The Conversion Process

While renaming a file extension (.dotx to .docx) is insufficient, this section details the programmatic process of converting a template file into a fully structured, ready-to-use DOCX document.

The conversion involves two critical steps: changing the file type and establishing the template relationship.

Change Document Type: You must explicitly inform the parser that the document is being converted into a final, executable document format.
Establish Relationship: You add an external relationship referencing the template's original location, which helps maintain document integrity.

Code sample

using (WordprocessingDocument template = WordprocessingDocument.Open(documentStream, true))
{
    // Step 1: Define the document as a final DOCX package.
    template.ChangeDocumentType(DocumentFormat.OpenXml.WordprocessingDocumentType.Document);

    MainDocumentPart mainPart = template.MainDocumentPart;
    // Step 2: Establish the relationship to the original template file path.
    mainPart.DocumentSettingsPart.AddExternalRelationship(
        "http://schemas.openxmlformats.org/officeDocument/2006/relationships/attachedTemplate",
        new Uri(templatePath, UriKind.Absolute));

    mainPart.Document.Save();
}