Make a ton of VS-recommended changes. Sure, why not.

Program.cs

@@ -2,7 +2,7 @@
 {
    class Program
    {
-      static void Main(string[] args)
+      static void Main()
       {
          var start = System.DateTime.Now;
          Q01.Go();

Q03.cs

@@ -25,7 +25,7 @@ namespace _2020
             list.Add(new List<bool>());
             for (int i = 0; i < line.Length; i++)
             {
-               list[list.Count - 1].Add(line[i] != '.');
+               list[^1].Add(line[i] != '.');
             }
          }
 

Q04.cs

@@ -7,11 +7,11 @@ namespace _2020
 {
    class Q04
    {
-      static List<Dictionary<string, string>> Passports = new List<Dictionary<string, string>>();
+      static readonly List<Dictionary<string, string>> Passports = new List<Dictionary<string, string>>();
 
-      static Regex PassportRegex = new Regex(@"(?<key>[a-z]{3}):(?<val>\S+)", RegexOptions.Compiled);
+      static readonly Regex PassportRegex = new Regex(@"(?<key>[a-z]{3}):(?<val>\S+)", RegexOptions.Compiled);
-      static Regex HTMLColor = new Regex("^#[0-9a-f]{6}$", RegexOptions.Compiled);
+      static readonly Regex HTMLColor = new Regex("^#[0-9a-f]{6}$", RegexOptions.Compiled);
-      static Regex PassportID = new Regex("^[0-9]{9}$", RegexOptions.Compiled);
+      static readonly Regex PassportID = new Regex("^[0-9]{9}$", RegexOptions.Compiled);
 
       public static void Go()
       {
@@ -35,7 +35,7 @@ namespace _2020
             var matches = PassportRegex.Matches(line);
             for (int i = 0; i < matches.Count; i++)
             {
-               Passports[Passports.Count - 1].Add(matches[i].Groups["key"].Value, matches[i].Groups["val"].Value);
+               Passports[^1].Add(matches[i].Groups["key"].Value, matches[i].Groups["val"].Value);
             }
          }
       }

Q06.cs

@@ -6,12 +6,12 @@ namespace _2020
 {
    class Q06
    {
-      class group
+      class Group
       {
          public int numInGroup;
          public Dictionary<char, int> answers;
       }
-      static List<group> list = new List<group>();
+      static readonly List<Group> list = new List<Group>();
 
       public static void Go()
       {
@@ -25,12 +25,12 @@ namespace _2020
          using var fs = new FileStream("06input.txt", FileMode.Open);
          using var sr = new StreamReader(fs);
          string line;
-         group entry = null;
+         Group entry = null;
          while ((line = sr.ReadLine()) != null)
          {
             if (line.Length == 0 || entry == null)
             {
-               entry = new group()
+               entry = new Group()
                {
                   numInGroup = 0,
                   answers = new Dictionary<char, int>(),

Q07.cs

@@ -14,9 +14,9 @@ namespace _2020
          public List<Tuple<string, int>> ValidContents = new List<Tuple<string, int>>();
       }
 
-      static Regex bagRegex = new Regex(@"(?: contain |, )?(?<numBags>[0-9]+ )?(?<bagType>.+?) bags?", RegexOptions.Compiled);
+      static readonly Regex bagRegex = new Regex(@"(?: contain |, )?(?<numBags>[0-9]+ )?(?<bagType>.+?) bags?", RegexOptions.Compiled);
 
-      static List<BagType> list = new List<BagType>();
+      static readonly List<BagType> list = new List<BagType>();
 
       public static void Go()
       {
@@ -82,11 +82,11 @@ namespace _2020
 
       static void Part1()
       {
-         Func<List<string>, BagType, List<string>> bagAgg = (accum, bagType) =>
+         static List<string> bagAgg(List<string> accum, BagType bagType)
          {
             accum.Add(bagType.BagName);
             return accum;
-         };
+         }
 
          var directContain = GetBagsCanContain(new BagType() { BagName = "shiny gold" });
          var totalContain = directContain.Aggregate(new List<string>(), bagAgg);

