if (variable == null)
{
   variable = expression; // C# 1..7
}

variable ??= expression; // C# 8

U+202A:   LEFT-TO-RIGHT EMBEDDING (LRE) 
U+202B:   RIGHT-TO-LEFT EMBEDDING (RLE) 
U+202D:   LEFT-TO-RIGHT OVERRIDE (LRO) 
U+202E:   RIGHT-TO-LEFT OVERRIDE (RLO) 
U+202C:   POP DIRECTIONAL FORMATTING (PDF) 

using System;
using System.Diagnostics;
using System.IO;
using iTextSharp.text;
using iTextSharp.text.pdf;

namespace RleTests
{
    class Program
    {
        const char RightToLeftEmbedding = (char)0x202B;
        const char PopDirectionalFormatting = (char)0x202C;

        static string FixWeakCharacters(string data)
        {
            if (string.IsNullOrWhiteSpace(data)) return string.Empty;
            var weakCharacters = new[] { @"\", "/", "+", "-", "=", ";", "$" };
            foreach (var weakCharacter in weakCharacters)
            {
                data = data.Replace(weakCharacter, RightToLeftEmbedding + weakCharacter + PopDirectionalFormatting);
            }
            return data;
        }

        static void Main(string[] args)
        {
            using (var pdfDoc = new Document(PageSize.A4))
            {
                PdfWriter.GetInstance(pdfDoc, new FileStream("Test.pdf", FileMode.Create));
                pdfDoc.Open();

                FontFactory.Register("c:\\windows\\fonts\\Arial.ttf");
                Font tahoma = FontFactory.GetFont("Arial", BaseFont.IDENTITY_H);

                var table1 = new PdfPTable(1);
                table1.WidthPercentage = 100;

                var pdfCell = new PdfPCell
                {
                    RunDirection = PdfWriter.RUN_DIRECTION_RTL,
                    Border = 0,
                    Phrase = new Phrase(FixWeakCharacters(
                        "تاریخ: " + "1390/11/18" + Environment.NewLine +
                        "شماره پروژه: " + "1/2/3/4/56" + Environment.NewLine +
                        "اسلش: " + " 12/A/13 " + Environment.NewLine +
                        "بک اسلش: " + "  12\\13\\14 " + Environment.NewLine +
                        "مساوی و جمع: " + " 2+3=5 " + Environment.NewLine +
                        "سمی کولون: " + " 2=1+1; " + Environment.NewLine +
                        "دلار: " + "12$" + Environment.NewLine +
                        "کاما: " + "12,34,67" + Environment.NewLine +
                        "نقطه: " + "12.34" + Environment.NewLine +
                        "پرانتز: " + "متن (ساده)"
                        ),
                        tahoma)
                };

                table1.AddCell(pdfCell);
                pdfDoc.Add(table1);

            }

            Process.Start("Test.pdf");
        }
    }
}

<span dir="ltr" style="display:inline">1390/11/19</span>