属性参数必须是常量表达式

Question

有人可以向我解释为什么以下接口定义在 Visual Studio 2010 中编译时出错？

   [IncompleteCodePort(SourceOriginType.Other, "This should be a GL abstraction depending on what OpenGL API will be used")]
    public interface IGL
    {
        /// <summary>
        /// Returns true if provided function is available or supported by graphics API
        /// </summary>
        /// <param name="funcName"></param>
        /// <returns></returns>
        bool IsFunctionAvailable(string funcName);

        /// <summary>
        /// Returns true if provided function is supported as extension by graphics API
        /// </summary>
        /// <param name="funcName"></param>
        /// <returns></returns>
        bool IsExtensionAvailable(string funcName);
    }



public class IncompleteCodePortAttribute : Attribute
    {
        public SourceOriginType SourceOriginType { get; private set; }
        public string SourceUrl { get; private set; }
        public string Reason { get; private set; }


        public IncompleteCodePortAttribute(SourceOriginType originType, string reason, string sourceUrl = null)
        {
            SourceOriginType = originType;
            SourceUrl = sourceUrl;
            Reason = reason;
        }
    }

    public enum SourceOriginType
    {
        CodePlex,
        WorldWindJdk,
        StackOverflow,
        Other
    }

我得到的错误是：

属性参数必须是属性参数类型的常量表达式、typeof表达式或数组创建表达式

如果我删除自定义属性，我不会收到任何编译错误。

Answer 1

这似乎是 VS2010 中 C# 编译器中的一个错误（代码在 VS2012 下编译良好）。看起来编译器没有将参数默认值中的null 视为常量。

此代码无法编译：

[IncompleteCodePort()]
interface IGL
{}

class IncompleteCodePortAttribute : Attribute
{
    public IncompleteCodePortAttribute(string sourceUrl = null)
    {}
}

带有提到的错误消息（“属性参数必须是属性参数类型的常量表达式、类型表达式或数组创建表达式”），但没有源代码位置会令人困惑。

一些有效的属性声明示例：

class IncompleteCodePortAttribute : Attribute
{
    public IncompleteCodePortAttribute(string sourceUrl = "")
    {}
}

class IncompleteCodePortAttribute : Attribute
{
    private const string Null = null;

    public IncompleteCodePortAttribute(string sourceUrl = Null)
    {}
}

class IncompleteCodePortAttribute : Attribute
{
    public IncompleteCodePortAttribute()
        : this(null)
    {}

    public IncompleteCodePortAttribute(string sourceUrl)
    {}
}