Why structs cannot have extern properties, but methods are OK?

Question

Dear ladies and sirs.

Observe the following code:

[AttributeUsage(AttributeTargets.All)]
public class XAttribute : Attribute
{
}

public struct A
{
  [X]
  public static extern int XX { get; }
}

This does not compile. The error message says

The modifier 'extern' is not valid for this item

But, the following code compiles OK:

[AttributeUsage(AttributeTargets.All)]
public class XAttribute : Attribute
{
}

public struct A
{
  [X]
  public static extern int GetXX();
}

Why ???

EDIT

Guys, guys. I would not have asked had I not had a real application for this. I understand, that purely academic interest to understand why some things are defined the way they are is not something that motivates some of us, so here is the down to earth motivation. I have a PostSharp attribute, that injects a certain logic into the attributed extern property. Specifically, the real code looks like so:

[LoggerAccessProperty]
private static extern ILog Logger { get; }

Where PostSharp processes the LoggerAccessProperty aspect and injects the actual getter method, which surfaces the private static compiler generated ILog instance. This is part of our in-house extension of the Log4PostSharp package. At the time, we have published the extended version of Log4PostSharp at the PostSharp google code site and this attribute is my recent addition, yet unpublished.

EDIT2

Note, that the following code compiles just fine:

[AttributeUsage(AttributeTargets.All)]
public class XAttribute : Attribute
{
}

public class A
{
  [X]
  public static extern int XX { get; }
}

The difference is that A is a class here, not struct.

EDIT3

I am using .NET 4.

Answer 1

extern properties are not allowed in structs.

Answer 2

The extern modifier is used to declare a method that is implemented externally.

The above is the first line of documentation for the extern keyword. (With emphasis added). Given that it only applies to methods (not properties), one has to ask: What are you even trying to do? The fact that it's a struct rather than a class, and the presence of an attribute in your code examples both seem to be red herrings.

Answer 3

Because an external method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no braces ({ }) following the signature.

So if you changed your code to:

[LoggerAccessProperty]
private static extern ILog Logger();

It should work fine in your struct.

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