Can not deserialize a javascript array of object into C# array

First thing you need to check if your JSON is correct. HFDiscount.Value property should contain something like that (structure, not values):

[  
   {  
      "productCode":"535854302-3",
      "discount":11
   },
   {  
      "productCode":"905491934-5",
      "discount":6
   }
]

For deserialization I suggest you to use JSON.Net (what was your first attempt). You don't need to have Serialization attribute and default constructor for your model:

public class RefDiscountPair
{
    public string ProductCode { get; set; }
    public int Discount { get; set; }
}

Now you can easily deserialize JSON:

var refDiscountPairs = 
    JsonConvert.DeserializeObject<IList<RefDiscountPair>>(HFDiscount.Value);

EDIT: You have some problems with your JSON. I created a simple demo and it worked fine. Model I used:

public class IndexModel
{
    public string HFDiscount { get; set; }
}

Cshtml:

@using (Html.BeginForm("Index", "Home"))
{
    @Html.HiddenFor(m => m.HFDiscount)
    <button type="submit">Submit</button>
}

Javascript:

var tabDiscount = [];
for (var i = 0; i < 2; i++) {
    var refdiscountpair = { productCode: "code" + i, discount: i }
    tabDiscount.push(refdiscountpair);
}
$("#HFDiscount").val(JSON.stringify(tabDiscount));

And controller method:

[HttpPost]
 public ActionResult Index(IndexModel model)
 {
     var refDiscountPairs = JsonConvert.DeserializeObject<IList<RefDiscountPair>>(model.HFDiscount);
     return View();
 }

EDIT: It turns out that you are actually using pure ASP.NET. I tried it too and it works well.

ASPX:

<asp:HiddenField ID="HFDiscount" ClientIDMode="Static" runat="server" />
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_OnClick" />

Codebehind:

protected void Button1_OnClick(object sender, EventArgs e)
{
    var refDiscountPairs =
        JsonConvert.DeserializeObject<IList<RefDiscountPair>>(HFDiscount.Value);
}

You need to verify all steps and find the place where JSON gets corrupted. Start from inspecting hidden field value in browser.

You don't need multiple deserialize, only try to convert it to list of object:

js.Deserialize<List<RefDiscountPair>>(HFDiscount.Value));

But make sure that JSON string in hiddenfield is right one. If you get any exception in this case, then should try to fix the JSON string in javascript, or remove some additional character in your code before desrialize it

EDIT:

Definitely your JSON string is not correct, you have to change it. You can do in JS code or in C#. If you want to fix it in code behind you can use below code:

var jsonStr = "\"[{\\\"productCode\\\": \\\"1111111\\\", \\\"discount\\\": \\\"5\\\"}, {\\\"productCode\\\": \\\"2222222\\\", \\\"discount\\\": \\\"5\\\"}]\"";
if (jsonStr.StartsWith("\""))
    jsonStr = jsonStr.Substring(1);
if(jsonStr.EndsWith("\""))
    jsonStr = jsonStr.Substring(0 , jsonStr.Length-1);    

jsonStr = jsonStr.Replace("\\", "");

var serializer = new JavaScriptSerializer();

serializer.Deserialize<List<RefDiscountPair>>(jsonStr);

I tested this and it's working fine

   string jsonInput = "Your json STRING";

                    Newtonsoft.Json.Linq.JContainer resp = (Newtonsoft.Json.Linq.JContainer)JsonConvert.DeserializeObject(jsonInput);
                    foreach (JObject str in resp)
                    {
                        JsonSerializer serializer = new JsonSerializer();

                        YourObject p = (YourObject)serializer.Deserialize(new JTokenReader(str), typeof(YourObject));
                        YourObjectAsListOfObject.Add(p);
                    }

You have some issues with your json.

Here is a working example : Demo.

Code :

// you can generate your class with http://json2csharp.com/
public class RefDiscountPair
{
    public string ProductCode { get; set; }
    public int Discount { get; set; }
}

public void Main()
{
    string json = "[{\"productCode\": \"1111111\", \"discount\": \"5\"}, {\"productCode\": \"2222222\", \"discount\": \"5\"}]";

    var m = JsonConvert.DeserializeObject<List<RefDiscountPair>>(json);

    Console.WriteLine(m[0].Discount);
}