COM interop marshalling format

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C#

COM

If a COM interface has a definition in C++:

interface Ixxx {
 ....
HRESULT fn([in]LPWSTR pszName, [in]REFIID riid, [out]void** ppv);

I am declaring it in my COMImport as

[ComImport, Guid(...)]
Interface Ixxx {
 ....
void fn(
    [In, MarshalAs(UnmanagedType.LPWStr)] string pszName,
    [In, MarshalAs(UnmanagedType.LPStruct)] Guid riid,
    [Out, MarshalAs(UnmanagedType.Interface, IidParameterIndex = 1)] out IInterface ppv);

Then, when I try to call the function...

IInterface obj;
Ixxx x = // object implementing the Ixxx interface;
string name="name";
try {
    x.fn(name, typeof(IInterface).GUID, out obj);
}
catch { // Handle the COM exception

...I am getting a protected memory violation. There is something amiss with the way I am declaring the void** parameter, I am sure, although it all seems to work ok when calling an API function with the same sort of signature.

Any ideas on how the marshalling should be declared?

What I have tried:

setting ppv as an IntPtr
passing riid by ref
leaving off the IidParameterIndex parameter

and, of course, google.