using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<ProdSize> size1 = new List<ProdSize>

            {

                new ProdSize { size = "M", stock =1 },

                new ProdSize { size = "S", stock =0 },

            };

​

            List<ProdSize> size2 = new List<ProdSize>

            {

                new ProdSize { size = "M", stock =0 },

                new ProdSize { size = "S", stock =1 },

            };

​

            List<ProdSize> size3 = new List<ProdSize>

            {

                new ProdSize { size = "M", stock =1 },

                new ProdSize { size = "S", stock =1 },

            };

​

            List<Product> productList = new List<Product>

            {

                new Product { productRef = "A", sizes = size1 },

                new Product { productRef = "B", sizes = size2 },

                new Product { productRef = "C", sizes = size3 },

            };

        var FinalList = productList.SelectMany(x => x.sizes, (prodObj, Sizes) => new { prodObj.productRef, Sizes })

                                       .Where(x => x.Sizes.size == "M" && x.Sizes.stock > 0)

                                       .Select(x => new

                                                    {

                                                        ProductName = x.productRef,

                                                        Size = x.Sizes.size,

                                                        Stock = x.Sizes.stock

                                                    }).ToList();

        foreach (var item in FinalList)

            {

                Console.WriteLine();

                Console.WriteLine("ProductName: {0}", item.ProductName);

                Console.WriteLine("Size: {0}", item.Size);

                Console.WriteLine("Stock: {0}", item.Stock);

            }

    }

}

​

public class Product

    {

        public string productRef  { get; set; }

        public int totalStock  { get; set; }

        public List<ProdSize> sizes  { get; set; }

    }

​

    public class ProdSize

    {

        public int stock  { get; set; }

        public string size  { get; set; }

    }