public static IEnumerable<Linq7CategoryGroup> Linq7(IEnumerable<Product> products)
        {
            _ = products ?? throw new ArgumentNullException();

            /* example of Linq7result

             category - Beverages
                UnitsInStock - 39
                    price - 18.0000
                    price - 19.0000
                UnitsInStock - 17
                    price - 18.0000
                    price - 19.0000
             */

            var result = products.GroupBy(category => category.Category)
               .Select(grp => (
               grp.Key,
               grp.GroupBy(uni => uni.UnitsInStock)
               .Select(stock =>
               (
               stock.Key,
               stock.OrderBy(price => price.UnitPrice)
               )
               )));

            return (IEnumerable<Linq7CategoryGroup>)result;
        }

IEnumerable<Linq7CategoryGroup>

var r = products.GroupBy(product => product.Category, (category, element) => (
                category,
                element.GroupBy(item => item.UnitsInStock, (units, products) =>
                (
                    units,
                    products.OrderBy(t => t.UnitPrice).Select(t => t.UnitPrice)
                ))
            ));

public class Linq7CategoryGroup
    {
        public string Category { get; set; }
        public IEnumerable<Linq7UnitsInStockGroup> UnitsInStockGroup { get; set; }
    }

    public class Linq7UnitsInStockGroup
    {
        public int UnitsInStock { get; set; }
        public IEnumerable<decimal> Prices { get; set; }
    }

public T Get<T>()
{
    throw new NotImplementedException();
}

public void AddTypeTandTBase_Import_ReturnCustomerBL2()
{
    c.AddType(typeof(cDAL), typeof(IcDAL));
    c.AddType(typeof(Logger));
    c.AddType(typeof(cBLL2));

    var сBLL = c.Get<cBLL2>();

    Assert.That(сBLL, Is.Not.Null);
    Assert.That(сBLL, Is.InstanceOf<cBLL2>());
    Assert.That(сBLL.cDAL, Is.Not.Null);
    Assert.That(сBLL.cDAL, Is.InstanceOf<IcDAL>());
    Assert.That(сBLL.Logger, Is.Not.Null);
    Assert.That(сBLL.Logger, Is.InstanceOf<Logger>());
}

public void AddType(Type type)
{
    if(type == null)
    {
        new ArgumentNullException();
    }
}

public void AddType(Type type, Type baseType)
{
     
}

public T Get<T>()
{
    throw new NotImplementedException();
}

Predicate<Product> predicate

public static int IndexOf(Product[] products, Predicate<Product> predicate)
        {
                    var product = products[i];
                    if (predicate(product)){ //завжди false
                    //do something
                    }

public void Test()
        {
            var products = new Product[]
            {
                new Product("Product 3", 30.0d),
            };
            var productToFind = new Product("Product 3", 30.0d);
 
            int index = Utilities.IndexOf(products, product => product.Equals(productToFind));