query = query.OrderBy(x => x."ProductId");

System.Reflection.PropertyInfo prop = typeof(YourType).GetProperty("PropertyName");

query = query.OrderBy(x => prop.GetValue(x, null));

PropertyDescriptor prop = TypeDescriptor.GetProperties(typeof(YourType)).Find("PropertyName");

query = query.OrderBy(x => prop.GetValue(x));

query = query.OrderBy(x => x.GetType().GetProperty("ProductId").GetValue(x, null));

typeof(YourType).GetProperty("ProductId").GetValue(theInstance);

query = query.OrderBy(x => x.GetType().GetProperty("ProductId").GetValue(x, null));

if (sortBy == "ProductId")
   query.OrderBy(x => x.ProductId);
else
   query.OrderBy(x => x.ProductName);