Remove AsQueryable() from left join code sample

[roji]

The LINQ join operations doc page has a section on left outer join, showing the following snippet for method syntax:

var query = students.GroupJoin(departments, student => student.DepartmentID, department => department.ID,
    (student, departmentList) => new { student, subgroup = departmentList.AsQueryable() })
    .SelectMany(joinedSet => joinedSet.subgroup.DefaultIfEmpty(), (student, department) => new
    {
        student.student.FirstName,
        student.student.LastName,
        Department = department.Name
    });

The AsQueryable() in the sample isn't needed, and is the source of some pretty dramatic perf overhead - this PR removes that (and reformats the code a bit for better readability). Note dotnet/runtime#110292, which is a proposal for adding a dedicated LeftJoin operator in .NET 10, which would replace the GroupJoin/SelectMany construct documented here.

BenchmarkDotNet v0.14.0, macOS Sequoia 15.1.1 (24B91) [Darwin 24.1.0]
Apple M2 Max, 1 CPU, 12 logical and 12 physical cores
.NET SDK 9.0.100
  [Host]     : .NET 9.0.0 (9.0.24.52809), Arm64 RyuJIT AdvSIMD
  DefaultJob : .NET 9.0.0 (9.0.24.52809), Arm64 RyuJIT AdvSIMD

Method	InnersPerOuter	OuterCount	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Gen2	Allocated	Alloc Ratio
LeftJoin	1	1	97.82 ns	0.292 ns	0.244 ns	0.001	0.00	0.0573	-	-	480 B	0.07
GroupJoin_SelectMany	1	1	67,935.22 ns	440.050 ns	411.623 ns	1.000	0.01	0.7324	0.3662	-	6735 B	1.00
LeftJoin	1	10	490.78 ns	1.131 ns	1.003 ns	0.001	0.00	0.2031	-	-	1704 B	0.03
GroupJoin_SelectMany	1	10	674,969.93 ns	4,314.797 ns	3,824.955 ns	1.000	0.01	6.8359	2.9297	-	63527 B	1.00
LeftJoin	1	100	4,791.48 ns	28.291 ns	22.088 ns	0.001	0.00	1.7090	0.0534	-	14344 B	0.02
GroupJoin_SelectMany	1	100	6,824,534.62 ns	36,758.120 ns	32,585.116 ns	1.000	0.01	70.3125	31.2500	-	631926 B	1.00
LeftJoin	1	1000	47,687.53 ns	124.164 ns	116.144 ns	0.001	0.00	16.2964	3.4790	-	136728 B	0.02
GroupJoin_SelectMany	1	1000	67,837,071.04 ns	451,666.644 ns	400,390.716 ns	1.000	0.01	750.0000	375.0000	-	6313457 B	1.00
LeftJoin	10	1	293.74 ns	0.499 ns	0.442 ns	0.004	0.00	0.1316	-	-	1104 B	0.15
GroupJoin_SelectMany	10	1	68,395.18 ns	594.304 ns	555.913 ns	1.000	0.01	0.8545	0.3662	-	7504 B	1.00
LeftJoin	10	10	2,598.17 ns	17.950 ns	16.791 ns	0.004	0.00	0.9460	0.0076	-	7944 B	0.11
GroupJoin_SelectMany	10	10	678,496.09 ns	7,117.783 ns	6,657.979 ns	1.000	0.01	7.8125	3.9063	-	69776 B	1.00
LeftJoin	10	100	30,906.21 ns	30.591 ns	27.118 ns	0.005	0.00	9.1553	0.7324	-	76744 B	0.11
GroupJoin_SelectMany	10	100	6,844,961.17 ns	27,512.584 ns	22,974.254 ns	1.000	0.00	78.1250	39.0625	-	694356 B	1.00
LeftJoin	10	1000	377,067.26 ns	1,734.244 ns	1,622.213 ns	0.006	0.00	90.8203	30.2734	-	760728 B	0.11
GroupJoin_SelectMany	10	1000	68,429,902.38 ns	579,735.477 ns	542,284.925 ns	1.000	0.01	714.2857	285.7143	-	6934177 B	1.00
LeftJoin	100	1	1,849.73 ns	4.218 ns	3.739 ns	0.03	0.00	0.6981	0.0019	-	5848 B	0.48
GroupJoin_SelectMany	100	1	68,215.49 ns	611.444 ns	510.583 ns	1.00	0.01	1.3428	0.6104	-	12104 B	1.00
LeftJoin	100	10	18,371.97 ns	367.054 ns	306.507 ns	0.03	0.00	6.5918	0.2747	-	55384 B	0.47
GroupJoin_SelectMany	100	10	684,697.64 ns	6,704.490 ns	5,598.553 ns	1.00	0.01	13.6719	6.8359	-	117230 B	1.00
LeftJoin	100	100	202,379.73 ns	3,950.849 ns	4,227.364 ns	0.03	0.00	65.6738	15.8691	-	551144 B	0.47
GroupJoin_SelectMany	100	100	6,863,089.50 ns	31,329.779 ns	26,161.785 ns	1.00	0.01	125.0000	62.5000	-	1168687 B	1.00
LeftJoin	100	1000	2,600,306.48 ns	4,152.430 ns	3,467.467 ns	0.04	0.00	656.2500	316.4063	-	5504731 B	0.47
GroupJoin_SelectMany	100	1000	71,509,367.47 ns	558,147.996 ns	522,091.982 ns	1.00	0.01	1285.7143	714.2857	-	11679688 B	1.00
LeftJoin	1000	1	17,042.46 ns	322.777 ns	301.926 ns	0.20	0.00	5.8594	0.1221	-	49056 B	0.89
GroupJoin_SelectMany	1000	1	86,478.90 ns	914.553 ns	855.473 ns	1.00	0.01	6.5918	2.1973	-	55318 B	1.00
LeftJoin	1000	10	170,578.63 ns	329.190 ns	291.819 ns	0.20	0.00	58.1055	11.9629	-	487464 B	0.89
GroupJoin_SelectMany	1000	10	853,239.40 ns	2,109.237 ns	1,761.309 ns	1.00	0.00	64.4531	25.3906	-	549354 B	1.00
LeftJoin	1000	100	1,937,489.58 ns	5,986.665 ns	4,999.136 ns	0.22	0.00	582.0313	251.9531	-	4871945 B	0.89
GroupJoin_SelectMany	1000	100	8,938,502.84 ns	50,181.279 ns	46,939.600 ns	1.00	0.01	656.2500	312.5000	-	5490176 B	1.00
LeftJoin	1000	1000	40,889,193.74 ns	241,213.807 ns	213,829.757 ns	0.36	0.01	5900.0000	1700.0000	100.0000	48712871 B	0.89
GroupJoin_SelectMany	1000	1000	112,797,755.31 ns	1,643,320.304 ns	2,653,656.189 ns	1.00	0.03	6333.3333	1666.6667	-	54892131 B	1.00

