This example shows how C# implements merging multiple word documents, which is a very useful technique. Share to everybody for everybody reference.

Specific implementation methods are as follows:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Microsoft.Office.Interop.Word;
using System.Reflection;
using System.IO;
using System.Diagnostics;

namespace driverexam.WordReport
{
  public class WordDocumentMerger
  {
    private ApplicationClass objApp = null;
    private Document objDocLast = null;
    private Document objDocBeforeLast = null;
    public WordDocumentMerger()
    {
      objApp = new ApplicationClass();
    }
    #region  Open the file
    private void Open(string tempDoc)
    {
      object objTempDoc = tempDoc;
      object objMissing = System.Reflection.Missing.Value;

   objDocLast = objApp.Documents.Open(
      ref objTempDoc, //FileName
      ref objMissing, //ConfirmVersions
      ref objMissing, //ReadOnly
      ref objMissing, //AddToRecentFiles
      ref objMissing, //PasswordDocument
      ref objMissing, //PasswordTemplate
      ref objMissing, //Revert
      ref objMissing, //WritePasswordDocument
      ref objMissing, //WritePasswordTemplate
      ref objMissing, //Format
      ref objMissing, //Enconding
      ref objMissing, //Visible
      ref objMissing, //OpenAndRepair
      ref objMissing, //DocumentDirection
      ref objMissing, //NoEncodingDialog
      ref objMissing //XMLTransform
      );
      objDocLast.Activate();
    }
    #endregion

    #region  Save the file to the output template
    private void SaveAs(string outDoc)
    {
      object objMissing = System.Reflection.Missing.Value;
      object objOutDoc = outDoc;
      objDocLast.SaveAs(
      ref objOutDoc, //FileName
      ref objMissing, //FileFormat
      ref objMissing, //LockComments
      ref objMissing, //PassWord
      ref objMissing, //AddToRecentFiles
      ref objMissing, //WritePassword
      ref objMissing, //ReadOnlyRecommended
      ref objMissing, //EmbedTrueTypeFonts
      ref objMissing, //SaveNativePictureFormat
      ref objMissing, //SaveFormsData
      ref objMissing, //SaveAsAOCELetter,
      ref objMissing, //Encoding
      ref objMissing, //InsertLineBreaks
      ref objMissing, //AllowSubstitutions
      ref objMissing, //LineEnding
      ref objMissing //AddBiDiMarks
      );
    }
    #endregion

    #region  Loop merge multiple files (copy merge duplicate files)
    /// <summary>
    ///  Loop merge multiple files (copy merge duplicate files)
    /// </summary>
    /// <param name="tempDoc"> Template file </param>
    /// <param name="arrCopies"> Files that need to be merged </param>
    /// <param name="outDoc"> The merged output file </param>
    public void CopyMerge(string tempDoc, string[] arrCopies, string outDoc)
    {
      object objMissing = Missing.Value;
      object objFalse = false;
      object objTarget = WdMergeTarget.wdMergeTargetSelected;
      object objUseFormatFrom = WdUseFormattingFrom.wdFormattingFromSelected;
      try
      {
        // Open the template file
        Open(tempDoc);
        foreach (string strCopy in arrCopies)
        {
          objDocLast.Merge(
          strCopy, //FileName
          ref objTarget, //MergeTarget
          ref objMissing, //DetectFormatChanges
          ref objUseFormatFrom, //UseFormattingFrom
          ref objMissing //AddToRecentFiles
          );
          objDocBeforeLast = objDocLast;
          objDocLast = objApp.ActiveDocument;
          if (objDocBeforeLast != null)
          {
            objDocBeforeLast.Close(
            ref objFalse, //SaveChanges
            ref objMissing, //OriginalFormat
            ref objMissing //RouteDocument
            );
          }
        }
        // Save to an output file
        SaveAs(outDoc);
        foreach (Document objDocument in objApp.Documents)
        {
          objDocument.Close(
          ref objFalse, //SaveChanges
          ref objMissing, //OriginalFormat
          ref objMissing //RouteDocument
          );
        }
      }
      finally
      {
        objApp.Quit(
        ref objMissing, //SaveChanges
        ref objMissing, //OriginalFormat
        ref objMissing //RoutDocument
        );
        objApp = null;
      }
    }
    /// <summary>
    ///  Loop merge multiple files (copy merge duplicate files)
    /// </summary>
    /// <param name="tempDoc"> Template file </param>
    /// <param name="arrCopies"> Files that need to be merged </param>
    /// <param name="outDoc"> The merged output file </param>
    public void CopyMerge(string tempDoc, string strCopyFolder, string outDoc)
    {
      string[] arrFiles = Directory.GetFiles(strCopyFolder);
      CopyMerge(tempDoc, arrFiles, outDoc);
    }
    #endregion

    #region  Loop merge multiple files (insert merge file)
    /// <summary>
    ///  Loop merge multiple files (insert merge file)
    /// </summary>
    /// <param name="tempDoc"> Template file </param>
    /// <param name="arrCopies"> Files that need to be merged </param>
    /// <param name="outDoc"> The merged output file </param>
    public void InsertMerge(string tempDoc, string[] arrCopies, string outDoc)
    {
      object objMissing = Missing.Value;
      object objFalse = false;
      object confirmConversion = false;
      object link = false;
      object attachment = false;
      try
      {
        // Open the template file
        Open(tempDoc);
        foreach (string strCopy in arrCopies)
        {
          objApp.Selection.InsertFile(
          strCopy,
          ref objMissing,
          ref confirmConversion,
          ref link,
          ref attachment
          );
        }
        // Save to an output file
        SaveAs(outDoc);
        foreach (Document objDocument in objApp.Documents)
        {
          objDocument.Close(
          ref objFalse, //SaveChanges
          ref objMissing, //OriginalFormat
          ref objMissing //RouteDocument
          );
        }
      }
      finally
      {
        objApp.Quit(
        ref objMissing, //SaveChanges
        ref objMissing, //OriginalFormat
        ref objMissing //RoutDocument
        );
        objApp = null;
      }
    }
    /// <summary>
    ///  Loop merge multiple files (insert merge file)
    /// </summary>
    /// <param name="tempDoc"> Template file </param>
    /// <param name="arrCopies"> Files that need to be merged </param>
    /// <param name="outDoc"> The merged output file </param>
    public void InsertMerge(string tempDoc, string strCopyFolder, string outDoc)
    {
      string[] arrFiles = Directory.GetFiles(strCopyFolder);
      InsertMerge(tempDoc, arrFiles, outDoc);
    }
    #endregion
  }
}

I believe that this article has a certain reference value for your C# programming.