Fun Weird Nasty C#

C# snippets That Make You Go Hmmm...

All snippets are linked to their original creators (or the people I found them from). I don't take any credit, just collating fun bits! Most also have SharpLab.io links so you can see them in action yourself.

Smallest C#

{}

Via nietras

SharpLab

It's all Greek

using System;
unsafe class Program
{
    delegate void bar(int* i);
    static Index ƛ(bar β) => default;
    static void Main(string[] args)
    {
        int[] ω = { };
        int Ʃ = 42;
        int? Φ = 10;
        var ϼ = ω;
        
        ϼ=ω[ƛ(β:Δ=>Φ??=ω[^++Ʃ]/Ʃ|*&Δ[-0%Ʃ]>>1^Φ??0!&~(δ:Ʃ,^Φ..).δ)..(1_0>.0?Ʃ:0b1)];
    }
}

Via Llewellyn Pritchard

SharpLab

Mutate ReadOnly fields

using System;
using System.IO;

Console.WriteLine(Path.DirectorySeparatorChar); // prints '\'

var f = (in char x) => { /* can't modify 'x' here */ };
f = (ref char x) => { x = 'A'; }; // but can here!

f(Path.DirectorySeparatorChar);

Console.WriteLine(Path.DirectorySeparatorChar); // prints 'A'

Via Aleksandr Shvedov

SharpLab

New DateTime formula, Euclidean affine functions

// Exactly the same as Year, Month, Day properties, except computing all of
// year/month/day rather than just one of them. Used when all three
// are needed rather than redoing the computations for each.
//
// Implementation based on article https://arxiv.org/pdf/2102.06959.pdf
//   Cassio Neri, Lorenz Schneiderhttps - Euclidean Affine Functions and Applications to Calendar Algorithms - 2021
internal void GetDate(out int year, out int month, out int day)
{
    // y400 = number of whole 400-year periods since 3/1/0000
    // r1 = day number within 400-year period
    (uint y400, uint r1) = Math.DivRem(((uint)(UTicks / TicksPer6Hours) | 3U) + 1224, DaysPer400Years);
    ulong u2 = (ulong)Math.BigMul(2939745, (int)r1 | 3);
    ushort daySinceMarch1 = (ushort)((uint)u2 / 11758980);
    int n3 = 2141 * daySinceMarch1 + 197913;
    year = (int)(100 * y400 + (uint)(u2 >> 32));
    // compute month and day
    month = (ushort)(n3 >> 16);
    day = (ushort)n3 / 2141 + 1;

    // rollover December 31
    if (daySinceMarch1 >= March1BasedDayOfNewYear)
    {
        ++year;
        month -= 12;
    }
}

30% optimization of DateTime.GetDate()/.Year/.Month/.Day/.DayOfYear by 'Euclidean affine functions' #72712

ReadOnly lists aren't always readonly

using System;
using System.Collections.Generic;

var list = new List<int>{1, 2, 3, 4};

IReadOnlyList<int> readonlyList = list;

// error CS1061: 'IReadOnlyList<int>' does not contain a definition for 'Add'...
// readonlyList.Add(5);

// Works as expected
((List<int>)readonlyList).Add(5);

// 5
Console.WriteLine(readonlyList.Count);

Via Nick Chapsas

Great explanation on SO

SharpLab

async async async (async async)

async async async(async async) => 
        await async;

Via Jared Parsons

Explainer from Jared

SharpLab

async async async (async async) Part 2

[async, async<async>] async async async([async<async>, async] (async async, async) async)
        => await async.async;

Via Lucas Trzesniewski

SharpLab

Floats are inaccurate

using System;
float x = 0.4f;
float y = 0.6f;
Console.WriteLine((double)x + (double)y == 1); // False
Console.WriteLine((float)((double)x + (double)y) == 1); // True

Via Ashley Hauck

Explainer by Ashley Hauck

SharpLab

Ensure finally with ThreadAbort

Note this is for .NET Framework.

try {}
finally
{
    if (!m_canceled)
    {
        m_canceled = true;
        TimerQueue.Instance.DeleteTimer(this);
    }
}

Via Roman Marusyk

Documentation

C# lunar cycle performance

An unsigned number bug in Environment.TickCount in a specific situation in a threadpool prevent more threads from being added to the pool.

Via Kevin Gosse

GitHub Issue

Dynamic var global using

global using var = dynamic;

Via Jared Parsons via Immo Landwerth