Why am I getting mojibake when I try to create a shell link?

A customer couldn't get the IShellLink interface to work. They tried to set the shortcut target to a path, but it came out as Chinese mojibake.

Here's a reduction of their code to its simplest form.

HRESULT CreateLink()
{
  HRESULT hr;
  IShellLinkA* psl;

  hr = CoCreateInstance(CLSID_ShellLink, NULL, CLSCTX_INPROC_SERVER,
                        IID_IShellLink, (LPVOID*)&psl);
  if (SUCCEEDED(hr)) {
    IPersistFile* ppf;

    psl->SetPath("C:\\Windows"); // this comes out as mojibake

    hr = psl->QueryInterface(IID_IPersistFile, (LPVOID*)&ppf);
    if (SUCCEEDED(hr)) {
      hr = ppf->Save(L"C:\\Test\\Test.lnk", TRUE);
      ppf->Release();
    }
    psl->Release();
  }
  return hr;
}

(You can see that this customer used to be a C programmer, because all variable declarations are at the start of blocks. Also, because they aren't using RAII.)

The problem is hidden in the call to Co­Create­Instance:

  hr = CoCreateInstance(CLSID_ShellLink, NULL, CLSCTX_INPROC_SERVER,
                        IID_IShellLink, (LPVOID*)&psl);
  //                    --------------  -------------

Observe that the requested interface is IID_IShell­Link, but the result is placed into a pointer to IShell­LinkA. This mismatch should raise a warning flag.

It appears that the program is being compiled with Unicode as the default character set, which means that IID_IShell­Link is really IID_IShell­LinkW. Consequently, the requested interface is IShell­LinkW, and the result is placed into a pointer to IShell­LinkA. As a result of this mismatch, the call to psl->SetPath thinks that it's calling IShell­LinkA::Set­Path, but in reality it is calling IShell­LinkW::Set­Path. (The IShell­LinkA and IShell­LinkW interfaces have the same methods in the same order, except that one uses ANSI strings and the other uses Unicode strings.)

That is where the mojibake is coming from. An ANSI string is passed where a Unicode string is expected.

Mismatches like this can be avoided by using the IID_PPV_ARGS macro. This macro looks at the type of the pointer you pass it and autogenerates the matching REFIID, as well as casting the pointer to void**.

  hr = CoCreateInstance(CLSID_ShellLink, NULL, CLSCTX_INPROC_SERVER,
                        IID_PPV_ARGS(&psl));

While they're at it, the customer should consider abandoning the ANSI version altogether and just using the Unicode one.