Reading and updating OpenXML in F# - f#

Question

I can't get this F# code to read and update Content Control text fields inside Word documents.
The second function does absolutely nothing and the first one produces this error: An unhandled exception of type 'System.InvalidOperationException' occurred in System.Core.dll
Additional information: Sequence contains no elements
namespace OpenXML
open DocumentFormat.OpenXml
open DocumentFormat.OpenXml.Packaging
open DocumentFormat.OpenXml.Wordprocessing
open System.Linq
// Add the DocumentFormat.OpenXml assembly
// Add the WindowsBase assembly
module public Word =
let query_plain_text_content_control document_path_and_file_name content_control_tag =
use theDoc = WordprocessingDocument.Open((document_path_and_file_name :string), true)
let mainPart = theDoc.MainDocumentPart
let block = mainPart.Document.Body.Descendants<SdtElement>().Where(fun r -> r.SdtProperties.GetFirstChild<Tag>().Val = content_control_tag).Single()
let t = block.Descendants<Text>().FirstOrDefault()
t.Text
let update_plain_text_content_control document_path_and_file_name content_control_tag new_text = async {
use theDoc = WordprocessingDocument.Open((document_path_and_file_name :string), true)
let mainPart = theDoc.MainDocumentPart
let block = mainPart.Document.Body.Descendants<SdtElement>().Where(fun r -> r.SdtProperties.GetFirstChild<Tag>().Val = content_control_tag).Single()
let t = block.Descendants<Text>().FirstOrDefault()
t.Text <- new_text
mainPart.Document.Save() |> ignore
}

Answer 1

Seems to work fine, with a couple of tweaks:
#r#"DocumentFormat.OpenXml.dll"
#r"WindowsBase.dll"
open DocumentFormat.OpenXml
open DocumentFormat.OpenXml.Packaging
open DocumentFormat.OpenXml.Wordprocessing
open System.Linq
// Add the DocumentFormat.OpenXml assembly
// Add the WindowsBase assembly
module public Word =
let query_plain_text_content_control document_path_and_file_name content_control_tag =
use theDoc = WordprocessingDocument.Open((document_path_and_file_name :string), true)
let mainPart = theDoc.MainDocumentPart
let block = mainPart.Document.Body.Descendants<SdtElement>().Where(fun r -> r.SdtProperties.GetFirstChild<Tag>().Val.ToString() = content_control_tag).Single()
let t = block.Descendants<Text>().FirstOrDefault()
t.Text
let update_plain_text_content_control document_path_and_file_name content_control_tag new_text = async {
use theDoc = WordprocessingDocument.Open((document_path_and_file_name :string), true)
let mainPart = theDoc.MainDocumentPart
let block = mainPart.Document.Body.Descendants<SdtElement>().Where(fun r -> r.SdtProperties.GetFirstChild<Tag>().Val.ToString() = content_control_tag).Single()
let t = block.Descendants<Text>().FirstOrDefault()
t.Text <- new_text
mainPart.Document.Save() |> ignore
}
let oldtext = query_plain_text_content_control #".\text.docx" "ctrltag"
let update = update_plain_text_content_control #".\text.docx" "ctrltag" "new text"
Async.RunSynchronously(update)
let newtext = query_plain_text_content_control #".\text.docx" "ctrltag"
.. on a document containing a single plaintext content control, with tag of 'ctrltag', with content 'old text', I get:
val oldtext : string = "Old text"
val update : Async<unit>
val newtext : string = "new text"
Without calling .ToString() on 'r.SdtProperties.GetFirstChild().Val', I got this error:
The type 'string' is not compatible with the type 'StringValue'.
Perhaps there is a problem with your document? The error you are getting would seem to suggest that there are no content controls with the specified Tag.