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Just finished reading the huge Performance Improvements in .NET 10 article.

There’s a ton of interesting stuff in there, and I might eventually write a “small” summary of it. But for now, here’s a new #random_PoK (random Piece of Knowledge) :

Finally .NET has dedicated LeftJoin and RightJoin LINQ methods!

No more monstrous GroupJoin + SelectMany + DefaultIfEmpty constructs — and for LINQ to Objects, it’s even faster:

[MemoryDiagnoser(displayGenColumns: false)]
[HideColumns("Job", "Error", "StdDev", "Median", "RatioSD")]
public partial class Tests
{
    private IEnumerable<int> Outer { get; } = Enumerable.Sequence(0, 1000, 2);
    private IEnumerable<int> Inner { get; } = Enumerable.Sequence(0, 1000, 3);

    [Benchmark(Baseline = true)]
    public int LeftJoin_Manual() =>
        ManualLeftJoin(Outer, Inner, o => o, i => i, (o, i) => o + i).Count();

    [Benchmark]
    public int LeftJoin_Linq() =>
        Outer.LeftJoin(Inner, o => o, i => i, (o, i) => o + i).Count();

    private static IEnumerable<TResult> ManualLeftJoin<TOuter, TInner, TKey, TResult>(
        IEnumerable<TOuter> outer, IEnumerable<TInner> inner,
        Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector,
        Func<TOuter, TInner?, TResult> resultSelector) =>
        outer
        .GroupJoin(inner, outerKeySelector, innerKeySelector, (o, inners) => (o, inners))
        .SelectMany(x => x.inners.DefaultIfEmpty(), (x, i) => resultSelector(x.o, i));
}

|Method         |Mean    |Ratio|Allocated|Alloc Ratio|
|---------------|--------|-----|---------|-----------|
|LeftJoin_Manual|29.02 us|1.00 |65.84 KB |1.00       |
|LeftJoin_Linq  |15.23 us|0.53 |36.95 KB |0.56       |

Also — .NET Conf kicks off today!
Don’t miss it: https://www.dotnetconf.net/ 🎉🎉🎉

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