Realm primary key not getting stored - database

Question

Data Model:
public class Product : RealmObject
{
[PrimaryKey,JsonProperty(PropertyName = "productId")]
public int ProductId { get; set; }
[JsonProperty(PropertyName = "parentId")]
public int ParentId { get; set; }
[JsonProperty(PropertyName = "productType")]
public string ProductType { get; set; }
}
Method used to add product to db:
public void AddSampleProduct(Product product)
{
var _realm = Realm.GetInstance();
using (var transaction = _realm.BeginWrite())
{
var entry = _realm.Add(product);
transaction.Commit();
}
}
Method used to retrieve product:
public Product GetProductById(int id)
{
Product product = null;
var _realm = Realm.GetInstance();
product = _realm.Find<Product>(id);
var p = _realm.Find<Product>(id);
return product;
}
Method to fetch list of products:
public List<Product> GetProductList()
{
List<Product> productList = new List<Product>();
var _realm = Realm.GetInstance();
productList = _realm.All<Product>().ToList();
return productList;
}
Whenever I try to fetch a product using an id it returns null, at the same time if i try to fetch the list am getting all the products but with the primary key property having the value 0 for all the products. Probably the primary key is not getting stored and hence am not able to fetch individual product using id.
Also, in the model class I had tried defining the primary key and Json Property in separate lines like mentioned below.
public class Product : RealmObject
{
[PrimaryKey]
[JsonProperty(PropertyName = "productId")]
public int ProductId { get; set; }
[JsonProperty(PropertyName = "parentId")]
public int ParentId { get; set; }
[JsonProperty(PropertyName = "productType")]
public string ProductType { get; set; }
}