如果抛出异常，则使extern C ++函数返回消息(Make extern C++ function return a message if an exception is thrown)

 我想让我的extern C ++函数在发生异常时返回一条消息。 像这样的东西： 
 
extern "C" __declspec(dllexport) const char* __stdcall Calculate(double &result, double a, double b) 
{
    try
    {
        result = InternalCalculation(a, b);
    }
    catch(std::invalid_argument& e)
    {
        return e.what();
    }
    return "";
}


double InternalCalculation(double a, double b)
{
    if(a < b)
    {
        const char* err = "parameters error!";
        throw std::invalid_argument(err);
    }
    return sqrt(a - b);
}
 
 另一方面，我从我的C＃程序调用该函数，我想在MessageBox显示错误： 
 
[DllImport(@"MyDll.dll", EntryPoint = "Calculate")]
private static extern IntPtr Calculate(out double result, double a, double b);

private void Calculate()
{
    IntPtr err;     
    double result = 0;
    err = Calculate(out result, out 2, out 3);
    var sErr = Marshal.PtrToStringAnsi(err);
    if (string.IsNullOrEmpty(sErr))
        MessageBox.Show(sErr);
    ...
}
 
 不幸的是，它不起作用.. MessageBox只显示随机字符。 
 
 我很惊讶因为如果我更换： 
 
return e.what();
 
 通过： 
 
const char* err = "parameters error!";
return err;
 
 然后“参数错误！” 将在C＃代码的消息框中正确显示。 err和e.what()都是相同的类型（ const char* ），所以e.what()有什么问题？ 

I would like to make my extern C++ function return a message when an exception occurs. Something like this:
 
extern "C" __declspec(dllexport) const char* __stdcall Calculate(double &result, double a, double b) 
{
    try
    {
        result = InternalCalculation(a, b);
    }
    catch(std::invalid_argument& e)
    {
        return e.what();
    }
    return "";
}


double InternalCalculation(double a, double b)
{
    if(a < b)
    {
        const char* err = "parameters error!";
        throw std::invalid_argument(err);
    }
    return sqrt(a - b);
}
 
On the other hand, I call the function from my C# program and I would like to show error in a MessageBox:
 
[DllImport(@"MyDll.dll", EntryPoint = "Calculate")]
private static extern IntPtr Calculate(out double result, double a, double b);

private void Calculate()
{
    IntPtr err;     
    double result = 0;
    err = Calculate(out result, out 2, out 3);
    var sErr = Marshal.PtrToStringAnsi(err);
    if (string.IsNullOrEmpty(sErr))
        MessageBox.Show(sErr);
    ...
}
 
Unfortunately, it doesn't work.. the MessageBox just shows random characters.
 
I'm surprised because if I replace:
 
return e.what();
 
by:
 
const char* err = "parameters error!";
return err;
 
Then "parameters error!" will be shown correctly in the messagebox of the C# code. Both err and e.what() are the same type (const char*), so what's wrong with e.what()??

原文：https://stackoverflow.com/questions/18162575