问题
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 becausestring.Emptyis 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!
回答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.
回答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.]
回答3:
From the original question:
I thought that we would use it to be sure that we have a system-independent means of referring to the empty string.
In what way can the empty string vary from system to system? It's always a string with no characters! I'd be really scared if I ever found an implementation where string.Empty == "" returned false :) This is not the same as something like Environment.NewLine.
From Counter Terrorist's bounty post:
I want String.Empty can be used as a default parameter in the next C# release. :D
Well that's certainly not going to happen.
While I would personally have liked a very different defaulting mechanism too, the way optional parameters work has been in .NET since the start - and it always means embedding a constant into the metadata, so that the calling code can copy that constant into the call site if no corresponding argument is provided.
With string.Empty it's really pointless - using "" will do what you want; is it that painful to use the string literal? (I use the literal everywhere - I never use string.Empty - but that's a different argument.)
That's what surprises me about this question - the complaint revolves around something which doesn't actually cause a real problem. It's for more important in cases where you want the default to be computed at execution time because it might actually vary. For example, I could imagine cases where you want to be able to call a method with a DateTime parameter and have it default to "the current time". At the moment, the only vaguely elegant workaround I know for that is:
public void RecordTime(string message, DateTime? dateTime = null)
{
var realDateTime = dateTime ?? DateTime.UtcNow;
}
... but that's not always appropriate.
In conclusion:
- I very much doubt that this will ever be part of C#
- For
string.Emptyit's pointless anyway - For other values which really don't always have the same value, it really can be a pain
回答4:
I never use string.Empty, I can't see the point of it. Maybe it makes it easier for people that are really new to programming, but I doubt it's useful even for that.
回答5:
I think the idea behind string.Empty is it enhances readability. It is not like newline where there is any difference between how it is represented on different platforms. It's ashame it can't be used in a default parameter. However, it will not cause any issues if you port between Windows and something like Mono on Linux.
回答6:
As a FYI, it looks like the same constraint is imposed on values passed to attribute constructors - they must be constant. Since string.empty is defined as :
public static readonly string Empty
rather than an actual constant, it cannot be used.
回答7:
I use string.Empty purely for readability.
If someone else needs to read/change my code later they know that I meant to check for or set something to an empty string. Using just "" can sometimes cause bugs and confusion because I may have just forgot to put the string that I wanted in there.
For example:
if(someString == string.Empty)
{
}
vs
if(someString == "")
{
}
The first if statement just seems so much more deliberate and readable to me. Because this is just a preference though, I really do not see the train-smash in having to use "" instead of string.Empty.
回答8:
Perhaps the best solution to this problem is an overload of this method, in this way:
public static void PrintString()
{
PrintString(string.Empty);
}
来源：https://stackoverflow.com/questions/2701314/cannot-use-string-empty-as-a-default-value-for-an-optional-parameter