You can search in textfiles using
RegularExpressions.Regex.IsMatch
First get the file in StreamReader and read it's content
System.IO.StreamReader reader = new System.IO.StreamReader(file);
String text = reader.ReadToEnd();
use Regular expression to search the text
if (System.Text.RegularExpressions.Regex.IsMatch(text, searchText))
{
Console.WriteLine("word found");
}