<p>I need to use C# programatically to append several preexisting <code>docx</code> files into a single, long <code>docx</code> file - including special markups like bullets and images.  Header and footer information will be stripped out, so those won't be around to cause any problems.</p>

<p>I can find plenty of information about manipulating an individual <code>docx</code> file with .NET Framework 3, but nothing easy or obvious about how you would merge files.  There is also a third-party program (Acronis.Words) that will do it, but it is prohibitively expensive.</p>

<h2>Update:</h2>

<p>Automating through Word has been suggested, but my code is going to be running on ASP.NET on an IIS web server, so going out to Word is not an option for me.  Sorry for not mentioning that in the first place.</p>

I wrote a little test app a while ago to do this. My test app worked with Word 2003 documents (.doc) not .docx, but I imagine the process is the same - I should think all you'd have to change is to use a newer version of the Primary Interop Assembly. This code would look a lot neater with the new C# 4.0 features...
```
```
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Microsoft.Office.Interop.Word;
using Microsoft.Office.Core;
using System.Runtime.InteropServices;
using System.IO;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            new Program().Start();
        }

        private void Start()
        {
            object fileName = Path.Combine(Environment.CurrentDirectory, @"NewDocument.doc");
            File.Delete(fileName.ToString());

            try
            {
                WordApplication = new ApplicationClass();
                var doc = WordApplication.Documents.Add(ref missing, ref missing, ref missing, ref missing);
                try
                {
                    doc.Activate();

                    AddDocument(@"D:\Projects\WordTests\ConsoleApplication1\Documents\Doc1.doc", doc, false);
                    AddDocument(@"D:\Projects\WordTests\ConsoleApplication1\Documents\Doc2.doc", doc, true);

                    doc.SaveAs(ref fileName,
                        ref missing, ref missing, ref missing, ref missing,     ref missing,
                        ref missing, ref missing, ref missing, ref missing, ref missing,
                        ref missing, ref missing, ref missing, ref missing, ref missing);
                }
                finally
                {
                    doc.Close(ref missing, ref missing, ref missing);
                }
            }
            finally
            {
                WordApplication.Quit(ref missing, ref missing, ref missing);
            }
        }

        private void AddDocument(string path, Document doc, bool lastDocument)
        {
            object subDocPath = path;
            var subDoc = WordApplication.Documents.Open(ref subDocPath, ref missing, ref missing, ref missing,
                ref missing, ref missing, ref missing, ref missing, ref missing,
                ref missing, ref missing, ref missing, ref missing, ref missing,
                ref missing, ref missing);
            try
            {

                object docStart = doc.Content.End - 1;
                object docEnd = doc.Content.End;

                object start = subDoc.Content.Start;
                object end = subDoc.Content.End;

                Range rng = doc.Range(ref docStart, ref docEnd);
                rng.FormattedText = subDoc.Range(ref start, ref end);

                if (!lastDocument)
                {
                    InsertPageBreak(doc);
                }
            }
            finally
            {
                subDoc.Close(ref missing, ref missing, ref missing);
            }
        }

        private static void InsertPageBreak(Document doc)
        {
            object docStart = doc.Content.End - 1;
            object docEnd = doc.Content.End;
            Range rng = doc.Range(ref docStart, ref docEnd);

            object pageBreak = WdBreakType.wdPageBreak;
            rng.InsertBreak(ref pageBreak);
        }

        private ApplicationClass WordApplication { get; set; }

        private object missing = Type.Missing;
    }
}
```
```