What is the logic behind while loop behavior in this case?

Question

My code simply Creates a refrence variable of StreamReader object that reads a TextFile, After that using a while loop to retrive data from the TextFile. 

 

StreamReader sr = new StreamReader("File.txt");

            string line = "";

           while (line != null)
           {
              line = sr.ReadLine();

               Console.WriteLine(line);

           }

So, forgive my innocent question : The condition in the While loop is TRUE, so why it doesn't return an infinite loop ? instead it retrives the data from "File.txt" then it breaks the loop?

 

All what I know as a newbie programmer is that if the condition of the loop is TRUE keep it going until it turns to FALSE then the loop will break; itself.

 

Could somebody explain the logic behind this scenario please!