返回char的C函数的PInvoke*

您必须将其作为IntPtr返回。返回一个系统。PInvoke函数的字符串类型需要非常小心。CLR必须将内存从本机表示转移到托管表示。这是一个简单且可预测的操作。

然而,问题在于如何处理从返回的本机内存 foo() CLR假设PInvoke函数有以下两项,该函数直接返回字符串类型

1. 需要释放本机内存
2. 使用CoTaskMemAlloc分配了本机内存

因此,它将封送字符串,然后调用 CoTaskMemFree(...) 在本机内存blob上。除非您实际使用CoTaskMemAlloc分配了此内存,否则这最多只会导致应用程序崩溃。

为了在这里获得正确的语义,您必须直接返回一个IntPtr。然后使用Marshal。PtrToString*,以获取托管字符串值。您可能仍然需要释放本机内存,但这取决于foo的实现。

你可以使用元帅。PtrToStringAuto方法。

IntPtr ptr = foo();
string str = Marshal.PtrToStringAuto(ptr);

目前的答案并不完整,所以我只想在一个地方发布完整的解决方案。返回IntPtr而不是字符串根本不能解决任何问题,因为您仍然需要释放C脚本中分配的本机内存。最好的解决方案是在托管端分配字节缓冲区,并将内存传递给C脚本,该脚本将字符串写入该缓冲区,而根本不分配内存。

从C返回字符串的过程如下:

//extern "C" __declspec(dllexport) uint32_t foo(/*[out]*/ char* lpBuffer, /*[in]*/ uint32_t uSize)
uint32_t __stdcall foo(/*[out]*/ char* lpBuffer, /*[in]*/ uint32_t uSize)
{
  const char szReturnString[] = "Hello World";
  const uint32_t uiStringLength = strlen(szReturnString);

  if (uSize >= (uiStringLength + 1))
  {
    strcpy(lpBuffer, szReturnString);
    // Return the number of characters copied.
    return uiStringLength;
  }
  else
  {
    // Return the required size
    // (including the terminating NULL character).
    return uiStringLength + 1;
  }
}

C#代码:

[DllImport(_dllLocation, CallingConvention=CallingConvention.StdCall, CharSet=CharSet.Ansi)]
private static extern uint foo(IntPtr lpBuffer, uint uiSize);

private static string foo()
{
    // First allocate a buffer of 1 byte.
    IntPtr lpBuffer = Marshal.AllocHGlobal(1);
    // Call the API. If the size of the buffer
    // is insufficient, the return value in
    // uiRequiredSize will indicate the required
    // size.
    uint uiRequiredSize = foo(lpBuffer, 1);

    if (uiRequiredSize > 1)
    {
        // The buffer pointed to by lpBuffer needs to be of a
        // greater size than the current capacity.
        // This required size is the returned value in "uiRequiredSize"
        // (including the terminating NULL character).
        lpBuffer = Marshal.ReAllocHGlobal(lpBuffer, (IntPtr)uiRequiredSize);
        // Call the API again.
        foo(lpBuffer, uiRequiredSize);
    }

    // Convert the characters inside the buffer
    // into a managed string.
    string str = Marshal.PtrToStringAnsi(lpBuffer);

    // Free the buffer.
    Marshal.FreeHGlobal(lpBuffer);
    lpBuffer = IntPtr.Zero;

    // Display the string.
    Console.WriteLine("GetString return string : [" + str + "]");

    return str;
}

使用StringBuilder在C#端管理内存分配/释放有一种更简单的方法:

[DllImport("TestDLL.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi)]
private static extern uint foo(StringBuilder lpBuffer, UInt32 uiSize);

private static string foo()
{
    StringBuilder sbBuffer = new StringBuilder(1);
    uint uiRequiredSize = foo(sbBuffer, (uint)sbBuffer.Capacity);

    if (uiRequiredSize > sbBuffer.Capacity)
    {
        // sbBuffer needs to be of a greater size than current capacity.
        // This required size is the returned value in "uiRequiredSize"
        // (including the terminating NULL character).
        sbBuffer.Capacity = (int)uiRequiredSize;
        // Call the API again.
        foo(sbBuffer, (uint)sbBuffer.Capacity);
    }

    return sbBuffer.ToString();
}

有一个 good topic 解释了从C/C++代码返回字符串的不同方法。