Merge documents

VSTO
Spire.Doc
Download Sample Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml.Linq;
using Word = Microsoft.Office.Interop.Word;
using Office = Microsoft.Office.Core;
using Microsoft.Office.Tools.Word;

namespace VSTO
{
    public partial class ThisAddIn
    {
        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {   
            //Open document
            Word.Application wordApp = Application;
            wordApp.Documents.Open("../../Sample.docx");

            wordApp.Selection.WholeStory();
            wordApp.Selection.EndOf();           
            wordApp.Selection.Range.InsertBreak();

            //Merge the file
            wordApp.Selection.InsertFile("../../Table.docx");
           
        }

        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {
        }

        #region VSTO generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
        }
        
        #endregion
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Spire.Doc;

namespace Spire.Doc
{
    class Program
    {
        static void Main(string[] args)
        {
            //Load source document
            Document srcDoc = new Document("../../Sample.docx");

            //Insert the new document at the end of the source document
            srcDoc.InsertTextFromFile("../../Table.docx", FileFormat.Docx);

            //Save and Launch
            srcDoc.SaveToFile("Des.docx");
            System.Diagnostics.Process.Start("Des.docx");
        }
    }
}

MergeDocument.zip