Nathan Evans' Nemesis of the Moment

Super skinny XML document generation with F#

Posted in .NET Framework, F# by Nathan B. Evans on April 15, 2015

I needed to generate some simple XML documents, in memory, from some F# script. From my C# days I was already familiar with the System.Xml.Linq namespace, which I still quite like. But it wasn’t particularly clean to use from F#. So I wrote a really simple F# wrapper for some of its most commonly used features.

XmlToolkit for F#

module nbevans.Util.XmlToolkit
open System.Text
open System.Xml
open System.Xml.Linq
open System.IO

let XDeclaration version encoding standalone = XDeclaration(version, encoding, standalone)
let XLocalName localName namespaceName = XName.Get(localName, namespaceName)
let XName expandedName = XName.Get(expandedName)
let XDocument xdecl content = XDocument(xdecl, content |> Seq.map (fun v -> v :> obj) |> Seq.toArray)
let XComment (value:string) = XComment(value) :> obj
let XElementNS localName namespaceName content = XElement(XLocalName localName namespaceName, content |> Seq.map (fun v -> v :> obj) |> Seq.toArray) :> obj
let XElement expandedName content = XElement(XName expandedName, content |> Seq.map (fun v -> v :> obj) |> Seq.toArray) :> obj
let XAttributeNS localName namespaceName value = XAttribute(XLocalName localName namespaceName, value) :> obj
let XAttribute expandedName value = XAttribute(XName expandedName, value) :> obj

type XDocument with
    /// Saves the XML document to a MemoryStream using UTF-8 encoding, indentation and character checking.
    member doc.Save() =
        let ms = new MemoryStream()
        use xtw = XmlWriter.Create(ms, XmlWriterSettings(Encoding = Encoding.UTF8, Indent = true, CheckCharacters = true))
        doc.Save(xtw)
        ms.Position <- 0L
        ms

The principle of this module is that it overrides the key type names like System.Xml.Linq.XElement and the others with F# functions that effectively provide the equivalent constructor behaviour but in a more functional signature. Then a XDocument type extension adds a useful Save() function (since the stock ones are so useless on their own). … and here is an example usage straight from my app (but hopefully you will get the idea):

let doc =
    XDocument (XDeclaration "1.0" "UTF-8" "yes") [
        XComment "This document was automatically generated by a configuration script."
        XElement "Metadata" [
            XElement "SystemMetadata" [
                XElement "ScannedBy" ["PCT"]
                XElement "GenerationDate" [DateTime.UtcNow.ToString("s")]
                XElement "IndexedBy" ["UNKNOWN"]
                XElement "IndexedOn" ["UNKNOWN"]
                XElement "FileName" [createPackageContentFileName cp.Id fileName]
                XElement "ScanInfo" [
                    XElement "NumberOfPagesScanned" [string formPdfPageCount]
                    XElement "IpAddress" ["UNKNOWN"]
                    XElement "MachineName" ["UNKNOWN"]
                    XElement "NumberOfBlankPages" ["0"]
                ]
            ]
            XElement "UserDefinedMetadata" [
                XElement "Address1" [defaultArg (gp.Fields.TryFind "property-address") "UNKNOWN"]
                XElement "Postcode" [defaultArg (gp.Fields.TryFind "property-postcode") "UNKNOWN"]
                XElement "Patchcode" ["1"]
                XElement "Reviewdate" [DateTime.UtcNow.AddYears(1).ToString("s")]
            ]
        ]
    ]

let ms = doc.Save()
// 'ms' at this point contains a System.IO.MemoryStream of the generated XML document.
// That was my use-case, but maybe you will want to adapt this code or the XmlToolkit module itself to use a different type of stream; perhaps a FileStream. I'm a firm believer in K.I.S.S to avoid over-engineering.
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