You can replace your default value with Null and then use null-coalescing operator, inside your method. Something like this:
public static string ToSummaryString<T>(this IEnumerable<T> items, string delimiter = null) { var realDelimiter = delimiter ?? Environment.NewLine; } As an alternative, you can also use Method-Overloading, as @Dennis_E also mentioned: write 2 methods; one with the delimiter and one without.