Concept

Closure is a function that references free variables in the lexical context ( I removed this article from Wikipedia ). That is, it is a function that references, for example, variables that are not declared in your body.

Normally 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 a cloister, which references the variable id declared in the body of FindById (another function).

A cloister can be assigned to a variable:

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

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

acao();

Use

Suppose I'm going to use the Distinct() method implemented in IEnumerable that requires a comparison class to be written to it 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 wordy. With a cloister, I can use not Distinct , but a class extension something like this:

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