C#: Replace in Files

Below shows the method that replace a text in files within a directory which is iteratively searched:

public void ReplaceInFiles(string dir_path, Dictionary<string, Dictionary<int, string>> results, string old_text, string new_text, string extension=null)
{
 DirectoryInfo dir = new DirectoryInfo(dir_path);
 DirectoryInfo[] sub_dirs = dir.GetDirectories();
 FileInfo[] files = dir.GetFiles();

 foreach (FileInfo file in files)
 {
  string file_path=file.FullName;
  if (!string.IsNullOrEmpty(extension) && file.Extension != extension) continue;

  Dictionary<int, string> found_locations = null;
  using (StreamReader reader = new StreamReader(file.OpenRead()))
  {
   string line;
   int line_index=0;
   while ((line = reader.ReadLine()) != null)
   {
    line_index++;
    if (line.Contains(old_text))
    {
     if (results.ContainsKey(file_path))
     {
      found_locations = results[file_path];
     }
     else
     {
      found_locations = new Dictionary<int, string>();
      results[file_path] = found_locations;
     }
     found_locations[line_index] = line;
    }
   }
  }

  if(found_locations!=null)
  {
   string file_content=null;
   using(StreamReader reader =new StreamReader(file.OpenRead()))
   {
    file_content=reader.ReadToEnd();
   }
   file_content=file_content.Replace(old_text, new_text).TrimEnd(Environment.NewLine.ToCharArray());
   using(StreamWriter writer=new StreamWriter(file.OpenWrite()))
   {
    writer.WriteLine(file_content);
    writer.Flush();
   }
  }
  
 }

 foreach (DirectoryInfo sub_dir in sub_dirs)
 {
  string sub_dir_path=sub_dir.FullName;
  ReplaceInFiles(sub_dir_path, results, old_text, new_text, extension);
 }
}

Below is the code snippet which shows how to use the method above:

string dir_path="C:\temp";
string text_to_find="SimuKit.DomainModels\";
string text_to_replace="packages\SimuKit.DomainModels\";
string extension=".sln";
Dictionary<string, Dictionary<int, string>> results=new Dictionary<string, Dictionary<int, string>>();

ReplaceInFiles(dir_path, results, text_to_find, text_to_replace, extension);
foreach (string file_path in results.Keys)
{
 Dictionary<int, string> line_content = results[file_path];
 
 foreach (int line_index in line_content.Keys)
 {
  Console.writeline("{0} is replaced in file {1} in line {2}, original content: {3}", text_to_find, file_path, line_index, line_content[line_index]);
 }
}