c# - C# 中的 C++ union — 奇怪的行为

Question

我正在尝试使用 C# 中的 VHD API 创建一些 vhd/vhdx 文件。

有一个看起来像这样的 C++ union :

typedef struct _CREATE_VIRTUAL_DISK_PARAMETERS
{
    CREATE_VIRTUAL_DISK_VERSION Version;

    union
    {
        struct
        {
            GUID                  UniqueId;
            ULONGLONG             MaximumSize;
            ULONG                 BlockSizeInBytes;
            ULONG                 SectorSizeInBytes;
            PCWSTR                ParentPath;
            PCWSTR                SourcePath;
        } Version1;

        struct
        {
            GUID                   UniqueId;
            ULONGLONG              MaximumSize;
            ULONG                  BlockSizeInBytes;
            ULONG                  SectorSizeInBytes;
            ULONG                  PhysicalSectorSizeInBytes;
            PCWSTR                 ParentPath;
            PCWSTR                 SourcePath;
            OPEN_VIRTUAL_DISK_FLAG OpenFlags;
            VIRTUAL_STORAGE_TYPE   ParentVirtualStorageType;
            VIRTUAL_STORAGE_TYPE   SourceVirtualStorageType;
            GUID                   ResiliencyGuid;
        } Version2;

        struct
        {
            GUID                   UniqueId;
            ULONGLONG              MaximumSize;
            ULONG                  BlockSizeInBytes;
            ULONG                  SectorSizeInBytes;
            ULONG                  PhysicalSectorSizeInBytes;
            PCWSTR                 ParentPath;
            PCWSTR                 SourcePath;
            OPEN_VIRTUAL_DISK_FLAG OpenFlags;
            VIRTUAL_STORAGE_TYPE   ParentVirtualStorageType;
            VIRTUAL_STORAGE_TYPE   SourceVirtualStorageType;
            GUID                   ResiliencyGuid;
            PCWSTR                 SourceLimitPath;
            VIRTUAL_STORAGE_TYPE   BackingStorageType;
        } Version3;
    };
} CREATE_VIRTUAL_DISK_PARAMETERS, *PCREATE_VIRTUAL_DISK_PARAMETERS;

我正在尝试将其转换为 C#，但运气不佳。我对 Version3 根本不感兴趣，所以我把它排除在外。

我已经尝试了很多事情，我能做到的最好的就是让版本 2 工作(通过做一些非常奇怪的事情)，但我从来没有设法让版本 1 和版本 2 同时工作。

到目前为止，取得最好结果的解决方案是这个，但肯定有问题，因为 Version1 根本不起作用，而 Version1 中的 SectorSizeInBytes 是一个 ulong 而不是 uint(如果我将它更改为 uint，我会破坏 Version2 而 Version1 仍然不起作用!)

[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParameters
{
    [FieldOffset(0)] public CreateVirtualDiskParametersVersion1 Version1;

    [FieldOffset(0)] public CreateVirtualDiskParametersVersion2 Version2;
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParametersVersion1
{
    public CreateVirtualDiskVersion Version;
    public Guid UniqueId;
    public ulong MaximumSize;
    public uint BlockSizeInBytes;
    public ulong SectorSizeInBytes;
    public string ParentPath;
    public string SourcePath;
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParametersVersion2
{
    public CreateVirtualDiskVersion Version;
    public Guid UniqueId;
    public ulong MaximumSize;
    public uint BlockSizeInBytes;
    public uint SectorSizeInBytes;
    public uint PhysicalSectorSizeInBytes;
    public string ParentPath;
    public string SourcePath;
    public OpenVirtualDiskFlags OpenFlags;
    public VirtualStorageType ParentVirtualStorageType;
    public VirtualStorageType SourceVirtualStorageType;
    public Guid ResiliencyGuid;
}

我知道理论上 Version 字段应该设置在 Version 结构之外，我也试过了，但它只是更有趣地打破了事情......

那么，有人可以建议如何将上面的内容正确地转换为 C#，而忽略不需要的 Version3 结构吗？

Answer 1

使用 Pack = 1 到 StructLayout 属性消除了 struct 成员之间的任何填充。在 TCP 连接中，结构通常在没有填充的情况下传递，以便所有使用该结构的程序都可以就其在内存中的布局达成一致。

然而，正如@David Heffernan 指出的那样，将结构传递给 Windows DLL 时情况可能并非如此。我没有测试对 CreateVirtualDisk 的实际调用，因为它看起来有点冒险，因为我以前没有使用过这个调用并且不想在我犯错时破坏我的磁盘。根据以下引用，看起来 8 字节的默认打包(Pack = 0 默认或 Pack = 8)可能是正确的设置。

参见 64-bit Windows API struct alignment caused Access Denied error on named pipe

Windows SDK 期望打包为 8 个字节。来自 Using the Windows Headers

Projects should be compiled to use the default structure packing, which is currently 8 bytes because the largest integral type is 8 bytes. Doing so ensures that all structure types within the header files are compiled into the application with the same alignment the Windows API expects. It also ensures that structures with 8-byte values are properly aligned and will not cause alignment faults on processors that enforce data alignment.

