How to: Modify an Office Open XML Document

public static class LocalExtensions
{
    public static string StringConcatenate(this IEnumerable<string> source)
    {
        StringBuilder sb = new StringBuilder();
        foreach (string s in source)
            sb.Append(s);
        return sb.ToString();
    }

    public static string StringConcatenate<T>(this IEnumerable<T> source,
        Func<T, string> func)
    {
        StringBuilder sb = new StringBuilder();
        foreach (T item in source)
            sb.Append(func(item));
        return sb.ToString();
    }

    public static string StringConcatenate(this IEnumerable<string> source, string separator)
    {
        StringBuilder sb = new StringBuilder();
        foreach (string s in source)
            sb.Append(s).Append(separator);
        return sb.ToString();
    }

    public static string StringConcatenate<T>(this IEnumerable<T> source,
        Func<T, string> func, string separator)
    {
        StringBuilder sb = new StringBuilder();
        foreach (T item in source)
            sb.Append(func(item)).Append(separator);
        return sb.ToString();
    }
}

class Program
{
    public static string ParagraphText(XElement e)
    {
        XNamespace w = e.Name.Namespace;
        return e
               .Elements(w + "r")
               .Elements(w + "t")
               .StringConcatenate(element => (string)element);
    }

    static void Main(string[] args)
    {
        const string fileName = "SampleDoc.docx";

        const string documentRelationshipType =
          "http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument";
        const string stylesRelationshipType =
          "http://schemas.openxmlformats.org/officeDocument/2006/relationships/styles";
        const string wordmlNamespace =
          "http://schemas.openxmlformats.org/wordprocessingml/2006/main";
        XNamespace w = wordmlNamespace;

        using (Package wdPackage = Package.Open(fileName, FileMode.Open, FileAccess.ReadWrite))
        {
            PackageRelationship docPackageRelationship =
              wdPackage.GetRelationshipsByType(documentRelationshipType).FirstOrDefault();
            if (docPackageRelationship != null)
            {
                Uri documentUri = PackUriHelper.ResolvePartUri(new Uri("/", UriKind.Relative),
                  docPackageRelationship.TargetUri);
                PackagePart documentPart = wdPackage.GetPart(documentUri);

                //  Load the document XML in the part into an XDocument instance.
                XDocument xDoc = XDocument.Load(XmlReader.Create(documentPart.GetStream()));

                //  Find the styles part. There will only be one.
                PackageRelationship styleRelation =
                  documentPart.GetRelationshipsByType(stylesRelationshipType).FirstOrDefault();
                PackagePart stylePart = null;
                XDocument styleDoc = null;

                if (styleRelation != null)
                {
                    Uri styleUri = PackUriHelper.ResolvePartUri(documentUri, styleRelation.TargetUri);
                    stylePart = wdPackage.GetPart(styleUri);

                    //  Load the style XML in the part into an XDocument instance.
                    styleDoc = XDocument.Load(XmlReader.Create(stylePart.GetStream()));
                }

                XElement paraNode = xDoc
                                    .Root
                                    .Element(w + "body")
                                    .Descendants(w + "p")
                                    .FirstOrDefault();

                string paraText = ParagraphText(paraNode);

                // remove all text runs
                paraNode.Descendants(w + "r").Remove();

                paraNode.Add(
                    new XElement(w + "r",
                        new XElement(w + "t", paraText.ToUpper())
                    )
                );

                //  Save the XML into the package
                using (XmlWriter xw =
                  XmlWriter.Create(documentPart.GetStream(FileMode.Create, FileAccess.Write)))
                {
                    xDoc.Save(xw);
                }

                Console.WriteLine("New first paragraph: >{0}<", paraText.ToUpper());
            }
        }
    }
}

New first paragraph: >PARSING WORDPROCESSINGML WITH LINQ TO XML<