C#: How do I prepend text to each line in a string?

Question

What would an implementation of 'MagicFunction' look like to make the following (nunit) test pass?

public MagicFunction_Should_Prepend_Given_String_To_Each_Line()
{
    var str = @"line1
line2
line3";

    var result = MagicFunction(str, "-- ");

    var expected = @"-- line1
-- line2
-- line3";

    Assert.AreEqual(expected, result);
}

Answer 1

Thanks all for your answers. I implemented the MagicFunction as an extension method. It leverages Thomas Levesque's answer but is enhanced to handle all major environments AND assumes you want the output string to use the same newline terminator of the input string.

I favored Thomas Levesque's answer (over Spencer Ruport's, Fredrik Mork's, Lazarus, and JDunkerley) because it was the best performing. I'll post performance results on my blog and link here later for those interested.

(Obviously, the function name of 'MagicFunctionIO' should be changed. I went with 'PrependEachLineWith')

public static string MagicFunctionIO(this string self, string prefix)
{
  string terminator = self.GetLineTerminator();
  using (StringWriter writer = new StringWriter())
  {
    using (StringReader reader = new StringReader(self))
    {
      bool first = true;
      string line;
      while ((line = reader.ReadLine()) != null)
      {
        if (!first)
          writer.Write(terminator);
        writer.Write(prefix + line);
        first = false;
      }
      return writer.ToString();
    }
  }
}

public static string GetLineTerminator(this string self)
{
  if (self.Contains("\r\n")) // windows
    return "\r\n";
  else if (self.Contains("\n")) // unix
    return "\n";
  else if (self.Contains("\r")) // mac
    return "\r";
  else // default, unknown env or no line terminators
    return Environment.NewLine;
}