Cannot use String.Empty as a default value for an optional parameter

Question

I am reading Effective C# by Bill Wagner. In Item 14 - Minimize Duplicate Initialization Logic, he shows the following example of using the new optional parameters feature in a constructor:

public MyClass(int initialCount = 0, string name = "")

Notice that he used "" instead of string.Empty.
He comments:

You'll note [in an example above] that the second constructor specified "" for the default value on the name parameter, rather than the more customary string.Empty. That's because string.Empty is not a compile-time constant. It is a static property defined in the string class. Because it is not a compile constant, you cannot use it for the default value for a parameter.

If we cannot use the string.Empty static in all situations, then doesn't that defeat the purpose of it? I thought that we would use it to be sure that we have a system-independent means of referring to the empty string. Is my understanding wrong? Thanks.

UPDATE
Just a follow up comment. According to MSDN:

Each optional parameter has a default value as part of its definition. If no argument is sent for that parameter, the default value is used. Default values must be constants.

Then we aren't be able to use System.Environment.NewLine either, or use newly instantiated objects as default values. I haven't used VS2010 yet, and this is disappointing!

Answer 1

As of the C# 2.0 compiler, there is very little point to String.Empty anyway, and in fact in many cases it's a pessimisation, since the compiler can inline some references to "" but can't do the same with String.Empty.

In C# 1.1 it was useful to avoid creating lots of independent objects all containing the empty string, but those days are gone. "" works just fine.

Answer 2

There's nothing to stop you from defining your own constant for the empty string if you really want to use it as an optional parameter value:

const string String_Empty = "";  public static void PrintString(string s = String_Empty) {     Console.WriteLine(s); } 

[As an aside, one reason to prefer String.Empty over "" in general, that hasn't been mentioned in the other answers, is that there are various Unicode characters (zero-width joiners, etc.) that are effectively invisible to the naked eye. So something that looks like "" isn't necessarily the empty string, whereas with String.Empty you know exactly what you're using. I recognise this isn't a common source of bugs, but it is possible.]