我试图用下面的代码创建一个自定义.NET属性,但是意外地离开了这个子类.这会在注释中显示一个容易修复的编译器错误.
// results in compiler error CS0641: Attribute 'AttributeUsage' is
// only valid on classes derived from System.Attribute
[AttributeUsage(AttributeTargets.Class)]
internal class ToolDeclarationAttribute
{
internal ToolDeclarationAttribute()
{
}
}
我的问题是编译器如何知道[AttributeUsage]属性只能应用于System.Attribute的子类？使用.NET反射器我没有看到任何特殊的AttributeUsageAttribute类声明本身.不幸的是,这可能只是编译器本身生成的一个特殊情况.
[Serializable,ComVisible(true),AttributeUsage(AttributeTargets.Class,Inherited=true)]
public sealed class AttributeUsageAttribute : Attribute
{
...
解决方法
I would like to be able to specify that my custom attribute can only be placed on subclasses of a particular class (or interface). Is this possible?
实际上,有一种方法可以使用protected(见Restricting Attribute Usage)为子类(而不是接口)执行此操作.要重现代码(但不是讨论)：
abstract class MyBase {
[AttributeUsage(AttributeTargets.Property)]
protected sealed class SpecialAttribute : Attribute {}
}
class ShouldBeValid : MyBase {
[Special] // works fine
public int Foo { get; set; }
}
class ShouldBeInvalid { // not a subclass of MyBase
[Special] // type or namespace not found
[MyBase.Special] // inaccessible due to protection level
public int Bar{ get; set; }
}
