c# - 静态对象为空/未在正确时间初始化

Question

我正在使用 type-safe enum pattern在这里概述。我需要将一个类型安全的枚举嵌套到另一个枚举中。在创建父构造函数时，子属性(静态对象)为 NULL。似乎子构造函数没有被调用，我遇到了一些错误。(父子关系我很困惑，但它解释了层次结构)

这是一个例子(我正在使用 netMF):

public class MyDeviceSetting //parent
{
        public readonly string Name;
        public MyUnit SettingUnit;
        public readonly MyUnit.UnitPurpose UnitPurpose;

          #region MY STATIC SETTINGS
        //UNIT SETTINGS
        public static MyDeviceSetting TempUnits = new MyDeviceSetting("TempUnits", MyUnit.mm); //MyUnit.mm is null. Why?
        public static MyDeviceSetting BLAH = new MyDeviceSetting("BLAH", MyUnit.inch);//MyUnit.inch is null. Why?
          #endregion



        /// <summary>
        /// This is the MAIN PRIVATE Constructor
        /// </summary>
        /// <param name="?"></param>
        private MyDeviceSetting(string name, MyUnit defaultUnit)
        {
            Name = name;
            SettingUnit = defaultUnit;//NULL
            UnitPurpose = SettingUnit.Purpose; //fails because SettingUnit is NULL


        }


    }


public sealed class MyUnit
{
    private static int Count = 0;

    //these are used to store and identify the unit in memory
    public readonly int UnitID;
    public readonly int TestID;

    public enum UnitPurpose
    {
        DISTANCE,
        SPEED,
        TEMPERATURE,
        TIME,
        CLOCK,
        NO_UNITS
    }

    public readonly string DisplayName;
    public readonly string Abbreviation;
    public readonly string Name;
    public readonly UnitPurpose Purpose;

    #region My Units
    public static readonly MyUnit mm = new MyUnit("Milimeters", "mm", "mm", UnitPurpose.DISTANCE, 1);
    public static readonly MyUnit inch = new MyUnit("inch", "inch", "in", UnitPurpose.DISTANCE, 2);



    #endregion

    private MyUnit(string name,
                   string displayName,
                   string abbreviation,
                   UnitPurpose unitPurpose,
                   int unitID)
    {
        Name = name;
        DisplayName = displayName;
        Abbreviation = abbreviation;
        Purpose = unitPurpose;
        UnitID = unitID;
        TestID = Count;
        Count++;

    }


}

如何确保 child 不为空？有解决方法吗？编辑:This Post确保这应该可以正常工作，但就我而言，它不起作用。

Answer 1

这似乎是 .Net Micro 框架的错误/限制。它不完全支持静态构造函数。这是报告相同问题的人:https://msdn.microsoft.com/en-us/library/Cc533032.aspx

documentation对于 NetCF 3.0 包含以下警告:

Do not use static constructors. They are not yet fully supported by the .NET Micro Framework.

从这里blog post似乎还说(至少从 2.0 开始)对静态构造函数的调用是序列化的:

There are some things that cannot be done in .NET Compact Framework in a static constructor which are possible in the full .NET Framework. Basically, all static constructors when are executed in a serialized fashion in .NET Compact Framework V2

那篇文章在死锁的背景下讨论了它，但我相信这就是它不起作用的原因。

不幸的是，这意味着您不能依赖静态，而必须自己处理初始化和锁定。这样的事情应该有效:

private static MyUnit inch;

public static MyUnit Inch
{
    get
    {
        if (inch == null)
            inch = new MyUnit("inch", "inch", "in", UnitPurpose.DISTANCE, 2);
         return inch;
    }
}

不幸的是，这失去了静态构造函数为您提供的线程安全性。这将很难修复，因为您不能依赖静态成员，因为正如我们所见，您不能依赖它们的初始化。