Why I Am Not Getting Output In My Textbox?

public class diamondController : Controller
   {
       public ActionResult Diamond()
       {
           DiamondModel model = new DiamondModel();
           return View(model);

       }
       [HttpGet]
       public ActionResult Sum()
       {
           DiamondModel model = new DiamondModel();

           return View(model);
       }
       [HttpPost]
       public ActionResult Diamond(string button, DiamondModel model)
       {
           if (model.textBox.Contains("4") && button == "Submit")
           {
               model.textBox1 = "I love Pakistan";

               model.n = Convert.ToInt32(model.textBox);

               for (int i = 1; i <= model.n; i++)
               {
                   for (int j = 0; j < (model.n - i); j++)
                       model.textBox1 = ("");
                   for (int j = 1; j <= i; j++)
                       model.textBox1 = ("*");
                   for (int k = 1; k < i; k++)
                       model.textBox1 = ("*");

               }

               for (int i = model.n - 1; i >= 1; i--)
               {
                   for (int j = 0; j < (model.n - i); j++)
                       model.textBox1 = (" ");
                   for (int j = 1; j <= i; j++)
                       model.textBox1 = ("*");
                   for (int k = 1; k < i; k++)
                       model.textBox1 = ("*");

               }

           }
               }
           }
           return View(model);


       }

   }