https://codereview.stackexchange.com/questions/161172/project-euler-problem-4-palindromic-number/161252#161252

161172.a.cs

using System;
using System.Collections.Generic;
using System.Linq;
using BenchmarkDotNet.Reports;
using BenchmarkDotNet.Running;
using BenchmarkDotNet.Attributes;

namespace Test {
	public class PalindromNumber {
		//Thanks to @Velial and @Denis
		private static bool IsPalindromicNumber(int i) {
			string testNumber = Convert.ToString(i);
			for (int j = testNumber.Length / 2; j >= 0; j--) {
				if (testNumber[j] != testNumber[testNumber.Length - 1 - j]) {
					return false;
				}
			}
			return true;
		}

		[Benchmark]
		public static string LargestPalindromeOriginal() {
			int test = 0;
			int left = 0;
			int right = 0;
			int biggestNumber = 0;
			int maxNumber = 999;
			for (left = maxNumber; left >= 99; left--) {
				for (right = maxNumber; right >= 99; right--) {
					test = left * right;
					if (IsPalindromicNumber(test)) {
						break;
					}
				}
				if (test <= biggestNumber) continue;
				biggestNumber = test;
			}
			return biggestNumber.ToString();
		}

		[Benchmark]
		public static string LargestPalindromeDenis() {
			int max = 0;
			for (int i = 100; i < 1000; i++) {
				for (int j = i + 1; j < 1000; j++) {
					int ij = i * j;
					if (max < ij && IsPalindromicNumber(ij)) {
						max = ij;
					}
				}
			}
			return max.ToString();
		}

		[Benchmark]
		public static string LargestPalindromeEric() {
			var query = from first in ThreeDigitNumbers()
						from second in ThreeDigitNumbers()
						let product = first * second
						where IsPalindromicNumber(product)
						select product;
			return query.Max().ToString();
		}

		private static IEnumerable<int> ThreeDigitNumbers() {
			for (int i = 100; i < 999; i++) {
				yield return i;
			}
		}

		[Benchmark]
		public static string LargestPalindromePgs() {
			int largestPalindrome = 0, b, delta;
			for (int a = 999; a >= 100; a--) {
				if (a % 11 == 0) {
					b = 999;
					delta = 1;
				} else {
					b = 990;
					delta = 11;
				}
				while (b >= a) {
					int ab = a * b;
					if (ab <= largestPalindrome)
						break;
					if (IsPalindromicNumber(ab)) {
						largestPalindrome = ab;
					}
					b -= delta;
				}
			}
			return largestPalindrome.ToString();
		}

		[Benchmark]
		public static string LargestPalindromeDavislor() => 
			LargestPalindromeDavislorImpl().First(HasFactors).ToString();

		private static IEnumerable<Int32> LargestPalindromeDavislorImpl() {
			for (Int32 a = 9; a >= 1; --a)
				for (Int32 b = 9; b >= 0; --b)
					for (Int32 c = 9; c >= 0; --c)
						yield return a * 100001 + b * 10010 + c * 1100;
		}
		private static bool HasFactors(Int32 p) {
			Int32 lower1 = (p / 999 + 10) / 11,
				lower = Math.Max(10, lower1);
			Int32 upper1 = p / 1100,
				  upper = Math.Min(90, upper1);
			Int32 count = Math.Max(upper - lower + 1, 0);
			return Enumerable.Range(lower, count).Any(x => p % x == 0);
		}

		[Benchmark]
		public static string LargestPalindromeDavislor2() => 
			LargestPalindromeDavislorImpl().First(p => XCandidates(p).Any(x => p % x == 0)).ToString();

		private static IEnumerable<int> XCandidates(int p) {
			int q = p / 11;
			int lower = Math.Max(10, (q + (999 - 1)) / 999);
			int upper = Math.Min(90, q / 100);
			int k = q % 6;

			if (k == 1 || k == 5) {
				int r = lower % 6;
				bool mod1 = r <= 1;
				int i = lower - r + (mod1 ? 1 : 5);

				while (i <= upper) {
					yield return i;
					i += mod1 ? 4 : 2;
					mod1 = !mod1;
				}
			} else if (k == 2 || k == 4) {
				int r = lower % 6;
				int m = (r == 3 || r == 0) ? r + 1 : r;
				int i = lower - r + m;

				while (i <= upper) {
					yield return i;
					if (m == 5) {
						i += 2;
						m = 1;
					} else if (m == 2) {
						i += 2;
						m = 4;
					} else {
						++i;
						++m;
					}
				}
			} else if (k == 3) {
				int i = lower + 1 - lower % 2;

				while (i <= upper) {
					yield return i;
					i += 2;
				}
			} else {
				for (int i = lower; i <= upper; ++i)
					yield return i;
			}
		}

		[Benchmark]
		public static string LargestPalindrome_t3chb0t() {
			var isPalindrom = new Func<(int left, int right, int product), bool>(t => {
				var str = t.product.ToString();
				for (int i = 0, j = str.Length - 1; i < str.Length; i++, j--) {
					if (str[i] != str[j]) return false;
				}
				return true;
			});

			int max = 2000;

			(int left, int right, int product) result =
				Enumerable
				.Range(0, max)
				.Select(x => Enumerable.Range(0, max).Select(y => (left: x, right: y, product: x * y)))
				.SelectMany(results => results)
				.AsParallel()
				.Where(isPalindrom)
				.OrderByDescending(x => x.product)
				.FirstOrDefault();
			return result.Item3.ToString();
		}
	}


	class Program {
		static void Main() {
			Summary summary = BenchmarkRunner.Run<PalindromNumber>();
			Console.ReadKey();
		}
	}
}