Why isn't String.Empty a constant?

The reason that static readonly is used instead of const is due to use with unmanaged code, as indicated by Microsoft here in the Shared Source Common Language Infrastructure 2.0 Release. The file to look at is sscli20\clr\src\bcl\system\string.cs.

The Empty constant holds the empty string value. We need to call the String constructor so that the compiler doesn't mark this as a literal.

Marking this as a literal would mean that it doesn't show up as a field which we can access from native.

I found this information from this handy article at CodeProject.

I think there is a lot of confusion and bad responses here.

First of all, const fields are static members (not instance members).

Check section 10.4 Constants of the C# language specification.

Even though constants are considered static members, a constant-declaration neither requires nor allows a static modifier.

If public const members are static, one could not consider that a constant will create a new Object.

Given this, the following lines of code do exactly the same thing in respect to the creation of a new Object.

public static readonly string Empty = "";
public const string Empty = "";

Here is a note from Microsoft that explains the difference between the 2:

The readonly keyword is different from the const keyword. A const field can only be initialized at the declaration of the field. A readonly field can be initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used. Also, while a const field is a compile-time constant, the readonly field can be used for runtime constants, ...

So I find that the only plausible answer here is Jeff Yates's.

This answer exists for historical purposes.

Originally:

Because String is a class and therefore cannot be a constant.

Extended Discussion:

A lot of useful dialog was hammered out in vetting this answer, and rather than deleting it, this content is reproduced directly:

In .NET, (unlike in Java) string and String are exactly the same. And yes, you can have string literal constants in .NET – DrJokepu Feb 3 '09 at 16:57

Are you saying that a Class cannot have constants? – StingyJack Feb 3 '09 at 16:58

Yes, objects have to use readonly. Only structs can do constants. I think when you use string instead of String the compiler changes the const into a readonly for you. All to do with keeping C programmers happy. – Garry Shutler Feb 3 '09 at 16:59

tvanfosson just explained it a little bit more verbose. "X cannot be a constant, because the containing Y is a class" was just a little bit too context-free ;) – Leonidas Feb 3 '09 at 17:01

string.Empty is static property that returns an instance of the String class, namely the empty string, not the string class itself. – tvanfosson Feb 3 '09 at 17:01

Empty is a readonly instance (it's not a property) of the String class. – senfo Feb 3 '09 at 17:02

Head hurting. I still think I'm right, but now I'm less certain. Research required tonight! – Garry Shutler Feb 3 '09 at 17:07

The empty string is an instance of the string class. Empty is a static field (not a property, I stand corrected) on the String class. Basically the difference between a pointer and the thing it points to. If it weren't readonly we could change which instance the Empty field refers to. – tvanfosson Feb 3 '09 at 17:07

Garry, you don't need to do any research. Think about it. String is a class. Empty is an instance of a String. – senfo Feb 3 '09 at 17:12

There is something I don't quite get: how on earth can the static constructor of the String class create an instance of the String class ? Isn't that some sort of "chicken or the egg" scenario? – DrJokepu Feb 3 '09 at 17:12 5

This answer would be correct for nearly any other class but System.String. .NET does a lot of performance special-casing for strings, and one of them is that you CAN have string constants, just try it. In this case, Jeff Yates has the correct answer. – Joel Mueller Feb 3 '09 at 19:25

As described in §7.18, a constant-expression is an expression that can be fully evaluated at compile-time. Since the only way to create a non-null value of a reference-type other than string is to apply the new operator, and since the new operator is not permitted in a constant-expression, the only possible value for constants of reference-types other than string is null. The previous two comments were taken directly from the C# language specification and reiterate what Joel Mueller mentioned. – senfo Feb 4 '09 at 15:05 5