test.cs

namespace MyClassLibrary
{
    public static class Class1
    {
        static Class1()
        {
            TXTextControl.ServerTextControl.EntryAssembly =
                typeof(Class1).Assembly;
        }

        public static string GetHtml()
        {
            using (TXTextControl.ServerTextControl tx = 
                new TXTextControl.ServerTextControl())
            {
                tx.Create();

                TXTextControl.Selection selection = 
                    new TXTextControl.Selection();
                selection.Text = "Hello, World!";
                selection.Start = 0;
                selection.Length = 5;
                selection.Bold = true;

                tx.Selection = selection;

                var html = "";
                tx.Save(out html, TXTextControl.StringStreamType.HTMLFormat);

                return html;
            }
        }
    }
}