OpenXML: Losing Custom Document Property After Editing Word Document - vb.net

Question

Using DocumentFormat.OpenXML, I am trying to add a custom property to a Word document and then later read the property. The following code "appears" to do just that:
Dim os As OpenSettings = New OpenSettings() With {
.AutoSave = False
}
Dim propVal As String = "Test Value"
Using doc As WordprocessingDocument = WordprocessingDocument.Open(filename, True, os)
Dim cPart As CustomFilePropertiesPart = doc.CustomFilePropertiesPart
If cPart Is Nothing Then
cPart = doc.AddCustomFilePropertiesPart
cPart.Properties = New DocumentFormat.OpenXml.CustomProperties.Properties()
End If
Dim cPart As CustomFilePropertiesPart = doc.CustomFilePropertiesPart
Dim cProps As Properties = cPart.Properties
For Each prop As CustomDocumentProperty In cProps
If prop.Name = "TranscriptID" Then
prop.Remove()
Exit For
End If
Next
Dim newProp As CustomDocumentProperty = New CustomDocumentProperty() With {
.Name = "TranscriptID"
}
newProp.VTBString = New VTBString(propVal)
newProp.FormatId = "{D5CDD505-2E9C-101B-9397-08002B2CF9AE}"
cProps.AppendChild(newProp)
Dim pid As Integer = 2
For Each item As CustomDocumentProperty In cProps
item.PropertyId = pid
pid += 1
Next
cProps.Save()
End Using
This code is modeled after code found here:
https://learn.microsoft.com/en-us/office/open-xml/how-to-set-a-custom-property-in-a-word-processing-document
It appears to work in this scenario:
Execute code from above.
Execute code from above again.
At #2 I expect to find the CustomFilePropertiesPart and the property value and my expectation is met.
The problem appears in this scenario:
Execute code from above.
Open document using Microsoft Word, save and close.
Execute code from above again.
What happens in this scenario is that the CustomFilePropertiesPart is missing, whereas it should be found. It is as if Microsoft Word does not successfully read this object, so when the document is save, the object is lost. This suggests to me that there is something that there is something wrong with my code. If you can see what it is, or if you have a comparable working example that I could compare it with, I would appreciate hearing from you. I feel like I correctly followed the Microsoft example, but obviously I did not and I am having trouble seeing where I departed. Thanks.

Answer 1

OK, I found this wonderful tool called the Office Productivity Tool. It has a code generation feature, so I was able to compare what I was doing with what Word does. Basically the problem was with setting the property value. This snippet does the trick:
Dim cProps As Properties = cPart.Properties
Dim val As DocumentFormat.OpenXml.VariantTypes.VTLPWSTR = New DocumentFormat.OpenXml.VariantTypes.VTLPWSTR
val.Text = tr.ID.ToString
Dim newProp As CustomDocumentProperty = New CustomDocumentProperty() With {
.Name = "TranscriptID",
.FormatId = "{D5CDD505-2E9C-101B-9397-08002B2CF9AE}"
}
newProp.Append(val)
cProps.AppendChild(newProp)