Are Lambda expressions in C# closures?
Are lambda expressions (and to a degree, anonymous functions) closures?
My understanding of closures are that they are functions that are treated as objects, which seems to be an accurate representation of what anonymous functions and Lambda expressions do.
And is it correct to call them closures? I understand that closures came about (or became popular) due to the lisp dialect, but is it also a general programming term?
Thanks for any clarification that you can provide!
A lambda may be implemented using a closure, but it is not itself necessarily a closure.
A closure is "a function together with a referencing environment for the non-local variables of that function.".
When you make a lambda expression that uses variables defined outside of the method, then the lambda must be implemented using a closure. For example:
int i = 42;
Action lambda = () => { Console.WriteLine(i); };
In this case, the compiler generated method must have access to the variable ( i ) defined in a completely different scope. In order for this to work, the method it generates is a "function together with the referencing environment" - basically, it's creating a "closure" to retrieve access to the variable.
However, this lambda:
Action lambda2 = () => { Console.WriteLine("Foo"); }
does not rely on any "referencing environment", since it's a fully contained method. In this case, the compiler generates a normal static method, and there is no closure involved at all.
In both cases, the lambda is creating a delegate ("function object"), but it's only creating a closure in the first case, as the lambda doesn't necessarily need to "capture" the referencing environment in all cases.
Reed's answer is correct; I would just add few additional details:
lambda expressions and anonymous methods both have closure semantics; that is, they "capture" their outer variables and extend the lifetimes of those variables.
anonymous function is the term we use when we mean a lambda expression or an anonymous method. Yes, that is confusing. Sorry. It was the best we could come up with.
a function that can be treated as an object is just a delegate. What makes a lambda a closure is that it captures its outer variables.
lambda expressions converted to expression trees also have closure semantics, interestingly enough. And implementing that correctly was a pain in the neck, I tell you!
"this" is considered an "outer variable" for the purpose of creating a closure even though "this" is not a variable.
It's "closure" not "clojure."
That is not what a closure is. A closure is basically a representation of a function together with any non-local variables that the function consumes.
In that sense, lambdas are not closures, but they do cause closures to be generated by the compiler if they close over any variables.
If you use ILDASM on an assembly that contains a lambda that closes over some variables, you will see in that assembly a compiler generated class that repsresents the function and those variables that were closed over. That is the closure.
When you say
functions that are treated as objects,
that's normally just "function object" (in C# we'd say "delegate") and is common in functional programming.
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