我是否需要释放用作函数参数的托管BSTR

Question

If I have managed COM interface called from unmanaged code, am I responsible for freeing up the memory after use or will it be handled by garbage collection?

 public void WriteOutFile([In] [MarshalAs(UnmanagedType.BStr)] String data)
 {
      File.WriteAllText(fileName, data);
      //do I need the line below??
      Marshal.FreeBSTR(data);
 }

Thanks

Answer 1

Marshalling involves copying the data from unmanaged memory to managed memory. You don not need to free the string instance because it's a managed object. But if you allocate memory in the native code before the callback call, you need to free the memory in the native code after the callback call.

Answer 2

You should not free the string because the caller can potentially reuse the data passed and if you free it is possible to have a fault. The reason is that FreeBSTR does not use any reference count mechanism and simply call SysFreeString , that by the way assumes that the string is allocated with one of the function Sys(Re)Alloc... , circumstance you are not aware of in the managed code. The example shown here is interesting, imagin the unmanaged code calling you is this one ( from the link before ):

// shows using the Win32 function 
// to allocate memory for the string: 
BSTR bstrStatus = ::SysAllocString(L"Some text");
if (bstrStatus != NULL)
{
   pBrowser->put_StatusText(bstrStatus);
   // Free the string:
   ::SysFreeString(bstrStatus);
}

and you have imlemented put_StatusText(...) in your managed code we are reproducing your situation. As you can see is the caller responsible on allocating/deallocating the parameter string, outside the callee.