Find longest span of characters in string, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string sample = RandomString(100); //Generates 100 random chars (see below), can change length or to whatever you want
            char lookFor = RandomIn(sample); //Picks a random char to look for, can change to any character
            
            Console.WriteLine("String: \"" + sample + "\"");
            
            int current = 0, max = 0;
            int index = -1, maxIndex = -1;
            
            //Start searching
            for (int at = 0; at < sample.Length; at++) 
            {
                char c = sample[at];
                if (c == lookFor) 
                {
                    if (current == 0) index = at;
                    current++;
                    if (current > max)
                    {
                        max = current;
                        maxIndex = index;
                    }
                }
                else current = 0;
            }
            
            //Print out result
            if (index == -1) Console.WriteLine("Could not find '" + lookFor + "'");
            else
            {
                Console.Write("Found '" + lookFor + "' " + max + " times at most, starting at index " + maxIndex);
                if (sample.Length > 15)
                {
                    Console.Write(", at \"");
                    if (maxIndex - 3 < 0) Console.Write(sample.Substring(0, max + 3));
                    else if (maxIndex + max + 3 >= sample.Length) Console.Write(sample.Substring(maxIndex - 3, max));
                    else Console.Write(sample.Substring(maxIndex - 3, max + 6));
                    Console.Write("\"");
                }
                Console.WriteLine();
            }
            
            //Console.ReadLine();
        }
        static string RandomString(int len)
        {
            string chars = "!@#$%^&*()!!!!!!"; //Can change to whatever you want, repeating chars will increase chance of being picked
            string result = "";
            Random rand = new Random();
            
            while (result.Length < len)
            {
                int index = rand.Next(chars.Length);
                result += chars[index];
            }
            return result;
        }
        static char RandomIn(string chars) 
        {
            Random r = new Random();
            return chars[r.Next(chars.Length)];
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,09 sec, cpu time: 0,09 sec, average memory usage: 12 Mb, average nr of threads: 2