如何将字符串参数从c ++ com传递到c＃？

Question

I have c++ code that has a parameter like this:

STDMETHODIMP TVinaImpl::test(BSTR Param1)
{
  try
  {
      Param1=(L"test1");
  }
  catch(Exception &e)
  {
    return Error(e.Message.c_str(), IID_IVina);
  }
  return S_OK;
}  

I use c++ builder:

When I call this com dll function in c# it shows me the error:

IntPtr a = new IntPtr();
vina.test(a);

it is null and did not get the value.
How can I pass variable from C# to c++ com and pass back it?

Answer 1

Since Param1 is declared as an [in, out] parameter, you need to declare it as a pointer to a BSTR: STDMETHODIMP TVinaImpl::test(BSTR* Param1)

Furthermore, you cannot simply assign a string literal to a BSTR. The correct way is to allocate memory using SysAllocString : *Param1 = SysAllocString(L"test1");

Answer 2

I'd recommend you declare the argument Param1 as [out, retval] . You are not using Param1 as any kind of input.

STDMETHODIMP TVinaImpl::test(BSTR* Param1)
{
  try
  {
      *Param1= SysAllocString(L"test1");
  }
  catch(Exception &e)
  {
    return Error(e.Message.c_str(), IID_IVina);
  }
  return S_OK;
}  

Then when you call the function from C# it is just,

string s = vina.test();

The .Net runtime manages marshalling of data from .Net to COM and vice versa.