test.cs

public IActionResult Index() {

  // load the JSON data
  string jsonData = System.IO.File.ReadAllText("App_Data/data.json");

  using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) {
    tx.Create();

    // load the template
    tx.Load("App_Data/template.tx", TXTextControl.StreamType.InternalUnicodeFormat);

    // use MailMerge to merge data
    using (TXTextControl.DocumentServer.MailMerge mm =
          new TXTextControl.DocumentServer.MailMerge()) {
      mm.TextComponent = tx;
      mm.MergeJsonData(jsonData);
    }

    // return result as HTML
    string result = "";
    tx.Save(out result, TXTextControl.StringStreamType.HTMLFormat);

    ViewBag.Document = result;
  }

  return View();
}