In C#, is there a way to write custom object initializers for new data-types?

Question

In C#, there's the "standard" initializer technique { Property1 = "a", Property2 = "b" }, and there are a couple of special variants for collections (list and dictionary). {value1, value2}, and { {"key1", value1 }, {"key2", value2} }.

I'd like to have a recursive object initializer for a tree data type, but I don't know if there's any way to customize that mechanism. I'd like something that looks like an s-expression. { item1 {item2 item3 item4 } {item5 item6} }

I'm doing this via constructors, but I'd like a terser syntax.

Answer 1

Using a variadic local lambda n that simply calls your constructor, you could get it as short as:

n(item1, n(item2, item3, item4), n(item5, item6))

Update: something like

var n = (params Node[] nodes) => new Node(nodes);

Answer 2

If you implement the ICollection IEnumerable interface and have a method called add. Incidentally, this is all included in the ICollection interface which is why I confused it.

Test test = new Test() {
    new Test2() {
     new Test3() {

     }
    },
    new Test() {
     new Test2() {
      { new Test(), new Test2() },
      { new Test(), new Test2() },
      { new Test(), new Test2() }
     }
    }
};

public class Test : IEnumerable
{
    public void Add(Test a){}
    public void Add(Test2 a){}
    public IEnumerator GetEnumerator(){}
}

public class Test2 : IEnumerable
{
    public void Add(Test a, Test2 b){}
    public void Add(Test3 a){}
    public IEnumerator GetEnumerator(){}
}

public class Test3 : IEnumerable
{
    public void Add(Test a) {}
    public void Add(Test2 a){}
    public IEnumerator GetEnumerator(){}
}

Answer 3

If the nodes of your tree are easy to construct, i.e. can be initialized from their value, then you can make things terser than ChaosPandion's answer, by adding an extra method:

class Tree : IEnumerable
{
    public string Value { get; set; }

    public void Add(Tree t) { ... }

    // Add this method
    public void Add(string s)
    {
        Add(new Tree { Value = s });
    }

    public IEnumerator GetEnumerator() { ... }
}

So:

{ item1 { item2 item3 item4 } { item5 item6 } }

Becomes:

new Tree { "item1", new Tree { "item2", "item3", "item4" }, new Tree { "item5", "item6" } };