LINQ 全联接 FULL JOIN 扩展源代码（一）

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace CRM.Web.TopTaobao.Extensions
{
    internal static class FullOuterExtensions
    {
        public static IDictionary<TK, IEnumerable<TV>> ToDictionary<TK, TV>(this IEnumerable<IGrouping<TK, TV>> grouping)
        {
            return grouping.ToDictionary(g => g.Key, g => g.AsEnumerable());
        }

        internal static IEnumerable<TV> OuterGet<TK, TV>(this IDictionary<TK, IEnumerable<TV>> dict, TK k)
        {
            IEnumerable<TV> result;
            if (!dict.TryGetValue(k, out result))
                yield break;

            foreach (var v in result)
                yield return v;
        }

        internal static IList<TR> FullOuterGroupJoin<TA, TB, TK, TR>(
            this IEnumerable<TA> a, 
            IEnumerable<TB> b,
            Func<TA, TK> selectKeyA, 
            Func<TB, TK> selectKeyB, 
            Func<IEnumerable<TA>, IEnumerable<TB>, TK, TR> projection)
        {
            var adict = a.GroupBy(selectKeyA).ToDictionary();
            var bdict = b.GroupBy(selectKeyB).ToDictionary();

            var keys = new HashSet<TK>(adict.Keys);
            keys.UnionWith(bdict.Keys);

            var join = from key in keys
                       let xa = adict.OuterGet(key)
                       let xb = bdict.OuterGet(key)
                       select projection(xa, xb, key);

            return join.ToList();
        }

        internal static IList<TR> FullOuterJoin<TA, TB, TK, TR>(
            this IEnumerable<TA> a,
            IEnumerable<TB> b,
            Func<TA, TK> selectKeyA, 
            Func<TB, TK> selectKeyB,
            Func<TA, TB, TK, TR> projection,
            TA defaultA = default(TA), TB defaultB = default(TB))
        {
            var adict = a.GroupBy(selectKeyA).ToDictionary();
            var bdict = b.GroupBy(selectKeyB).ToDictionary();

            var keys = new HashSet<TK>(adict.Keys);
            keys.UnionWith(bdict.Keys);

            var join = from key in keys
                       from xa in adict.OuterGet(key).DefaultIfEmpty(defaultA)
                       from xb in bdict.OuterGet(key).DefaultIfEmpty(defaultB)
                       select projection(xa, xb, key);

            return join.ToList();
        }
    }
}

class Product
    {
        public string Name { get; set; }
        public int CategoryID { get; set; }
    }

    class Category
    {
        public string Name { get; set; }
        public int ID { get; set; }
    }

void FullOuterJoinEx()
        {
            // Specify the first data source.
            List<Category> categories = new List<Category>()
            { 
                new Category(){Name="Beverages", ID=001},
                new Category(){ Name="Condiments", ID=002},
                new Category(){ Name="Vegetables", ID=003},
                new Category() {  Name="Grains", ID=004},
                new Category() {  Name="Fruit", ID=005}            
            };

            // Specify the second data source.
            List<Product> products = new List<Product>()
            {
                new Product{Name="Cola",  CategoryID=001},
                new Product{Name="Tea",  CategoryID=001},
                new Product{Name="Mustard", CategoryID=002},
                new Product{Name="Pickles", CategoryID=002},
                //new Product{Name="Carrots", CategoryID=003},
                //new Product{Name="Bok Choy", CategoryID=003},
                new Product{Name="Peaches", CategoryID=005},
                new Product{Name="Melons", CategoryID=005},
                new Product{Name="Carrots222", CategoryID=006},
            };

            var query111 = categories.FullOuterGroupJoin(products, a => a.ID, b => b.CategoryID, (a, b, id) => new { a, b }).ToList();
            var query222 = categories.FullOuterJoin(products, a => a.ID, b => b.CategoryID,
            (a, b, ID) => new { a, b },
            new Category { ID = -1, Name = "(no firstname)" },
            new Product { CategoryID = -2, Name = "(no surname)" }).ToList();
            //var ax = new[] { 
            //new { id = 1, name = "John" },
            //new { id = 2, name = "Sue" } };
            //var bx = new[] { 
            //new { id = 1, surname = "Doe" },
            //new { id = 3, surname = "Smith" } };

            //ax.FullOuterGroupJoin(bx, a => a.id, b => b.id, (a, b, id) => new { a, b })
            //    .ToList().ForEach(Console.WriteLine);

            //ax.FullOuterJoin(bx, a => a.id, b => b.id,
            //(a, b, id) => new { a.name, b.surname },
            //new { id = -1, name = "(no firstname)" },
            //new { id = -2, surname = "(no surname)" }).ToList().ForEach(Console.WriteLine);
        }