using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

public class Product
{
  public string ProductDetails { get; set; }
  public Dictionary<string, string> ProductDetailList
  {
    get
    {
      if (string.IsNullOrWhiteSpace(ProductDetails))
      {
        return new Dictionary<string, string>();
      }
      try
      {
        var obj = JToken.Parse(ProductDetails);
      }
      catch (Exception)
      {
        throw new FormatException("ProductDetails不符合json格式.");
      }
      return JsonConvert.DeserializeObject<Dictionary<string, string>>(ProductDetails);
    }
  }
}

public void Main(string[] args)
{
  var product = new Product();
  product.ProductDetails = "{'size':'10', 'weight':'10kg'}";

  foreach(var item in product.ProductDetailList)
  {
    Console.WriteLine(item.Key + " " + item.Value);
  }

  Console.Read();
}