C# what passes arguments to anonymous functions?

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I'm reading a book and I have problems understanding this code:

List<int> evenNumbers = list.FindAll(delegate(int i) 
{ 
   return (i % 2) == 0;
};

I cannot understand what will happen with this code. Who passes the argument to the parameter i and how? I'm really confused.

What I have tried:

Thinking about this but I cannot understand what's happening exactly.

Posted 23-Mar-18 7:10am

The_Unknown_Member

Updated 23-Mar-18 7:31am

v2

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Richard Deeming 23-Mar-18 14:58pm

When in doubt, you can almost always view the source of the built-in methods to see how they're implemented:
List<T>.FindAll[^]

2 solutions

Solution 1

Google is your friend.

Anonymous Functions (C# Programming Guide) | Microsoft Docs[^]

Posted 23-Mar-18 7:14am

#realJSOP

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User 7407470 · Accepted Answer · 2018-03-23T07:32:00

Solution 2

For every element in the List 'list' the .NET method 'FindAll' calls the delegate method to get a value (true or false) indicating whether the value should be added to the List 'evenNumbers'. So, the .NET method 'FindAll' passes the value i to the delegate method.

https://msdn.microsoft.com/nl-nl/library/windows/desktop/fh1w7y8z(v=vs.85).aspx[^]

Posted 23-Mar-18 7:32am

User 7407470

Comments

The_Unknown_Member 23-Mar-18 13:33pm

Ohhh, I got it... I'm passing the anonymous method to the Predecate<t> parameter of the method. Am I right?

[no name] 23-Mar-18 13:35pm

Yes, you're right!

The_Unknown_Member 23-Mar-18 13:40pm

Thanks!!!

[no name] 23-Mar-18 13:42pm

You're welcome! Thanks for the answer and vote.

The_Unknown_Member 23-Mar-18 15:23pm

One last question:How does FindAll not accept anything other than a boolean value. I want to know this particular detail of the implementation. It's a generic method but it works only with boolean values and when I try to return an int for example the compiler gives error: "Cannot convert lambda expression to intended delegate type because some of the return types in the block are not implicitly convertible to the delegate return type".

The delegate is generic too.

[no name] 23-Mar-18 15:39pm

The 'FindAll' specification says: 'Retrieves all the elements that match the conditions defined by the specified predicate'.
A predicate is a delegate method that returns a bool value. The delegate is generic for the the passed argument (int, float, double, etc.) but the return value of a predicate method is always 'bool' - it's like short for 'bool MatchMethod<T>'. So, if you specify another return value, you get a compiler error just like when you do something like that with a 'normal' method:

void Test()
{
bool result = GetResult(); // error! - GetResult() does not return bool.
}

int GetResult()
{
return 10;
}