Day1 and structure

.gitignore

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+bin/
+obj/
+/packages/
+riderModule.iml
+/_ReSharper.Caches/
+.idea

AoC2021.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>Exe</OutputType>
+        <TargetFramework>net5.0</TargetFramework>
+    </PropertyGroup>
+
+</Project>

AoC2021.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "AoC2021", "AoC2021.csproj", "{8EB1C70D-5BC2-4FD5-AC52-A7FCC272C2FA}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{8EB1C70D-5BC2-4FD5-AC52-A7FCC272C2FA}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{8EB1C70D-5BC2-4FD5-AC52-A7FCC272C2FA}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{8EB1C70D-5BC2-4FD5-AC52-A7FCC272C2FA}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{8EB1C70D-5BC2-4FD5-AC52-A7FCC272C2FA}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+EndGlobal

Day1.cs

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+#define Day1
+#if Day1
+using System;
+using System.IO;
+using System.Linq;
+
+var scans = File.ReadAllText("day1/input").Split("\n", StringSplitOptions.RemoveEmptyEntries).Select(l => int.Parse(l)).ToList();
+Console.WriteLine(Enumerable.Range(1, scans.Count - 1).Count(i => scans[i - 1] < scans[i]));
+Console.WriteLine(Enumerable.Range(3, scans.Count - 3).Count(i => scans[i - 3] < scans[i]));
+#endif

Day2.cs

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+// #define Day2
+#if Day2
+using System;
+using System.IO;
+using System.Linq;
+
+var toBeDefined = File.ReadAllText("day2/input").Split("\n", StringSplitOptions.RemoveEmptyEntries);
+#endif

README.md

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+# Advent of Code 2021
+
+## How to use
+
+All source files use [top-level statements](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9#top-level-statements).
+To run a specific file, uncomment the first line that contains a preprocessor directive and in turn allows compilation of the code below.
+Before moving to the next file, comment out the first line again, as only one file is allowed to have top-level statements.
+
+by Stefan Forstenlechner
+