How to search all files in folders/subfolders for one or more strings?

Question

I have a list of files (full path and file name) in Column D of my worksheet.  I'm trying to figure out how to run some code to open each file in this array (starting on D14 and going down), search for one or more strings, and then put an "X"
in Column I, in the same row as the file name.
How can I do that?
I experimented with a few samples of code.  It seems pretty easy to do it in one folder, but not all sub folders.  The folder/subfolder is setup a specific way for a specific purpose.
Knowledge is the only thing that I can give you, and still retain, and we are both better off for it.

Answer 1

Below is an example that allow you to search a directory and its subdirs txt files(you can change this), read its lines and search for specific word:
using System;
using System.IO;
using System.Linq;
class Program
static void Main(string[] args)
try
var files = from file in Directory.EnumerateFiles(@"c:\", "*.txt", SearchOption.AllDirectories)
from line in File.ReadLines(file)
where line.Contains("Microsoft")
select new
File = file,
Line = line
foreach (var f in files)
Console.WriteLine("{0}\t{1}", f.File, f.Line);
Console.WriteLine("{0} files found.", files.Count().ToString());
catch (UnauthorizedAccessException UAEx)
Console.WriteLine(UAEx.Message);
catch (PathTooLongException PathEx)
Console.WriteLine(PathEx.Message);
https://msdn.microsoft.com/en-us/library/dd997370%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396
Fouad Roumieh