c# – 与不安全的代码一起使用时,ref的安全性如何？

使用Microsoft Visual C#2010,我最近注意到您可以通过引用将对象传递给非托管代码.所以我自己尝试编写一些非托管代码,使用回调托管代码将C char *转换为C#字符串.我做了两次尝试.

尝试1：调用存储ref参数的非托管函数.然后,一旦该函数返回到托管代码,调用另一个非托管函数,该函数调用将char *转换为托管字符串的回调函数.

C++
typedef void (_stdcall* CallbackFunc)(void* ManagedString,char* UnmanagedString);

CallbackFunc UnmanagedToManaged = 0;
void* ManagedString = 0;

extern "C" __declspec(dllexport) void __stdcall StoreCallback(CallbackFunc X) {
    UnmanagedToManaged = X;
}
extern "C" __declspec(dllexport) void __stdcall StoreManagedStringRef(void* X) {
    ManagedString = X;
}
extern "C" __declspec(dllexport) void __stdcall CallCallback() {
    UnmanagedToManaged(ManagedString,"This is an unmanaged string produced by unmanaged code");
}

C#
[DllImport("Name.dll",CallingConvention = CallingConvention.StdCall)]
public static extern void StoreCallback(CallbackFunc X);
[DllImport("Name.dll",CallingConvention = CallingConvention.StdCall)]
public static extern void StoreManagedStringRef(ref string X);
[DllImport("Name.dll",CallingConvention = CallingConvention.StdCall)]
public static extern void CallCallback();

[UnmanagedFunctionPointer(CallingConvention.StdCall)]
public delegate void CallbackFunc(ref string Managed,IntPtr Native);

static void Main(string[] args) {
    string a = "This string should be replaced";

    StoreCallback(UnmanagedToManaged);
    StoreManagedStringRef(ref a);
    CallCallback();
}

static void UnmanagedToManaged(ref string Managed,IntPtr Unmanaged) {
    Managed = Marshal.PtrToStringAnsi(Unmanaged);
}

尝试2：将字符串ref传递给将字符串ref传递给托管回调的非托管函数.

C++
typedef void (_stdcall* CallbackFunc)(void* ManagedString,char* UnmanagedString);

CallbackFunc UnmanagedToManaged = 0;

extern "C" __declspec(dllexport) void __stdcall StoreCallback(CallbackFunc X) {
    UnmanagedToManaged = X;
}
extern "C" __declspec(dllexport) void __stdcall DoEverything(void* X) {
    UnmanagedToManaged(X,CallingConvention = CallingConvention.StdCall)]
public static extern void DoEverything(ref string X);

[UnmanagedFunctionPointer(CallingConvention.StdCall)]
public delegate void CallbackFunc(ref string Managed,IntPtr Unmanaged);

static void Main(string[] args) {
    string a = "This string should be replaced";

    StoreCallback(UnmanagedToManaged);
    DoEverything(ref a);
}

static void UnmanagedToManaged(ref string Managed,IntPtr Unmanaged) {
    Managed = Marshal.PtrToStringAnsi(Unmanaged);
}

尝试1不起作用但尝试2起作用.在尝试1中,似乎只要在存储ref之后非托管代码返回,ref就变为无效.为什么会这样？

鉴于尝试1的结果,我怀疑尝试2是否可行.那么,当与非托管代码一起使用时,代码的非托管端是否安全？或者换句话说,在使用ref时,什么在非托管代码中不起作用？

我想知道的是：

使用ref将代码传递给非托管代码时究竟会发生什么？

在非托管代码中使用ref时,它是否保证对象将保留在内存中的当前位置？

在非托管代码中,ref(我不能用ref做什么)有什么限制？