private void GetStepname(string[] files, string stepname)
{
foreach (string file in files)
{
  sr = new StreamReader(file);
  using (sr)
 {
     while (!sr.EndOfStream)
    {
       line = sr.ReadLine();
       if (line.Contains(stepname))  //search for a limit  
       //и так далее           
   }

}
}

private void GetStepname(string[] files, string stepname)
{
    foreach (string file in files)
   {
       string[] text = File.ReadAllLines(file);
      for (int i = 0; i < text.Length; i++)
      {
           if (text[i].Contains(stepname))
          //и так далее           
      }
}
}

private void buttonGetFields_Click(object sender, EventArgs e)
{
    string[] files = new string[listBoxFiles.SelectedItems.Count];
    listBoxFiles.SelectedItems.CopyTo(files, 0);
    GetStepname(files, stepname);
}

private void buttonGetFields_Click(object sender, EventArgs e)
{
    foreach (string file in listBoxFiles.SelectedItems)
    {
        GetStepname(file, stepname);
    }
}