add Day base class like 2019's

Day.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+
+namespace aoc2020
+{
+    public abstract class Day
+    {
+        public abstract int DayNumber { get; }
+        public virtual IEnumerable<string> Input =>
+            File.ReadLines($"input/day{DayNumber}.in");
+
+        public abstract string Part1();
+        public abstract string Part2();
+
+        public virtual void AllParts(bool verbose = false)
+        {
+            Console.WriteLine($"Day {DayNumber}:");
+            var s = new Stopwatch();
+
+            if (verbose)
+            {
+                s.Start();
+            }
+
+            var part1 = Part1();
+
+            if (verbose) 
+            {
+                s.Stop();
+                Console.WriteLine($"Part 1 elapsed ticks: {s.ElapsedTicks}");
+            }
+
+            Console.WriteLine(part1);
+
+            if (verbose)
+            {
+                s.Reset();
+                s.Start();
+            }
+
+            var part2 = Part2();
+
+            if (verbose)
+            {
+                s.Stop();
+                Console.WriteLine($"Part 2 elapsed ticks: {s.ElapsedTicks}");
+            }
+
+            Console.WriteLine(part2);
+            Console.WriteLine();
+        }
+    }
+}
+
+

Day1.cs

@@ -0,0 +1,16 @@
+namespace aoc2020
+{
+    internal class Day1 : Day
+    {
+        public override int DayNumber => 1;
+        public override string Part1()
+        {
+            return "";
+        }
+
+        public override string Part2()
+        {
+            return "";
+        }
+    }
+}

Program.cs

@@ -1,12 +1,37 @@
 using System;
+using System.Linq;
+using System.Reflection;
 
 namespace aoc2020
 {
-    class Program
+    internal class Program
     {
-        static void Main(string[] args)
+        private static void Main(string[] args)
         {
-            Console.WriteLine("Hello World!");
+            var days = Assembly.GetExecutingAssembly().GetTypes()
+                .Where(t => t.BaseType == typeof(Day))
+                .Select(t => (Day) Activator.CreateInstance(t))
+                .OrderBy(d => d.DayNumber);
+
+            if (args.Length == 1 && int.TryParse(args[0], out var dayNum))
+            {
+                var day = days.FirstOrDefault(d => d.DayNumber == dayNum);
+                if (day != null)
+                {
+                    day.AllParts();
+                }
+                else
+                {
+                    Console.WriteLine($"Day {dayNum} invalid or not yet implemented");
+                }
+            }
+            else
+            {
+                foreach (var d in days)
+                {
+                    d.AllParts();
+                }
+            }
         }
     }
 }

aoc2020.sln

@@ -0,0 +1,25 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 16
+VisualStudioVersion = 16.0.30709.132
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "aoc2020", "aoc2020.csproj", "{3D60F1A0-0DD9-4FF9-B85B-D491B13ACAE7}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{3D60F1A0-0DD9-4FF9-B85B-D491B13ACAE7}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{3D60F1A0-0DD9-4FF9-B85B-D491B13ACAE7}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{3D60F1A0-0DD9-4FF9-B85B-D491B13ACAE7}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{3D60F1A0-0DD9-4FF9-B85B-D491B13ACAE7}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {F1ECEA7D-4A0E-4B78-8D3C-29D4E1D09C0D}
+	EndGlobalSection
+EndGlobal