Q08.cs

@@ -7,7 +7,7 @@ namespace _2020
 {
    class Q08
    {
-      static List<Tuple<string, int>> instructions = new List<Tuple<string, int>>();
+      static readonly List<Tuple<string, int>> instructions = new List<Tuple<string, int>>();
 
       public static void Go()
       {
@@ -21,7 +21,7 @@ namespace _2020
          foreach (var line in File.ReadAllLines("08input.txt"))
          {
             var parts = line.Split(' ');
-            if (parts.Count() != 2)
+            if (parts.Length != 2)
             {
                throw new Exception("malformed input");
             }

Q09.cs

@@ -6,7 +6,7 @@ namespace _2020
 {
    class Q09
    {
-      static List<long> list = new List<long>();
+      static readonly List<long> list = new List<long>();
 
       const int chunkLength = 25;
 

Q10.cs

@@ -6,7 +6,7 @@ namespace _2020
 {
    class Q10
    {
-      static List<long> list = new List<long>();
+      static readonly List<long> list = new List<long>();
       static List<long> sortedList = null;
 
       public static void Go()
@@ -51,8 +51,10 @@ namespace _2020
 
       static void Part2()
       {
-         var pathsToIndices = new List<long>();
+         var pathsToIndices = new List<long>
-         pathsToIndices.Add(1);
+         {
+            1
+         };
          for (int i = 1; i < sortedList.Count; i++)
          {
             long pathLen = 0;

Q11.cs

@@ -100,7 +100,7 @@ namespace _2020
       {
          int numOccupied = 0;
 
-         Func<int, int, bool> checkSeat = (checkRow, checkCol) =>
+         bool checkSeat(int checkRow, int checkCol)
          {
             if (seatList[checkRow, checkCol] == SeatState.Occupied)
             {
@@ -113,7 +113,7 @@ namespace _2020
             }
 
             return false;
-         };
+         }
 
          // ul
          for (int checkRow = row - 1, checkCol = col - 1; checkRow >= 0 && checkCol >= 0; checkRow--, checkCol--)

Q12.cs

@@ -18,7 +18,7 @@ namespace _2020
       static Vec2 waypointPosition = new Vec2() { x = 10, y = 1 };
       static int facingDir = 90;
 
-      static List<Tuple<char, int>> instructions = new List<Tuple<char, int>>();
+      static readonly List<Tuple<char, int>> instructions = new List<Tuple<char, int>>();
 
       public static void Go()
       {
@@ -119,7 +119,7 @@ namespace _2020
          }
          else if (dir == 'L')
          {
-            facingDir = (facingDir - amount);
+            facingDir -= amount;
             while (facingDir < 0)
             {
                facingDir = 360 + facingDir;

Q13.cs

@@ -10,7 +10,7 @@ namespace _2020
    class Q13
    {
       static int leaveTime;
-      static List<int> busses = new List<int>();
+      static readonly List<int> busses = new List<int>();
 
       public static void Go()
       {
@@ -116,7 +116,9 @@ namespace _2020
       }
 
       // this is how i initially solved it. took ~30mins. i'm not proud of it, but the "real" answer apparently required specialized knowledge i did not have.
+#pragma warning disable IDE0051 // Remove unused private members
       static void Part2BruteForce()
+#pragma warning restore IDE0051 // Remove unused private members
       {
          int root = busses.First(x => x > 0);
          var sorted = busses.OrderBy(x => x);

Q14.cs

@@ -21,7 +21,7 @@ namespace _2020
          public List<byte> mask = new List<byte>();
          public List<WriteInstruction> writeInstructions = new List<WriteInstruction>();
       }
-      static List<Instruction> instructions = new List<Instruction>();
+      static readonly List<Instruction> instructions = new List<Instruction>();
 
       public static void Go()
       {
@@ -64,9 +64,11 @@ namespace _2020
             {
                var inst = instructions[^1];
 
