class Program
    {

        static void Main()
        {
            if (File.Exists("Input.txt"))
            {
                var InputLines = new List<string>(File.ReadAllLines("Input.txt"));
                for (int numberLine = 0; numberLine < InputLines.Count; numberLine++)
                {
                    for (int i = 0; i < InputLines[numberLine].Length; i++)
                        if (InputLines[numberLine].Substring(i, 1).Equals("#"))
                        {
                            InputLines[numberLine] = InputLines[numberLine].Remove(i - 1, 2);
                            i = i - 2;
                        }
                }
                File.AppendAllLines("Out.txt", InputLines);
            }
        }
    }

In file: abc#kek###d#