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?

Answer 1

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);         };     } } 

Output:

counter=1 counter=2 

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.