GuessTheNumber.cs

using System;

namespace GuessTheNumber {
	class Program {
		const int MAX_VALUE = 100;

		static void Main(string[] args) {
			Console.WriteLine("Guess the Number!");

			Console.Write("Would you like to play? [y/n]: ");

			// Declare input, and assign it the value of whatever the user types
			string input = Console.ReadLine();

			// Seed our random number generator
			// (You should only ever do this once per program lifecycle, unless you have a reason to make more than one)
			Random rnd = new Random();

			// Ensures we actually got something from ReadLine, and that what we got is y or Y
			// string.Equals is the same thing as == if you're comparing two strings, but it lets you pass in some extra options
			// For instance, StringComparsion.InvariantCultureIgnoreCase is a (very long) option that basically says the strings only have
			// to match by contents, but not by case. So, comparing Y and y would be true!
			// The extra check, input != null, is required to make sure input actually exists, because Console.ReadLine may return null
			// in some rare cases.
			while (input != null && input.Equals("y", StringComparison.InvariantCultureIgnoreCase)) {
				// The user hasn't won when the game starts, of course
				// This could just as easily be called notWin and be true, which would save me the ! later here, but
				// I prefer calling it win cus I'm weird
				bool win = false;

				int guesses = 0;

				// Random.Next's upper bound is EXCLUSIVE, so we need to add 1 to our MAX_VALUE to get a range of [1, MAX_VALUE]
				int num = rnd.Next(1, MAX_VALUE + 1);

				// While the user has **not** won...
				while (!win) {
					// Increment our guess counter
					guesses++;

					// The number the user entered
					int inNum = 0;

					// Whether or not the text the user inputted was a number
					bool valid = false;

					// While the user has **not** inputted a valid number...
					while (!valid) {
						Console.Write($"What number am I thinking of? [1-{MAX_VALUE}]: ");
						input = Console.ReadLine();
						valid = Int32.TryParse(input, out inNum);

						// Bounds checking - ensures 1 <= inNum <= MAX_VALUE
						if (valid) {
							valid = inNum >= 1 && inNum <= MAX_VALUE;
						}

						// Fancy error message - uses Console.Error
						// Console actually has two outputs: normal out (standard out, stdout), which is what Console.WriteLine defaults to,
						// and error (standard error, stderr), which we can access using Console.Error.Write/WriteLine
						if (!valid) {
							Console.Error.WriteLine("Invalid/out-of-range number entered. Try again!");
						}
					}

					if (inNum == num) {
						Console.WriteLine("You win!");
						win = true;
					} else if (inNum < num) {
						Console.WriteLine("Too low!");
					} else {
						// The else case above is the same as writing else if (inNum > num), but programmers are lazy
						Console.WriteLine("Too high!");
					}
				}

				Console.WriteLine($"That took you {guesses} guesses.");
				
				// Re-prompt the user if they'd like to play
				Console.Write("Would you like to play agin? [y/n]: ");
				input = Console.ReadLine();
			}
		}
	}
}