using System;
using System.Linq;
using System.Collections.Generic;

public class Test{
  static void Main(){
    Console.WriteLine(Replacer.sValueReplacer("0000##81#059671####=1811", 3, '#'));
  }
}

public class Replacer{

  public static string sValueReplacer(string source, int index, char char_to_replace)
  {
    string res = source;

    try
    {
      if (!string.IsNullOrEmpty(char_to_replace.ToString()))
      {
        if (res.Contains(char_to_replace.ToString()))
        {
           // Get ALL Indexes position of character
           var Indexes = GetIndexes(res, char_to_replace.ToString());

           int max = GetMaxValue(Indexes.Count);

           while (index >= max) 
           { 
            index -= max; 
           }

           var new_value = index.ToString().PadLeft(Indexes.Count, '0');

           for (int i = 0; i < Indexes.Count; i++)
           {
            res = res.Remove(Indexes[i], 1).Insert(Indexes[i], new_value[i].ToString());
           }
        }
      }
     }
     catch (Exception)
     {
      res = source;
     }

     return res;
  }
  
  private static List<int> GetIndexes(string mainString, string toFind)
  {
    var Indexes = new List<int>();

    for (int i = mainString.IndexOf(toFind); i > -1; i = mainString.IndexOf(toFind, i + 1))
    {
      // for loop end when i=-1 (line.counter not found)
      Indexes.Add(i);
    }

    return Indexes;
   }

   private static int GetMaxValue(int numIndexes)
   {
      int max = 0;

      for (int i = 0; i < numIndexes; i++)
      {
        if (i == 0)
          max = 9;
        else
          max = max * 10 + 9;
      }

      return max;
    }
}