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?

Answer 1

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

public delegate T Iterator() where T : class;

public Iterator CreateIterator(IList 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.