如何从DLL返回未知大小的字符串到Visual Basic [英] How to return a string of unknown size from DLL to Visual Basic

问题描述

我有一个可视的基本脚本，该脚本调用DLL，该DLL进行网络请求并以字符串形式返回一个请求的结果。调用DLL之前，结果的长度未知。 DLL是我自己用C / C ++编写的。

I have a visual basic script that calls to a DLL which does network requests and returns the result of one request as a string. The length of the result is unknown before calling the DLL. The DLL is written in C/C++ by myself.

据我所知，从DLL返回字符串的最常用方法是将预分配的字符串对象的引用传递给DLL。然后，DLL函数将返回的字符串填充到此内存中。
问题是分配的缓冲区必须足够大，以确保结果适合该缓冲区。

As far as I see, the most often used way to return strings from a DLL is to pass a reference of a preallocated string object as arguement to the DLL. The DLL function then just fills this memory with the returning string. The problem is that the allocated buffer has to be large enough, to make sure the result fits into it.

是否可以直接从DLL返回结果字符串？还是有其他方法根据结果的长度动态分配字符串对象并将其返回给VB调用者？

Is it possible to directly return the resulting string from the DLL? Or is there any other way to dynamically allocate a string object depending on the length of the result and return it to a VB caller?

我尝试了不同的方法，例如这（丑陋的例子）：

I tried different approaches, for example like this (ugly, just examples):

__declspec(dllexport) const char* sendCommand(const char* cmd, const char* ipAddress)
{
    // do request stuff... 
    long lengthOfResult = ...
    const char* result = new char[lengthOfResult];
    return result;
}

或类似。.

__declspec(dllexport) BSTR sendCommand(const char* cmd, const char* ipAddress)
{
    _bstr_t result("Test string.");
    BSTR bstrResult = result.copy();
    return bstrResult;
}

视觉基本面：

Declare Function sendCommand Lib "magicdll.dll" (cmd as String, ip as String) As String
result = sendCommand("any command", "192.168.1.1")

都没有成功-VB中的结果字符串充满了垃圾。

Both without success - the resulting string in VB is filled with garbage.

推荐答案

大多数DLL都不返回字符串。他们接受一个字符串作为参数，并将一个char数组复制到该缓冲区中。尝试这样的事情：

Most DLLs don't return a string. They accept a string as a param and copy a char array into that buffer. Try something like this:

_declspec(dllexport) int sendCommand(const char* cmd, 
                                     const char* ipAddress, 
                                     char* pszBuffer, 
                                     int nBufferSize)

然后将字符串复制到该缓冲区并返回字符数：

And then copy your string into that buffer and return the number of chars:

int nSize = nBufferSize;
if (lstrlen(szMyResult) < nBufferSize)
    nSize = lstrlen(szMyResult);

lstrcpyn(pszBuffer, szMyResult, nSize);
return nSize;

从VB调用时，分配一个字符串并指定其大小：

When calling from VB, allocate a string and specify its size:

Dim s As String, intChars As Long
s = Space$(128)

intChars = sendCommand("...", "...", s, Len(s))
s = Left$(s, intChars) 

编辑：

如果必须通过函数调用返回字符串，则可以尝试创建BSTR （VB样式的字符串）并返回。您需要将字符串转换为Unicode，然后使用 SysAllocString（）创建BSTR。例如：

If you must return a string as the result of your function call, you can try creating a BSTR (a VB-style string) and returning that. You'll need to convert your string to Unicode and then use SysAllocString() to create the BSTR. For example:

BSTR ReturnVBString() 
{
    return SysAllocString(L"This string is from a C DLL.");
} 

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