Add text to footer using openxml

Quote:

The headers and footers aren't in the MainDocumentPart.Document, they are subparts in the HeaderParts and FooterParts[^] collections off the MainDocumentPart[^]. So you'll need to process each part separately. Fortunately, all of the parts share a common base class, OpenXmlPart[^], so you can put them together into a list and process them in a loop. Since you'll be dealing with the base class, you'll need to use RootElement[^] property instead of Document as a starting point to get the Descendants.

var partsToUpdate = new List<OpenXmlPart> { doc.MainDocumentPart }
    .Concat(doc.MainDocumentPart.HeaderParts)
    .Concat(doc.MainDocumentPart.FooterParts);

foreach (var part in partsToUpdate)
{
    var elements = part.RootElement
        .Descendants<SdtElement>()
        .Where(s => s.SdtProperties.ChildElements.Count > 0 &&
                    s.SdtProperties.GetFirstChild<Tag>()?.Val == contentControlTag);

    foreach (var element in elements)
    {
        if (element == null)
        {
            continue;
        }

        var elementText = element.Descendants<Text>();

        if (elementText != null)
        {
            var elementTextValue = elementText.FirstOrDefault();

            if (elementTextValue != null)
            {
                elementTextValue.Text = text;
            }
            elementText.Skip(1).ToList().ForEach(t => t.Remove());
        }
    }
}