wpf listcollectionview grouping by multiple columns and calculating the amount and quantity

public class ProductSummary
{
    public int ProductId { get; set; }
    public string ProductName { get; set; }
    public double Amount { get; set; }
    public double Quantity { get; set; }
}

private List<Product> products;
public List<Product> Products
{
    get { return products; }
    set { products = value; RaisePropertyChanged("Products"); }
}

public ListCollectionView ProductsCollectionView { get; set; }

private void Grouping()
{
    ProductsCollectionView = new ListCollectionView(Products);
    ProductsCollectionView.GroupDescriptions.Add(new PropertyGroupDescription("CategoryId"));
    ProductsCollectionView.GroupDescriptions.Add(new PropertyGroupDescription("TypeId"));
    ProductsCollectionView.GroupDescriptions.Add(new PropertyGroupDescription("VendorId"));

    ProductsCollectionView.CustomSort = new ProductComparer();

    ProductsCollectionView.GroupSummary.Add(new GroupSummary<Product, ProductSummary>()
    {
        PropertySelector = (x