As a further example, in static List<string> AppendOnes(List<string> li) { return li.ConvertAll(x => x + 1); } the arrow becomes an instance of System.Converter<string, string>. Historically, the generic delegate Converter<,> existed before the BCL had "funcs", i.e. the type family Func<>, Func<,>, Func<,,>, etc. – Tower 6/1, 2015 at 13:21

My IDE (ReSomething ;-) ) mostly uses Action in the simplest lambda case. – Triciatrick 4/10, 2022 at 0:43

This is because () => 5 can be compatible with various delegate types (e.g. you may have a custom delegate that takes nothing and returns int). And in order to create a delegate, compiler has to know the exact type. It can't just pick a random type that suits your needs. So that's why unless you cast it to an actual delegate type, you can't Invoke it.

In cases like where a method expects a delegate, that conversion is implicitly done by the compiler:

void Foo(Func<int> func) {  }

Foo(() => 5);

And it is also important to know that delegates are actually classes behind the scenes. And each time you create a delegate instance, compiler creates an instance of that class. So either way you have to specify a type so the compiler will know which type to use.

Sathrum answered 6/1, 2015 at 9:43 Comment(2)

when we put ()=> in Select() or Where() what it means ? please explain – Finfoot 6/1, 2015 at 9:50

@EhsanSajjad I was just adding that to the answer :) – Ventilate 6/1, 2015 at 9:50

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