PsAccelerator: LINQ to XML, when lazy is too lazy in F# (and C# )

Today a F# program did not work as expected. This is a small example I created:

let xml ="<a><b>1</b><b>2</b></a>"
let doc = XDocument.Parse(xml)

let getXElements_b (doc:XDocument) =
let els = doc.Root.Elements(XName.Get("b"))
els

//test
let result1 = doc |> getXElements_b |> Seq.toList
//val result1 : XElement list = [<b>1</b>; <b>2</b>]
//the expected result


let proces (el:XElement) =
XElement(XName.Get("c")) |> el.ReplaceWith

doc |> getXElements_b |> Seq.iter proces

let result2 =  doc.ToString();;
//val result2 : string = "<a>
//  <c />
//  <b>2</b>
//</a>"

This is not the expected result. I expected:

val result2 : string = "<a>
<c />
<c />
</a>"

I can fix it by changing the getXElements_b function:

let getXElements_b (doc:XDocument) =
let els = doc.Root.Elements(XName.Get("b")) |> Seq.toList
els

With this adjustment everything works fine. It seems that I have to force the the computation.

This is a problem in C# too.

private void GetXml()
{
var xml = "<a><b>1</b><b>2</b></a>";
var doc = XDocument.Parse(xml);
var els = doc.Root.Elements(XName.Get("b"));

foreach (var el in els)
{
el.ReplaceWith(new XElement(XName.Get("c")));
}

txtResult.Text = doc.ToString();
}