如何使用extern＆QUOT; C＆QUOT; DLL函数以字符**在C＃应用程序的参数？ [英] How to use extern "C" dll function taking char** as an argument in C# application?

问题描述

我有一个功能DLL：

extern "C"
int
doJob(char** buffer);

及其与C ++使用情况是这样的：

Its usage with C++ looks like this:

char* buf;
int status = doJob(&buf);

我应该有什么的定义在C＃中这个功能呢？
我怎样才能使用此功能在C＃？

What definition should I have for this function in C#? How can I use this function in C#?

推荐答案

一个可能的模式是：

[DllImport("containsdojob.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern Int32 doJob(out IntPtr buffer);

[DllImport("containsdojob.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void freeMemory(IntPtr buffer);

和

IntPtr buffer = IntPtr.Zero;
string str = null;

try
{
    doJob(out buffer);

    if (buffer != IntPtr.Zero)
    {
        str = Marshal.PtrToStringAnsi(buffer);
    }
}
finally
{
    if (buffer != IntPtr.Zero)
    {
        freeMemory(buffer);
    }
}

请注意，你需要一个 freeMemory 方法以释放 doJob 分配的内存。

Note that you'll need a freeMemory method to free the memory allocated by doJob.

有其他可能的模式，例如基于 BSTR 和 SysAllocString 更易于实现C＃-side （但较难实现的C面）

There are other possible patterns, for example based on BSTR and SysAllocString that are easier to implement C#-side (but more difficult to implement C-side)

模式，使用BSTR：

C-端：

char *str = "Foo"; // your string
int len = strlen(str);
int wslen = MultiByteToWideChar(CP_ACP, 0, str, len, 0, 0);
BSTR bstr = SysAllocStringLen(NULL, wslen);
MultiByteToWideChar(CP_ACP, 0, str, len, bstr, wslen);
// bstr is the returned string

C＃ - 侧：

C#-side:

[DllImport("containsdojob.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern Int32 doJob([MarshalAs(UnmanagedType.BStr)] out string buffer);

string str;
doJob(out str);

该内存由CLR自动处理（释放）。

The memory is automatically handled (freed) by the CLR.

如果您使用Visual C ++，你甚至可以

If you are using Visual C++ you can even

char *str = "Foo"; // your string
_bstr_t bstrt(str);
BSTR bstr = bstrt.Detach(); 
// bstr is the returned string

或C端，你可以用它可以释放C＃ - 侧两个分配器之一：的 LocalAlloc 或<一个href=\"https://msdn.microsoft.com/en-us/library/windows/desktop/ms692727%28v=vs.85%29.aspx?f=255&MSPPError=-2147217396\"相对=nofollow> CoTaskMemAlloc ：

Or C-side you could use one of the two allocators that can be freed C#-side: LocalAlloc or CoTaskMemAlloc:

char *str = "Foo"; // your string
char *buf = (char*)LocalAlloc(LMEM_FIXED, strlen(str) + 1);
// or char *buf = (char*)CoTaskMemAlloc(strlen(str) + 1);
strcpy(buf, str);
// buf is the returned string

然后，可以使用第一个例子，但不是叫

Then you use the first example, but instead of calling

freeMemory(buffer);

您拨打：

Marshal.FreeHGlobal(buffer); // for LocalAlloc

或

Marshal.FreeCoTaskMem(buffer); // for CoTaskMemAlloc

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