如何在C ++和C#之间传递字符串参数?

问题描述

我已经在我的C#项目中导入了一个C ++库.我可以获得没有参数的C ++方法的答案.我要做的是将一些字符串参数发送到C ++并获取其结果.

I have imported a C++ library in my C# project. I can get the answer of a C++ method that has no parameters. What I want to do is to send some string parameters to C++ and get the result of it.

在C ++中:

public String SomeMethod(String param1, String param2)
{
     //Some code
}

---

在C#中:

---

In C#:

[DllImport("Library Path")]
public static extern string SomeMethod(string param1, string param2);

我该怎么做?

推荐答案

对于从C#传递到C ++的输入参数的字符串，您可以在以下位置使用LPCWSTR(即const wchar_t*) C ++一侧的界面，然后使用C#一侧的 UnmanagedType.LPWstr 选项进行封送.

For strings that are input parameters passed from C# to C++, you could just use LPCWSTR (i.e. const wchar_t*) at the interface on the C++ side, and marshal using the UnmanagedType.LPWstr option on the C# side.

但是要将字符串从C ++返回到C#时必须注意.
实际上，必须以某种方式在C ++中分配字符串，然后必须将字符串指针传递给C#. C#和CLR必须能够在不再需要此字符串时正确地释放，这可能会引起问题.例如:C#/CLR应该能够使用C ++代码用于分配字符串的相同分配器释放字符串内存.

But you must pay attention when you want to return strings from C++ to C#.
In fact, the string must somehow be allocated in C++, then the string pointer must be passed to C#. C# and the CLR must be able to properly release this string when it's no needed anymore, and this point can present issues. For example: C#/CLR should be able to release the string memory using the same allocator used by the C++ code to allocate the string.

对于在C ++中创建并在C#中使用的字符串使用相同的内存分配器，可以使用 BSTR s 解决.
BSTR是一个COM字符串，使用COM分配器分配，可以使用 same COM分配器释放.因此，C ++和C#端都可以为该字符串共享相同的内存分配器.

This problem of using the same memory allocator for strings created in C++ and consumed in C# can be solved using BSTRs.
A BSTR is a COM string, that is allocated using the COM allocator, and can be freed using the same COM allocator. So, both the C++ and the C# side can share the same memory allocator for the string.

要返回BSTR，可以使用 MarshalAs.UnmanagedTypeBStr 选项.

To return a BSTR, you can use the MarshalAs.UnmanagedTypeBStr option.

有关C#中的字符串封送处理的更多详细信息，请参见此MSDN页面.

More details on string marshaling in C# can be found in this MSDN page.

在 C ++ 中:

//
// Note: Export using a pure C interface from C++ DLL.
// (You can use C++ *inside* the DLL boundaries.)
// __stdcall is a widely used calling convention for
// C DLL functions (e.g. lots of Win32 APIs use it).
//
extern "C" BSTR __stdcall GetSomeString(void)
{
    //
    // Note: BSTRs are allocated using SysAllocString()
    // (...and freed using SysFreeString()).
    //
    // You could also use a convenient ATL RAII wrapper
    // (instead of using raw BSTRs inside C++ code),
    // like CComBSTR.
    //
    return ::SysAllocString(L"Hello from C++!");
}

在 C#中:

[DllImport(@"YourDll.dll", CallingConvention = CallingConvention.StdCall)]
[return: MarshalAs(UnmanagedType.BStr)]
private static extern string GetSomeString();

如果您选择使用BSTR作为返回字符串，则对于C ++函数接口代码的一致性，您可能还希望使用BSTR s作为输入字符串(即使并非绝对必要).

If you choose to have BSTRs as the returned string, for coherence of the C++ function interface code, you may also want to use BSTRs as the input strings (even if that is not strictly necessary).

您还有一个选择，就是请求调用者提供适当大小的缓冲区，本机C ++函数将用返回的字符串填充该缓冲区.这有点类似于几种Win32 API(例如 GetWindowText() ).
例如，在此处，您可以找到GetWindwoText()的相应P/Invoke :

Another option you have is to request the caller to provide a properly sized buffer that the native C++ function will fill with the returned string. This is somewhat similar to what several Win32 APIs (like e.g. GetWindowText()) do.
As an example, here you can find the corresponding P/Invoke for GetWindwoText():

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);

但是，我认为从C#调用的角度来看，返回BSTR更好(实际上，例如，在C#端，您不需要对输出字符串使用StringBuilder等).

However, I think that returning a BSTR is nicer from a C# calling perspective (in fact, e.g. on the C# side you are not required to use a StringBuilder for the output string, etc.).

最后但并非最不重要的，您可能感兴趣的另一种选择是构建一个很小的 C ++/CLI桥接层，以将本机C ++代码公开给C#，并在本机和托管使用之间编组字符串C ++/CLI(例如System::String类).

Last but not least, another option that you might be interested in is to build a tiny C++/CLI bridging layer to expose your native C++ code to C#, and marshal strings between native and managed using C++/CLI (and e.g. the System::String class).

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