How do i get transaction id from Paypal using mvc 4 c#

Question

How do i get transaction id from Paypal using mvc 4 c#. I have tried a lot of the code but not working. Here is the sample of the code:

in web.config: 

<add key="CancelPurchaseUrl" value="http://localhost:5799/Unit/Index" />
<add key="ReturnUrl" value="http://localhost:5799/Unit/GetResponse" />
<add key="NotifyUrl" value="http://localhost:5799/Unit/GetResponse" />
<add key="CurrencyCode" value="USD" />
<add key="UseSandbox" value="true" />
<add key="SendToReturnURL" value="true" />

Here is the code:

public ActionResult GetResponse()
        {

try
            {
                //ID, TransactionID, TransactionDate, Amount, UnitID, UserEmail, TransactionType
                string TransactionID = "NO";
                string TransactionDate = System.DateTime.Now.ToString();
                string UnitID = Session["UnitID"].ToString();
                string Amount = Session["payamount"].ToString();
                string UserEmail = Session["UserEmailMain"].ToString();
                string TransactionType = "Paypal";
                string Quantity = Session["Quantity"].ToString();

                PayPalModel model = new PayPalModel();
                model.TransactionID = TransactionID;
                model.TransactionDate = TransactionDate;
                model.UnitId = UnitID;
                model.Amount = Amount;
                model.UserEmail = UserEmail;
                model.TransactionType = TransactionType;
                model.Quantity = Quantity;

                Transaction items = new Transaction();
                items.TransactionID = model.TransactionID;
                items.TransactionDate = Convert.ToDateTime(model.TransactionDate);
                items.UnitID = Convert.ToInt32(model.UnitId);
                items.Amount = model.Amount;
                items.UserEmail = model.UserEmail;
                items.TransactionType = model.TransactionType;
                items.Quantity = Convert.ToInt32(model.Quantity);

                //int insertresult = objAdmin.InsertTransaction(items);
            }
            catch { }
            return RedirectToAction("Index");
        }

here i want to return the transactionid ..

so any suggestions???