What are closures and what is their use?

Question

The answer to this question on lambda What are lambda Expressions? And what’s the point of using them? talks about closures and Tree Xpressions, however, does not have a clear example of what they are in fact and what their main use is. So what are closures and in what situation it is important to use them?

Answer 1

Concept

Closure (or in Portuguese, enclosure) is a function that references free variables in the lexical context (I took it from this article on Wikipedia). That is, it is a function that references, for example, variables that are not declared in its body.

Usually closures are anonymous and exist only in the body of another method.

For example:

public Fruta FindById(int id)
{
    return this.Find(delegate(Fruta f)
    {
        return (f.Id == id);
    });
}

In this case, delegate(Fruta f) { ... } is an enclosure, which references the variable id stated in the body of FindById (another function).

An enclosure can be assigned to a variable:

Action acao = delegate { Console.WriteLine(umaStringQualquer); };

And activated at some specific point in the function or method:

acao();

Utilizing

Suppose I use the method Distinct() implemented in IEnumerable that requires that a class of comparison be written for him like this:

class Comparador : IEqualityComparer<Fruta>
{
    public bool Equals(Fruta x, Fruta y)
    {
        if (x.Nome == y.Nome)
        {
            return true;
        }
        else { return false; }
    }

    public int GetHashCode(Fruta fruta)
    {
        return 0;
    }
}

It works, but it’s a little long. With an enclosure, I can use not the Distinct, but a more or less like class extension:

public List<Fruta> AcharMinhaFruta(Fruta fruta) 
{
    return minhasFrutas.Select(f => delegate (f) { return f.Nome == fruta.Nome; } ).ToList();
}