How are going to read a text file and auto save in c #

Question

ex: if the text file has the apple - Messagebox.Show("This word is exist");

else

Messagebox.Show("The word does not exist!");
and if none of these words in the text file will be automatically saved and automatic make a new line for word does not exist in the text file.

Answer 1

read all text

  string fileText = File.ReadAllText("filePath");
  if (fileText.Contains("apple"))
  {
       Messagebox.Show("This word is exist");
  }
  else
  {
       Messagebox.Show("The word does not exist!");
       File.AppendAllText(@"filePAth", "The word does not exixt in the text file" + Environment.NewLine);
  }