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c#anonymous-types
Konstantin Spirin
Konstantin Spirin

Reputation: 21321

Create hierarchical anonymous type

Is there any way to create anonymous type that references instances of itself?

var root = new { Name = "Root", Parent = ??? };
var child = new { Name = "Child", Parent = root };
var childOfChild = new { Name = "Grand child", Parent = child };

For example, we can reference delegate from itself:

Action run = null;
run = () => run();

Another example, we can create generic Stack of anonymous types:

static Stack<T> CreateStack<T>(params T[] values)
{
    var stack = new Stack<T>();

    foreach (var value in values)
        stack.Add(value);

    return stack;
}

Can you think of any ways to reference anonymous type from itself?

Upvotes: 3

Views: 899

Answers (2)

chakrit
chakrit

Reputation: 61568

It seemed... that the C# compiler will simply refuses to infer the type recursively. Take this sample code for example:

(From @Eric: Correct; the type inference engine requires that all the "input" types of a lambda be known before the "output" type of the lambda is inferred)

public void Run()
{
  var k = Generator((str, parent) => new {
    Name = str,
    Parent = parent
  });
}

public Func<string, T, T> Generator<T>(Func<string, T, T> generator)
{
  return (str, obj) => generator(str, obj);
}

This snippet fails to compile with an error that the compiler cannot infer the <T> to use with Generator<T>... thus I think it's impossible.

Upvotes: 1

Eric Lippert
Eric Lippert

Reputation: 660493

Anonymous types in C# are immutable. Therefore all of their field values must have been present before the creation of the object and will never change. And therefore it is impossible to have a directly circular referencing anonymous type in C#.

Anonymous types in VB are mutable; you could probably figure out some way to do it in VB.

There might be a way to make an anonymous type indirectly reference itself, by, say, containing a delegate that when invoked, returns the instance of the anonymous type. I don't see any way off the top of my head to easily do that, but I also don't see a proof that doing so is impossible. Give it some thought and see what you come up with!

I assume this question is for entertainment purposes only. If you want to make a circularly-referential object, please make a nominal type.

Upvotes: 9

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