C# Max or default?

How do I do IEnumerable.Max with a default value for empty collections in C#?

Here is what I want to do in Java 8:

public static void doTest(List<String> l) {
    System.out.println("max string is: " + l.stream().max((a, b) -> a.compareToIgnoreCase(b)).orElse("default"));
    System.out.println("max length is: " + l.stream().mapToInt(s -> s.length()).max().orElse(-1));
}

public static void main(String[] args) throws Exception {
    System.out.println("Test with non-empty list");
    doTest(Arrays.asList("alpha", "beta", "charlie", "gamma"));

    System.out.println("");

    System.out.println("Test with empty list");
    doTest(Arrays.asList());
}

Here is what I have in C# so far:

static void DoTest(List<String> l) {
    Console.WriteLine("max string is: " + l.Max());
    Console.WriteLine("max length is: " + l.Select(s => s.Length).Max());
}

static void Main() {
    Console.WriteLine("Test with non-empty list");
    DoTest(new List<String> { "alpha", "beta", "charlie", "gamma" });

    Console.WriteLine("");

    Console.WriteLine("Test with empty list");
    DoTest(new List<String> { });
}