Version 被移动到 CreateVirtualDiskParameters 的顶部。然后两个 union 紧随其后。两者具有相同的偏移量 sizeof(CREATE_VIRTUAL_DISK_VERSION)。

此外，SectorSizeInBytes 是 uint 而不是 ulong。

您可以让编码器使用属性完成填充string 成员的工作，例如

[MarshalAs(UnmanagedType.LPWStr)] public string ParentPath;

或者，您可以表示它在内存中出现的样子，这是一个指向 Unicode 字符串的指针:

public IntPtr ParentPath;

然后自己提取字符串

Marshal.PtrToStringAuto(vdp.Version1.ParentPath)

如果您将 C# 结构传递给外部 DLL，请使用非托管字符串填充它

vdp.Version1.ParentPath = (IntPtr)Marshal.StringToHGlobalAuto("I am a managed string");

然后在完成后释放非托管字符串

Marshal.FreeHGlobal(vdp.Version1.ParentPath);

试试这个。

public enum CREATE_VIRTUAL_DISK_VERSION
{
    CREATE_VIRTUAL_DISK_VERSION_UNSPECIFIED = 0,
    CREATE_VIRTUAL_DISK_VERSION_1 = 1,
    CREATE_VIRTUAL_DISK_VERSION_2 = 2
};
public enum OPEN_VIRTUAL_DISK_FLAG
{
    OPEN_VIRTUAL_DISK_FLAG_NONE = 0x00000000,
    OPEN_VIRTUAL_DISK_FLAG_NO_PARENTS = 0x00000001,
    OPEN_VIRTUAL_DISK_FLAG_BLANK_FILE = 0x00000002,
    OPEN_VIRTUAL_DISK_FLAG_BOOT_DRIVE = 0x00000004,
    OPEN_VIRTUAL_DISK_FLAG_CACHED_IO = 0x00000008,
    OPEN_VIRTUAL_DISK_FLAG_CUSTOM_DIFF_CHAIN = 0x00000010
};

[StructLayout(LayoutKind.Sequential, Pack = 8, CharSet = CharSet.Unicode)]
public struct VIRTUAL_STORAGE_TYPE
{
    uint DeviceId;
    Guid VendorId;
};

[StructLayout(LayoutKind.Explicit, Pack = 8, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParameters
{
    [FieldOffset(0)]
    public CREATE_VIRTUAL_DISK_VERSION Version;

    [FieldOffset(8))]
    public CreateVirtualDiskParametersVersion1 Version1;

    [FieldOffset(8))]
    public CreateVirtualDiskParametersVersion2 Version2;
}

[StructLayout(LayoutKind.Sequential, Pack = 8, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParametersVersion1
{
    public Guid UniqueId;
    public ulong MaximumSize;
    public uint BlockSizeInBytes;
    public uint SectorSizeInBytes;
    //public IntPtr ParentPath;   // PCWSTR in C++ which is a pointer to a Unicode string
    //public IntPtr SourcePath;   //string
    [MarshalAs(UnmanagedType.LPWStr)] public string ParentPath;
    [MarshalAs(UnmanagedType.LPWStr)] public string SourcePath;
}

[StructLayout(LayoutKind.Sequential, Pack = 8, CharSet = CharSet.Unicode)]
public struct CreateVirtualDiskParametersVersion2
{
    public Guid UniqueId;
    public ulong MaximumSize;
    public uint BlockSizeInBytes;
    public uint SectorSizeInBytes;
    public uint PhysicalSectorSizeInBytes;
    //public IntPtr ParentPath;   //string
    //public IntPtr SourcePath;   //string
    [MarshalAs(UnmanagedType.LPWStr)] public string ParentPath;
    [MarshalAs(UnmanagedType.LPWStr)] public string SourcePath;
    public OPEN_VIRTUAL_DISK_FLAG OpenFlags;
    public VIRTUAL_STORAGE_TYPE ParentVirtualStorageType;
    public VIRTUAL_STORAGE_TYPE SourceVirtualStorageType;
    public Guid ResiliencyGuid;
}