[In, Out, MarshalAs(UnmanagedType.IUnknown)] ref object ppvObject);    

    <MarshalAs(UnmanagedType.IUnknown)> ByRef ppvObject As Object) As Int32

    <MarshalAs(UnmanagedType.IUnknown)> ByRef ppvObject As Object) As Int32

    [MarshalAs(UnmanagedType.IUnknown)]

        //public unsafe IUnknown* punkVal;

        //public unsafe IUnknown** ppunkVal;

    'public unsafe IUnknown* punkVal;

    'public unsafe IUnknown** ppunkVal;

   [MarshalAs(UnmanagedType.IUnknown)] object pUnk, IntPtr lpvReserved);

[return: MarshalAs(UnmanagedType.IUnknown)]

static extern object CoCopyProxy([MarshalAs(UnmanagedType.IUnknown)] object pProxy);

[return: MarshalAs(UnmanagedType.IUnknown)]

   [MarshalAs(UnmanagedType.IUnknown)] object punkOuter);

   [MarshalAs(UnmanagedType.IUnknown)] object pUnkOuter,

   [MarshalAs(UnmanagedType.IUnknown)] object pUnkOuter,

   [MarshalAs(UnmanagedType.IUnknown)] object pUnk,

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

   pclsid, [MarshalAs(UnmanagedType.IUnknown)] object pUnkOuter, uint dwClsCtx,

   Guid riid, [MarshalAs(UnmanagedType.IUnknown)] out object ppv);

   [MarshalAs(UnmanagedType.IUnknown)] object pUnk, MSHCTX dwDestContext,

   [MarshalAs(UnmanagedType.IUnknown)] object pUnk, out IStream ppStm);

    [MarshalAs(UnmanagedType.IUnknown)] object pUnk,

static extern int CoSetProxyBlanket([MarshalAs(UnmanagedType.IUnknown)]

Please look at http://msdn2.microsoft.com/en-us/library/aa302389.aspx#secnetch09_callingservicedcompaspnet, which shows how to call CoSetProxyBlanket in C#. As this is published on the MS Patterns and Practices web site I would assume this would be OK and is safe. This reference has code and discusses the issues about transition from .net object to COM IUnknown reference using the Marshal functions.

   [MarshalAs(UnmanagedType.IUnknown)] out object ppv);

   [MarshalAs(UnmanagedType.IUnknown)] out object ppv);

static extern int CreateObjrefMoniker([MarshalAs(UnmanagedType.IUnknown)]

static extern int CreatePointerMoniker([MarshalAs(UnmanagedType.IUnknown)] object

        pIUnk = Marshal.GetIUnknownForObject((object)objComCtrl); // returns a pointer to a pointer of the object’s IUnknown Interface

            ErrorMsg = "GetIUnknownForObject was zero.";

            IPersistStreamInit pPersistStream = (IPersistStreamInit)Marshal.GetObjectForIUnknown(pIPStream);

       string iid = "{00000000-0000-0000-C000-000000000046}"; // IUnknown

       string iid = "{00000000-0000-0000-C000-000000000046}"; // IUnknown

    int cObjects, [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 4, ArraySubType = UnmanagedType.IUnknown)] object[] lplpUnk,

    <MarshalAs(UnmanagedType.Interface, ArraySubType:=UnmanagedType.IUnknown)> _

        [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 4, ArraySubType = UnmanagedType.IUnknown)] object[] lplpUnk,

    UInt32 cObjects, [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 4, ArraySubType = UnmanagedType.IUnknown)] object[] lplpUnk,

    <MarshalAs(UnmanagedType.Interface, ArraySubType:=UnmanagedType.IUnknown)> _

       static extern int OleDraw([MarshalAs(UnmanagedType.IUnknown)] object pUnk,

           IntPtr pUnk = Marshal.GetIUnknownForObject(c.GetOcx());

static extern int OleGetClipboard([MarshalAs(UnmanagedType.IUnknown)]out object ppDataObj);

[return: MarshalAs(UnmanagedType.IUnknown)]

    static extern int OleGetClipboard([MarshalAs(UnmanagedType.IUnknown)]out object ppDataObj);