Benchmark code

[MemoryDiagnoser]
public class AsQueryableBenchmarks
{
    [Params(1, 10, 100, 1000, Priority = 1)]
    public int InnersPerOuter { get; set; }

    [Params(1, 10, 100, 1000, Priority = 2)]
    public int OuterCount { get; set; }

    private Outer[] _outers = null!;
    private Inner[] _inners = null!;

    class Outer
    {
        public int Id { get; set; }
        public string? OuterPayload { get; set; }
    }

    class Inner
    {
        public int Id { get; set; }
        public int OuterId { get; set; }
        public string? InnerPayload { get; set; }
    }

    private const int RandomSeed = 42;

    [GlobalSetup]
    public void Setup()
    {
        _outers = new Outer[OuterCount];
        _inners = new Inner[OuterCount * InnersPerOuter];

        var remainingInners = new List<Inner>(OuterCount * InnersPerOuter);
        for (var outerId = 0; outerId < OuterCount; outerId++)
        {
            _outers[outerId] = new Outer { Id = outerId, OuterPayload = $"Outer{outerId}" };

            for (var j = 0; j < InnersPerOuter; j++)
            {
                var innerId = outerId * j + j;
                remainingInners.Add(new Inner { Id = innerId, OuterId = outerId, InnerPayload = $"Inner{innerId}" });
            }
        }

        var random = new Random(RandomSeed);

        for (var i = 0; i < _inners.Length; i++)
        {
            var j = random.Next(0, remainingInners.Count);
            _inners[i] = remainingInners[j];
            remainingInners.RemoveAt(j);
        }

        Debug.Assert(remainingInners.Count == 0);
    }

    [Benchmark(Baseline = true)]
    public int GroupJoin_SelectMany_with_AsQueryable()
        => _outers
            .GroupJoin(_inners, o => o.Id, i => i.OuterId, (o, inners) => new { Outer = o, Inners = inners.AsQueryable() })
            .SelectMany(
                joinedSet => joinedSet.Inners.DefaultIfEmpty(),
                (o, i) => new
                {
                    o.Outer.OuterPayload,
                    i?.InnerPayload
                })
            .Count();

    [Benchmark]
    public int GroupJoin_SelectMany_without_AsQueryable()
        => _outers
            .GroupJoin(_inners, o => o.Id, i => i.OuterId, (o, inners) => new { Outer = o, Inners = inners })
            .SelectMany(
                joinedSet => joinedSet.Inners.DefaultIfEmpty(),
                (o, i) => new
                {
                    o.Outer.OuterPayload,
                    i?.InnerPayload
                })
            .Count();
}

/cc @jeffhandley @dotnet/area-system-linq @dotnet/efteam

[BillWagner]

Thanks for making this fix.

I'll now.