This one seemed hard AF. Then seemed pretty easy, then seemed hard again.

Then after looking at some different hints & approaches (is this cheating?) I came up with my own flavor of solution:

using System;
using System.Collections.Generic;
using System.Linq;
namespace advent_of_code_2020
{
    class Day3
    {

        public int TreeCount = 0;

        public string Tree = "#";

        public void solve(){
            part1();
            part2();
        }

        public void part1()
        {
            ThoseTreesTho(3, 1);
            Console.WriteLine(TreeCount);
        }

        public void part2()
        {
            var mValues = new int[] {
                ThoseTreesTho(1, 1),
                ThoseTreesTho(3, 1),
                ThoseTreesTho(5, 1),
                ThoseTreesTho(7, 1),
                ThoseTreesTho(1, 2)
            };

            Int64 answer = 1;

            foreach(int val in mValues)
            {       
                answer *= val;
            }

            Console.WriteLine(answer);
        }

        private int ThoseTreesTho(int xSlope, int ySlope)
        {
            TreeCount = 0;
            var x = xSlope;
            var y = ySlope;
            while (y < PuzzleInput.Count)
            {
                if (PuzzleInput[y][x] == Convert.ToChar(Tree)) 
                {
                    TreeCount++;
                }
                y += ySlope;
                x = (x + xSlope) % PuzzleInput[0].Count();
            }
            return TreeCount;
        }

        protected List<string> PuzzleInput = new List<string>() { 
            //...
         };

    }
}