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

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.