[c#] Easiest way to split a string on newlines in .NET?

To split on a string you need to use the overload that takes an array of strings:

string[] lines = theText.Split(
    new[] { Environment.NewLine },
    StringSplitOptions.None
);

Edit:
If you want to handle different types of line breaks in a text, you can use the ability to match more than one string. This will correctly split on either type of line break, and preserve empty lines and spacing in the text:

string[] lines = theText.Split(
    new[] { "\r\n", "\r", "\n" },
    StringSplitOptions.None
);