    [return: MarshalAs(UnmanagedType.IUnknown)]

   [MarshalAs(UnmanagedType.IUnknown)] out object ppvObj);

      if (varType == 13) //Object support IUnknown

        Guid IID_IUnknown = new Guid("00000000-0000-0000-C000-000000000046");

        int ret = OleLoadFromStream(pStm, ref IID_IUnknown, out obj);

   [MarshalAs(UnmanagedType.IUnknown)] out object ppvObj);

      if (varType == 13) //Object support IUnknown

        Guid IID_IUnknown = new Guid("00000000-0000-0000-C000-000000000046");

        int ret = OleLoadFromStream(stream, ref IID_IUnknown, out obj);

static extern int OleRun([MarshalAs(UnmanagedType.IUnknown)] object pUnknown);

IntPtr pUnknown = Marshal.GetIUnknownForObject(fooObject);

   [MarshalAs(UnmanagedType.IUnknown)] out object ppObjectOpen);

   [MarshalAs(UnmanagedType.IUnknown)] out object ppObjectOpen);

   [MarshalAs(UnmanagedType.IUnknown)] out object ppObjectOpen);

        [MarshalAs(UnmanagedType.IUnknown)]

    [MarshalAs(UnmanagedType.IUnknown)] object pCaller,

    [MarshalAs(UnmanagedType.IUnknown)] object pCaller,

    [MarshalAs(UnmanagedType.IUnknown)] object pCaller,

Private Shared Function URLDownloadToFile(<MarshalAs(UnmanagedType.IUnknown)> pCaller As Object, <MarshalAs(UnmanagedType.LPWStr)> szURL As String, <MarshalAs(UnmanagedType.LPWStr)> szFileName As String, dwReserved As Int32, lpfnCB As IntPtr) As Int32

    [MarshalAs(UnmanagedType.IUnknown)]    object pCaller,

   [MarshalAs(UnmanagedType.IUnknown)] out Object ppunk);

   [MarshalAs(UnmanagedType.IUnknown)] out Object ppunk);

   <MarshalAs(UnmanagedType.IUnknown), Out> ppunk As Object) As Integer

[Guid("c46ca590-3c3f-11d2-bee6-0000f805ca57"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

          [MarshalAs(UnmanagedType.IUnknown)] object pUnkOuter,

           <MarshalAs(UnmanagedType.IUnknown)> ByVal pUnkOuter As Object, _

    [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

[InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]

<InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)> _

    InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

[InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]

        Object obj = Marshal.GetObjectForIUnknown( SfParent );

    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

        public const int BFFM_IUNKNOWN = 5; // provides IUnknown to client. lParam: IUnknown*

    [InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]

Object obj = Marshal.GetTypedObjectForIUnknown(ppMalloc, mallocType);

'TODO: Insert <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _ if this doesn't derive from IDispatch

//TODO: Insert [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)] if this doesn't derive from IDispatch

'TODO: Insert <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _ if this doesn't derive from IDispatch

//TODO: Insert [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)] if this doesn't derive from IDispatch

'TODO: Insert <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _ if this doesn't derive from IDispatch

'TODO: Insert <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _ if this doesn't derive from IDispatch

'TODO: Insert <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _ if this doesn't derive from IDispatch

//TODO: Insert [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)] if this doesn't derive from IDispatch

'TODO: Insert <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _ if this doesn't derive from IDispatch

    <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

<ComImport(), InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("F490EB00-1240-11D1-9888-006097DEACF9")> _

        [return: MarshalAs(UnmanagedType.IUnknown)]

'TODO: Insert <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _ if this doesn't derive from IDispatch

        [return: MarshalAs(UnmanagedType.IUnknown)]

'TODO: Insert <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _ if this doesn't derive from IDispatch

[InterfaceTypeAttribute(ComInterfaceType.InterfaceIsIUnknown)]

'TODO: Insert <InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _ if this doesn't derive from IDispatch

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]

<InterfaceType(ComInterfaceType.InterfaceIsIUnknown)>