[DefaultMemberAttribute("Item")] 
[SerializableAttribute()] 
public class ProductGroupFieldValueCollection : System.Collections.ObjectModel.Collection<ProductGroupFieldValue> 

class RenderProduct
{
   public void RenderProductGroupCustomFields(ProductGroupFieldValueCollection ProductGroupFieldValues)
   {
       ProductFieldValueCollection pfvCol = new ProductFieldValueCollection();

       foreach (ProductGroupFieldValue pgfv in ProductGroupFieldValues)
       {
           ProductField pf = new ProductField();
           ProductFieldValue pfv = new ProductFieldValue(pf, pgfv.Value);
 
           pfv.ProductField.Types = pgfv.ProductGroupField.Types;
           pfv.ProductField.TemplateName = pgfv.ProductGroupField.TemplateName;
           pfv.ProductField.SystemName = pgfv.ProductGroupField.SystemName;
           pfv.Value = pgfv.Value;

           pfvCol.Add(pfv);
       }

       if (pfvCol.Count > 0)
       {
           //TODO: insert your code here
       }
   }
}