What are anonymous methods and what is their main purpose?

Question

I know they’re used to working together with delegates, but the sources I found were a little confused in my opinion.

Answer 1

It is a way to create a delegate as well as lambda expressions.

The C# along its evolution has been introducing new ways to create delegates, which practically makes the previous versions unused, almost unused I would say... is the case here.

An anonymous method is called so, because it is not referential via a static name (I mean, a compile-time name, statically analysable =D ).

In the microsoft website have examples about this evolution.

C# 1:

// Sintaxe original de delegate requer a
// inicialização com um método nomeado.
TestDelegate testDelA = new TestDelegate(M);

The M above is a named method:

static void M(string s)
{
    Console.WriteLine(s);
}

// Chamando o delegate.
testDelA("Delegate criado com argumento, usando um Method Group.");

C# 2:

// C# 2.0: Um delegate pode ser inicializado com
// código inline, chamado da "método anônimo."
// Nesse formato, o tipo dos argumentos é explícito.
// Note que o método não possui um nome.
TestDelegate testDelB = delegate(string s) { Console.WriteLine(s); };

C# 3:

// C# 3.0. O delegate pode ser inicializado a partir
// de uma expressão lambda. Neste caso, já se evoluiu para haver
// inferência de tipos. O tipo de `x` é inferido a partir do
// tipo de delegate TestDelegate.
TestDelegate testDelC = (x) => { Console.WriteLine(x); };

The guy TestDelegate:

delegate void TestDelegate(string s);

testDelB("That's nothing. I'm anonymous and ");
testDelC("I'm a famous author.");
testDelD("I'm a famous author.");

C# 7:

And now C# 7, has local functions, which advances a little more in the ways of defining a delegate, which is not in this Microsoft document (it is outdated by the visa).

static void Main(string[] args)
{
    testDelD(Exemplo usando função local.");

    void testDelD(string x) { Console.WriteLine(x); }
}

The interesting thing is that local functions bring back the names... left aside for anonymous methods and lambda expressions.

Answer 2

Anonymous method is a nameless method, it is a method that does not have a symbol associated with it at compile time.

You have only one reference to the code and this reference can be assigned to a variable of any type, passed as argument to another method or returned from a method.

The delegate is just a syntax to define an anonymous method. But the delegate may have an associated name as well. The delegate in direct use is virtually obsolete in normal applications.

delegate(string i) { WriteLine(i); };

Today we use the syntax of lambda that is always anonymous.

(i) => WriteLine(i);

For example can do:

public Action<string> Metodo() {
    Action<string> func = (i) => WriteLine(i);
    func("teste"); //imprime teste
    return func;
}

I put in the Github for future reference.

This way the method has no name, but the variable that supports it has. Obviously, at the moment the anonymous method returns it will probably be assigned to another variable, then the name is provisional, is linked to its support and not to the method.

More details on What is the difference between a lambda expression, a closure and a delegate?.