-               var writeInst = new Instruction.WriteInstruction();
+               var writeInst = new Instruction.WriteInstruction
-               writeInst.idx = Convert.ToUInt64(line["mem[".Length..line.IndexOf("]")]);
+               {
-               writeInst.value = Convert.ToUInt64(line[(line.IndexOf('=') + 1)..]);
+                  idx = Convert.ToUInt64(line["mem[".Length..line.IndexOf("]")]),
+                  value = Convert.ToUInt64(line[(line.IndexOf('=') + 1)..])
+               };
 
                inst.writeInstructions.Add(writeInst);
             }

Q15.cs

@@ -6,7 +6,7 @@ namespace _2020
 {
    class Q15
    {
-      static List<int> list = new List<int>();
+      static readonly List<int> list = new List<int>();
 
       public static void Go()
       {

Q18.cs

@@ -30,17 +30,12 @@ namespace _2020
 
       static long DoOp(long left, long right, char op)
       {
-         switch (op)
+         return op switch
          {
-            case '+':
+            '+' => left + right,
-               return left + right;
+            '*' => left * right,
-
+            _ => throw new Exception("unrecognized op"),
-            case '*':
+         };
-               return left * right;
-
-            default:
-               throw new Exception("unrecognized op");
-         }
       }
 
       static void Part1()

Q19.cs

@@ -7,8 +7,8 @@ namespace _2020
 {
    class Q19
    {
-      static Dictionary<int, Rule> rules = new Dictionary<int, Rule>();
+      static readonly Dictionary<int, Rule> rules = new Dictionary<int, Rule>();
-      static List<string> messages = new List<string>();
+      static readonly List<string> messages = new List<string>();
 
       [DebuggerDisplay("ch={ch} | num rules={ruleIdxs?.Count}")]
       class Rule
@@ -141,10 +141,11 @@ namespace _2020
 
       static void Part2()
       {
-         var rulesUpdated = new Dictionary<int, Rule>(rules);
+         var rulesUpdated = new Dictionary<int, Rule>(rules)
-         rulesUpdated[8] = new Rule()
          {
-            ruleIdxs = new List<List<int>>()
+            [8] = new Rule()
+            {
+               ruleIdxs = new List<List<int>>()
             {
                new List<int>()
                {
@@ -156,10 +157,10 @@ namespace _2020
                   8,
                },
             },
-         };
+            },
-         rulesUpdated[11] = new Rule()
+            [11] = new Rule()
-         {
+            {
-            ruleIdxs = new List<List<int>>()
+               ruleIdxs = new List<List<int>>()
             {
                new List<int>()
                {
@@ -173,6 +174,7 @@ namespace _2020
                   31,
                },
             },
+            }
          };
 
          int numMatched = 0;

Q21.cs

@@ -8,8 +8,8 @@ namespace _2020
 {
    class Q21
    {
-      static List<Tuple<List<string>, List<string>>> list = new List<Tuple<List<string>, List<string>>>();
+      static readonly List<Tuple<List<string>, List<string>>> list = new List<Tuple<List<string>, List<string>>>();
-      static Dictionary<string, int> ingredientCounts = new Dictionary<string, int>();
+      static readonly Dictionary<string, int> ingredientCounts = new Dictionary<string, int>();
       static IEnumerable<string> allAllergens = new List<string>();
       static IEnumerable<string> allIngredients = new List<string>();
 

Q22.cs

@@ -7,7 +7,7 @@ namespace _2020
 {
    class Q22
    {
-      static Dictionary<int, List<int>> list = new Dictionary<int, List<int>>();
+      static readonly Dictionary<int, List<int>> list = new Dictionary<int, List<int>>();
 
       public static void Go()
       {