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Comment by Leszek | Hacker News Reader

    var (ret, err) = Foo();
    if (err != null) {
      ...
    }

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So Foo() has to return a class which has a destructor? Seems like just returning a Tuple would be better in that case.

Tuple in C# is a class IIRC. Without destructor, you will have to pull out individual members separately. For example for a tuple of form <Int, Int, Int>, you will do something like `var r = c.getFirst(); var g = c.getSecond();`. With deconstructor, you can do the same via `var (r, g, b) = color;`. In other words, you can say that tuples are way to combine data, while deconstructors tell how to explode it. Since these deconstructors are themselves functions, you can do computation each time, and get different results due to side-effects (probably not good idea).

EDIT: I don't know C#, so syntax is definitely not right.

Basically yes. Except, with the new tuple syntax support in C# it's using a value type internally instead of the old Tuple class (I'm pretty sure Tuple's going to be gaining a set of deconstructors to allow it to participate though). And the pure tuple path apparently doesn't use the deconstructors, there's something fancier going on under the hood which is presumably faster.

But yes you can write a deconstructor for anything you like so it can participate in deconstructing assignment. And yes it can have side effects. And that would be VERY BAD.

You would just check t.Item2, no need to assign to variables I'm this example. However, the deconstructor syntax is nice, and now tour functions don't all return Tuple with vaguely named members.

Right. Not great example. What I wanted to say was deconstructor is all about creating new bindings with nicer syntax. You could just do that by calling getters / accessing fields, but you have to write more when you doing that.

Well, it allows you to return multiple objects without using an opaque Tuple or creating your own class. Sure, the other options work, this is just a nicer syntax. I mean, I can all of what C# does in C if I want to, but I appreciate the syntactical sugar.

OK, but that's not what the TFA describes as a deconstructor.

It's linked to providing these tuple types out of a method call.

So instead of having to declare that Foo returns a FooReturn, it can be declared as (say)

    public (int ret, string err) Foo() { return (5, "something"); }

That's not a deconstructor though, which is what the GP was asking about.

EDIT: Looks like there's confusion... Yes, you can deconstruct a Tuple like this, but C# 7 also introduces a specific Deconstructor pattern in which ANY class can be deconstructed as though it were a Tuple.

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