pass large structs by ref for performance

[bbarry]

assuming the readme is correct,

public int Method1()
{
    int[] arr = new[] { 1, 2, 3, 4 };
    int q = 2;
    return this.Method1_ProceduralLinq1(arr, q);
}
private int Method1_ProceduralLinq1(int[] _linqitems, int q)
{
    if (_linqitems == null) throw new ArgumentNullException();

    int num = 0;
    for (int i = 0; i < _linqitems.Length; i++)
    {
        if (_linqitems[i] > q)
            num += num2 + 3;
    }
    return num;
}

should instead be:

...
    return this.Method1_ProceduralLinq1(arr, ref q);
}
private int Method1_ProceduralLinq1(int[] _linqitems, ref int q)
...

It doesn't really matter in this particular case, but it could in cases where the type of the closure was a larger struct or was modified. For example

public void Main()
{
    var arr = new[] { 1, 2, 3, 4 };
    var q = 2;
    var l = arr.Where(x => x > q).Select(x => (q+=x) + 3).Last();
    Console.WriteLine("q: {0}, l: {1}", q, l);
}

should output q: 5, l: 8

[antiufo]

Yes, when a variable is modified by the lambda (flow analysis), the parameter is made ref.

https://github.com/antiufo/roslyn-linq-rewrite/blob/master/RoslynLinqRewrite/RoslynLinqRewrite/LinqRewriter.cs#L320
https://github.com/antiufo/roslyn-linq-rewrite/blob/master/RoslynLinqRewrite/RoslynLinqRewrite/LinqRewriter.cs#L453

I just checked with your example and it correctly generates a ref parameter.

[bbarry]

heh yeah I guess I was unclear, it should be ref if it changes OR it is a struct type. Otherwise a copy is made which can be significant if the struct is large (large is relative here; a Guid benefits from being passed by ref in a hot path):

public class Program {
    static void Main(string[] args) {
        var summary = BenchmarkRunner.Run<Tests>();
    }
}
public class Tests
{
    Guid guid = Guid.NewGuid();

    bool ByValFn(Guid g) { return g == guid; }

    [Benchmark]
    public bool ByVal()
    {
        return ByValFn(guid);
    }

    bool ByRefFn(ref Guid g) { return g == guid; }

    [Benchmark]
    public bool ByRef()
    {
        return ByRefFn(ref guid);
    }
}

 Method |    Median |    StdDev |
------- |---------- |---------- |
  ByVal | 4.9627 ns | 0.2178 ns |
  ByRef | 4.5439 ns | 0.1089 ns |

~9% improvement for passing a Guid by reference (numbers via BenchmarkDotNet).