how to Find string in txt file and delete line with this string

using System;

using System.IO;

using System.Collections.Generic;

class Test

{

   static void Main()

   {

      string filePath = "sasa.txt";

      string searchWord = "COMMENT";

      string[] oldLines  = File.ReadAllLines(filePath);

      List newLines = new List(); 

    

      foreach(string line in oldLines)

      {         

         if (!line.Contains(searchWord)) newLines.Add(line);

      }

      File.WriteAllLines(filePath, newLines.ToArray());

   }

}

You might want to use StartsWith() instead of Contains() and to do a case insensitive search by converting everything to (say) lower case.