How to parse an arbitrary XML string into a structured object

It sounds pretty much like you want what LINQ to XML offers. Parse XML like so:

var doc = XDocument.Parse(@"<foo><bar>baz</bar></foo>");

Then you could query it in a similar way to your suggested syntax:

var barValue = (string)doc.Elements("foo").Elements("bar").Single()

or:

var barValue = (string)doc.Descendants("bar").Single()

See the docs for more info.

Personally, I agree with the other answers that a LINQ to XML based solution is best. Something like:

string xml = "<root><foo><bar>baz</bar></foo></root>";
string s = XElement.Parse(xml).Element("foo").Element("bar").Value;

But if you really wanted behaviour like your example, you could write a small wrapper class such as:

EDIT: Example updated to be indexable using a multidimensional indexer^note.

class MyXmlWrapper
{
    XElement _xml;

    public MyXmlWrapper(XElement xml)
    {
        _xml = xml;
    }

    public MyXmlWrapper this[string name, int index = 0]
    {
        get
        {
            return new MyXmlWrapper(_xml.Elements(name).ElementAt(index));
        }
    }

    public static implicit operator string(MyXmlWrapper xml)
    {
        return xml._xml.Value;
    }
}

And use that exactly like you wanted:

string xml = "<root><foo><bar>baz</bar></foo></root>";
MyXmlWrapper wrapper = new MyXmlWrapper(XElement.Parse(xml));
string s = wrapper["foo"]["bar"];

Edited example for returning an element from a collection:

string xml = "<root><foo><bar><baz>1</baz><baz>2</baz></bar></foo></root>";
MyXmlWrapper wrapper = new MyXmlWrapper(XElement.Parse(xml));
string baz1 = wrapper["foo"]["bar"]["baz", 0];
string baz2 = wrapper["foo"]["bar"]["baz", 1];

XDocument.Parse(string) is your friend:

https://msdn.microsoft.com/en-us/library/bb345532(v=vs.110).aspx