(C#) Encounter format trouble in concatenating String

The file format is look like

abc def 
    ghy jk
          lmp

And here is my code

 System.IO.StreamReader file =
        new System.IO.StreamReader("./test.txt");
        string afterreplace = "";
        while ((line = file.ReadLine()) != null)
        {
            Console.WriteLine(line);
            afterreplace = String.Concat(afterreplace, line); 

        }
        Console.WriteLine(afterreplace);

However I got this in my output

It doesn't match the original file

abc def     ghy jk      lmp

Of cause I can add some newline character, I really want to know why it cause

Does

String.Concat

ignore the '/n' ?

How could I achieve my expectation?

thx a lot.