不能声明指向非非托管类型的指针

Question

我正试图学习如何编写一个DLL包装器，并且我碰到了一点障碍。我有这样一个结构声明：

[StructLayout(LayoutKind.Sequential)] 
unsafe struct SDL_Surface 
{ 
    public readonly UInt32 flags;    
    public readonly SDL_PixelFormat* format; 
    public readonly int w, h;     
    public readonly int pitch;    
    public void* pixels;    

    /// <summary>Application data associated with the surface</summary> 
    public void* userdata;   

    /// <summary>information needed for surfaces requiring locks</summary> 
    public readonly int locked;    
    public readonly void* lock_data;   

    /// <summary>clipping information</summary> 
    public readonly SDL_Rect clip_rect;   

    /// <summary>info for fast blit mapping to other surfaces</summary> 
    private SDL_BlitMap *map; // <--- Cannot take the address of, get the size of, or declare a pointer to a managed type 

    /// <summary>Reference count -- used when freeing surface</summary> 
    public int refcount;    
} 

当我尝试编译我的项目，它给出了上述错误。

但你会注意到它上面，我有一个指向另一个结构的指针。我试图弄清楚这两个结构之间的区别是什么使一个工作，但另一个没有，但我不知道;他们都是不安全的结构。他们如下：

[StructLayout(LayoutKind.Sequential)] 
unsafe struct SDL_PixelFormat 
{ 
    UInt32 format; 
    SDL_Palette *palette; 
    byte BitsPerPixel; 
    byte BytesPerPixel; 
    fixed byte padding [2]; 
    UInt32 Rmask; 
    UInt32 Gmask; 
    UInt32 Bmask; 
    UInt32 Amask; 
    byte Rloss; 
    byte Gloss; 
    byte Bloss; 
    byte Aloss; 
    byte Rshift; 
    byte Gshift; 
    byte Bshift; 
    byte Ashift; 
    int refcount; 
    SDL_PixelFormat *next; 
} 

unsafe internal delegate int SDL_blit(SDL_Surface* src, SDL_Rect* srcrect, SDL_Surface* dst, SDL_Rect* dstrect); 

[StructLayout(LayoutKind.Sequential)] 
unsafe struct SDL_BlitMap 
{ 
    SDL_Surface* dst; 
    int identity; 
    SDL_blit blit; 
    void* data; 
    SDL_BlitInfo info; 

    /* the version count matches the destination; mismatch indicates 
     an invalid mapping */ 
    UInt32 dst_palette_version; 
    UInt32 src_palette_version; 
} 

[StructLayout(LayoutKind.Sequential)] 
struct SDL_Rect 
{ 
    int x, y; 
    int w, h; 
} 

那么，我必须改变，以使这个编译？

---

我相信这是SDL_BlitMap是造成这一问题的参考SDL_blit。我已经宣布它是一个代表;有什么我应该被宣布为？它的定义如下：C：

typedef int (*SDL_blit) (struct SDL_Surface * src, SDL_Rect * srcrect, 
         struct SDL_Surface * dst, SDL_Rect * dstrect); 

Answer 1

任何包含托管类型的结构都无法获取其地址。 代表是一个引用类型，因此它们也是一个托管类型。 这意味着SDL_Blitmap是一个托管类型，因为它包含对SDL_blit的托管引用，因此如果不先修复它，您将无法获取指向它的指针。

如果你试图调用该函数是在DLL已经上市，我建议你看看DllImportAttribute。（http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportattribute(v=vs.100).aspx）

通过结合一个DllImportAttribute公共静态外部声明，你可以调用任何你在互操作的dll中声明的全局函数。

或者您需要创建一个C/C++函数指针并调用它，这可能会变得混乱。

Answer 2

如果您不依赖不安全的代码会怎么样？尽管代码对性能/速度至关重要，但它可能会影响性能。沿此线的东西：

[StructLayout(LayoutKind.Sequential)] 
struct SDL_PixelFormat 
{ 
    UInt32 format; 
    IntPtr palettePtr; 
    byte BitsPerPixel; 
    byte BytesPerPixel; 
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)] 
    byte padding[]; 
    UInt32 Rmask; 
    UInt32 Gmask; 
    UInt32 Bmask; 
    UInt32 Amask; 
    byte Rloss; 
    byte Gloss; 
    byte Bloss; 
    byte Aloss; 
    byte Rshift; 
    byte Gshift; 
    byte Bshift; 
    byte Ashift; 
    int refcount; 
    IntPtr nextPtr; 
} 

[StructLayout(LayoutKind.Sequential)] 
struct SDL_Surface 
{ 
    public readonly UInt32 flags; 
    public readonly IntPtr format; 
    public readonly int w, h;     
    public readonly int pitch;    
    public IntPtr pixels;    

    /// <summary>Application data associated with the surface</summary> 
    public IntPtr userdata;   

    /// <summary>information needed for surfaces requiring locks</summary> 
    public readonly int locked;    
    public readonly IntPtr lock_data;   

    /// <summary>clipping information</summary> 
    public readonly SDL_Rect clip_rect;   

    /// <summary>info for fast blit mapping to other surfaces</summary> 
    private IntPtr mapPtr; 

    /// <summary>Reference count -- used when freeing surface</summary> 
    public int refcount;    
} 

[StructLayout(LayoutKind.Sequential)] 
struct SDL_BlitMap 
{ 
    IntPtr dstPtr; 
    int identity; 
    SDL_blit blit; 
    IntPtr data; 
    SDL_BlitInfo info; 

    /* the version count matches the destination; mismatch indicates 
     an invalid mapping */ 
    UInt32 dst_palette_version; 
    UInt32 src_palette_version; 
} 

[StructLayout(LayoutKind.Sequential)] 
struct SDL_Rect 
{ 
    int x, y; 
    int w, h; 
} 

[UnmanagedFunctionPointer(CallingConvention.ToBeAdded)] 
internal delegate int SDL_blit(ref SDL_Surface src, ref SDL_Rect srcrect, ref SDL_Surface dst, ref SDL_Rect dstrect); 

当你需要阅读的任何结构指针：

var palette = (SDL_Palette) Marshal.PtrToStructure(surface.palettePtr, typeof (SDL_Pallete)); 

否则，如果你想坚持到您当前密码，尝试声明函数指针作为IntPtr而不是委托，并在运行时使用Marshal.GetDelegateForFunctionPointer创建委托。