C# 平台调用，具有引用和值类型的 C 风格 union

Question

我正在尝试整理以下结构

struct OpalMessage {
  OpalMessageType m_type;   ///< Type of message
  union {
    const char *             m_commandError;       ///< Used by OpalIndCommandError
    OpalParamGeneral         m_general;            ///< Used by OpalCmdSetGeneralParameters
    OpalParamProtocol        m_protocol;           ///< Used by OpalCmdSetProtocolParameters
    OpalParamRegistration    m_registrationInfo;   ///< Used by OpalCmdRegistration
    OpalStatusRegistration   m_registrationStatus; ///< Used by OpalIndRegistration
    OpalParamSetUpCall       m_callSetUp;          ///< Used by OpalCmdSetUpCall/OpalIndProceeding/OpalIndAlerting/OpalIndEstablished
    const char *             m_callToken;          ///< Used by OpalCmdHoldcall/OpalCmdRetrieveCall/OpalCmdStopRecording
    OpalStatusIncomingCall   m_incomingCall;       ///< Used by OpalIndIncomingCall
    OpalParamAnswerCall      m_answerCall;         ///< Used by OpalCmdAnswerCall/OpalCmdAlerting
    OpalStatusUserInput      m_userInput;          ///< Used by OpalIndUserInput/OpalCmdUserInput
    OpalStatusMessageWaiting m_messageWaiting;     ///< Used by OpalIndMessageWaiting
    OpalStatusLineAppearance m_lineAppearance;     ///< Used by OpalIndLineAppearance
    OpalStatusCallCleared    m_callCleared;        ///< Used by OpalIndCallCleared
    OpalParamCallCleared     m_clearCall;          ///< Used by OpalCmdClearCall
    OpalStatusMediaStream    m_mediaStream;        ///< Used by OpalIndMediaStream/OpalCmdMediaStream
    OpalParamSetUserData     m_setUserData;        ///< Used by OpalCmdSetUserData
    OpalParamRecording       m_recording;          ///< Used by OpalCmdStartRecording
    OpalStatusTransferCall   m_transferStatus;     ///< Used by OpalIndTransferCall
    OpalStatusIVR            m_ivrStatus;          ///< Used by OpalIndCompletedIVR
  } m_param;
};

转到 C#。明显的问题是这两个字符串不可避免地是引用类型。

所以，我尝试了这个:

    [StructLayout(LayoutKind.Explicit)]
    public struct OpalMessageStrUnion
    {
      [FieldOffset(0)]
      [MarshalAs(UnmanagedType.LPStr)]
      public string m_commandError;       ///< Used by OpalIndCommandError

      [FieldOffset(0)]
      [MarshalAs(UnmanagedType.LPStr)]
      public string m_callToken;          ///< Used by OpalCmdHoldcall/OpalCmdRetrieveCall/OpalCmdStopRecording
    }

    [StructLayout(LayoutKind.Explicit)]
    public struct OpalMessageUnion
    {
      [FieldOffset(0)]      
      public OpalParamGeneral m_general;            ///< Used by OpalCmdSetGeneralParameters

      [FieldOffset(0)]
      public OpalParamProtocol m_protocol;           ///< Used by OpalCmdSetProtocolParameters

      [FieldOffset(0)]
      public OpalParamRegistration m_registrationInfo;   ///< Used by OpalCmdRegistration

      [FieldOffset(0)]
      public OpalStatusRegistration m_registrationStatus; ///< Used by OpalIndRegistration

      [FieldOffset(0)]
      public OpalParamSetUpCall m_callSetUp;          ///< Used by OpalCmdSetUpCall/OpalIndProceeding/OpalIndAlerting/OpalIndEstablished     

      [FieldOffset(0)]
      public OpalStatusIncomingCall m_incomingCall;       ///< Used by OpalIndIncomingCall

      [FieldOffset(0)]
      public OpalParamAnswerCall m_answerCall;         ///< Used by OpalCmdAnswerCall/OpalCmdAlerting

      [FieldOffset(0)]
      public OpalStatusUserInput m_userInput;          ///< Used by OpalIndUserInput/OpalCmdUserInput

      [FieldOffset(0)]
      public OpalStatusMessageWaiting m_messageWaiting;     ///< Used by OpalIndMessageWaiting

      [FieldOffset(0)]
      public OpalStatusLineAppearance m_lineAppearance;     ///< Used by OpalIndLineAppearance

      [FieldOffset(0)]
      public OpalStatusCallCleared m_callCleared;        ///< Used by OpalIndCallCleared

      [FieldOffset(0)]
      public OpalParamCallCleared m_clearCall;          ///< Used by OpalCmdClearCall

      [FieldOffset(0)]
      public OpalStatusMediaStream m_mediaStream;        ///< Used by OpalIndMediaStream/OpalCmdMediaStream

      [FieldOffset(0)]
      public OpalParamSetUserData m_setUserData;        ///< Used by OpalCmdSetUserData

      [FieldOffset(0)]
      public OpalParamRecording m_recording;          ///< Used by OpalCmdStartRecording

      [FieldOffset(0)]
      public OpalStatusTransferCall m_transferStatus;     ///< Used by OpalIndTransferCall

      [FieldOffset(0)]
      public OpalStatusIVR m_ivrStatus;          ///< Used by OpalIndCompletedIVR

    }

    /// <summary>
    /// Message to/from OPAL system.
    /// This is passed via the OpalGetMessage() or OpalSendMessage() functions.
    /// </summary> 
    [StructLayout(LayoutKind.Explicit)]
    public struct OpalMessage
    {
      //this guy is an enumeration b.t.w.
      /// <summary>
      /// type of message
      /// </summary> 
      [FieldOffset(0)]
      public OpalMessageType m_type;

      [FieldOffset(4)]
      public OpalMessageUnion m_param;

      [FieldOffset(4)]
      public OpalMessageStrUnion m_strParam;
    }

但是，我仍然收到编码错误，告诉我这行不通，因为我在同一内存位置混合了一个对象和非对象类型。我现在假设结构本身(即 OpalParamGeneral 等)也不能混合引用和值类型，即使它们是按顺序排列的？

顺便说一下，对每个结构进行单独的函数调用不是一种选择。我宁愿写一个 COM 包装器也不愿那样做。

Answer 1

[StructLayout(LayoutKind.Explicit)] 属性是不寻常的，它不仅在编码(marshal)时影响结构的布局，而且还影响结构的托管版本的布局。这可能非常方便，它允许在托管语言中实现 union ，即使该语言本身不支持 union 。

它们会带来麻烦，但是垃圾收集器对它们有问题。它需要能够找到对引用类型的引用，但在 union 中不能可靠地做到这一点。因为它无法准确判断引用的是什么类型。这也是一个可靠性问题，您可以将引用类型与 IntPtr 重叠，并以这种方式发现对象的地址。

CLR 类加载器检查此条件并在发生重叠时抛出 TypeLoadException。

解决方法很明确，您需要确保只使用 blittable 类型。您的所有 OpalXxx 类型都需要是本身仅包含可 blittable 类型的结构。字符串的解决方法是改为将它们声明为 IntPtr 并使用 Marshal 类来转换值。在此 MSDN library article 中更多关于 blittable 类型的信息.