What are 'closures' in .NET?

Question

What is a 'closure'? Do we have them in .NET?

If they do exist in .NET, could you please provide a code snippet (preferably in C#) explaining it?

EDIT: I went through Jon Skeet's article to understand what closures are and how to use them in .NET.
EDIT: I found another interesting article here

Answer 1

Closures are functional values that hold onto variable values from their original scope. C# can use them in the form of anonymous delegates.

For a very simple example, take this C# code:

    delegate int testDel();

    static void Main(string[] args)
    {
        int foo = 4;
        testDel myClosure = delegate()
        {
            return foo;
        };
        int bar = myClosure();

    }

At the end of it, bar will be set to 4, and the myClosure delegate can be passed around to be used elsewhere in the program.

Closures can be used for a lot of useful things, like delayed execution or to simplify interfaces - LINQ is mainly built using closures. The most immediate way it comes in handy for most developers is adding event handlers to dynamically created controls - you can use closures to add behavior when the control is instantiated, rather than storing data elsewhere.

Answer 2

I have an article on this very topic. (It has lots of examples.)

In essence, a closure is a block of code which can be executed at a later time, but which maintains the environment in which it was first created - i.e. it can still use the local variables etc of the method which created it, even after that method has finished executing.

The general feature of closures is implemented in C# by anonymous methods and lambda expressions.

Here's an example using an anonymous method:

using System;

class Test
{
    static void Main()
    {
        Action action = CreateAction();
        action();
        action();
    }

    static Action CreateAction()
    {
        int counter = 0;
        return delegate
        {
            // Yes, it could be done in one statement; 
            // but it is clearer like this.
            counter++;
            Console.WriteLine("counter={0}", counter);
        };
    }
}

Here we can see that the action returned by CreateAction still has access to the counter variable, and can indeed increment it, even though CreateAction itself has finished.

Answer 3

A closure is a function, defined within a function, that can access the local variables of it as well as its parent.

public string GetByName(string name)
{
   List<things> theThings = new List<things>();
  return  theThings.Find<things>(t => t.Name == name)[0];
}

so the function inside the find method.

 t => t.Name == name

can access the variables inside its scope, t, and the variable name which is in its parents scope. Even though it is executed by the find method as a delegate, from another scope all together.

Answer 4

Func<int, int> GetMultiplier(int a)
{
     return delegate(int b) { return a * b; }
}
//...
var fn2 = GetMultiplier(2);
var fn3 = GetMultiplier(3);
Console.WriteLine(fn2(2));  //outputs 4
Console.WriteLine(fn2(3));  //outputs 6
Console.WriteLine(fn3(2));  //outputs 6
Console.WriteLine(fn3(3));  //outputs 9

A closure is an anonymous function passed outside of the function in which it is created. It maintains any variables from the function in which it is created that it uses.

Answer 5

Closures are chunks of code that reference a variable outside themselves, (from below them on the stack), that might be called or executed later, (like when an event or delegate is defined, and could get called at some indefinite future point in time)... Because the outside variable that the chunk of code references may gone out of scope (and would otherwise have been lost), the fact that it is referenced by the chunk of code (called a closure) tells the runtime to "hold" that variable in scope until it is no longer needed by the closure chunk of code...

Answer 6

Here is a contrived example for C# which I created from similar code in JavaScript:

public delegate T Iterator<T>() where T : class;

public Iterator<T> CreateIterator<T>(IList<T> x) where T : class
{
        var i = 0; 
        return delegate { return (i < x.Count) ? x[i++] : null; };
}

So, here is some code that shows how to use the above code...

var iterator = CreateIterator(new string[3] { "Foo", "Bar", "Baz"});

// So, although CreateIterator() has been called and returned, the variable 
// "i" within CreateIterator() will live on because of a closure created 
// within that method, so that every time the anonymous delegate returned 
// from it is called (by calling iterator()) it's value will increment.

string currentString;    
currentString = iterator(); // currentString is now "Foo"
currentString = iterator(); // currentString is now "Bar"
currentString = iterator(); // currentString is now "Baz"
currentString = iterator(); // currentString is now null

Hope that is somewhat helpful.

Answer 7

Basically closure is a block of code that you can pass as an argument to a function. C# supports closures in form of anonymous delegates.

Here is a simple example:
List.Find method can accept and execute piece of code (closure) to find list's item.

// Passing a block of code as a function argument
List<int> ints = new List<int> {1, 2, 3};
ints.Find(delegate(int value) { return value == 1; });

Using C#3.0 syntax we can write this as:

ints.Find(value => value == 1);