search for a string in a file

How to search for a string in a file. As string might be somewhere in the middle of text file.

Code:

Console.WriteLine("Input a string ");
String data = Console.ReadLine();
sr = File.OpenText(@"C:\username.txt");

String st = sr.ReadToEnd();
?????

help please?

thanks

 

Shabbir Brother you are the star. Thanks a million

 

One more question is

"How to find a position of that string in a file"?

Rgds,

 

I got the answer for position thanks anyway