C# Signature:

    [DllImport("shell32.dll", EntryPoint = "SHOpenWithDialog", CharSet = CharSet.Unicode)]
    private static extern int SHOpenWithDialog(IntPtr hWndParent, ref tagOPENASINFO oOAI);

User-Defined Types:

    // http://msdn.microsoft.com/en-us/library/windows/desktop/bb773363(v=vs.85).aspx
    private struct tagOPENASINFO
    {
        [MarshalAs(UnmanagedType.LPWStr)]
        public string cszFile;

        [MarshalAs(UnmanagedType.LPWStr)]
        public string cszClass;

        [MarshalAs(UnmanagedType.I4)]
        public tagOPEN_AS_INFO_FLAGS oaifInFlags;
    }

    [Flags]
    private enum tagOPEN_AS_INFO_FLAGS
    {
           OAIF_ALLOW_REGISTRATION =  0x00000001,   // Show "Always" checkbox
           OAIF_REGISTER_EXT =    0x00000002,   // Perform registration when user hits OK
           OAIF_EXEC =        0x00000004,   // Exec file after registering
           OAIF_FORCE_REGISTRATION =  0x00000008,   // Force the checkbox to be registration
           OAIF_HIDE_REGISTRATION =   0x00000020,   // Vista+: Hide the "always use this file" checkbox
           OAIF_URL_PROTOCOL =    0x00000040,   // Vista+: cszFile is actually a URI scheme; show handlers for that scheme
           OAIF_FILE_IS_URI =     0x00000080    // Win8+: The location pointed to by the pcszFile parameter is given as a URI
    }

Alternative Managed API:

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Notes:

Requires Windows Vista or later; on earlier platforms, use rundll32 to run OpenAs

Tips & Tricks:

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Sample Code:

    public static void DoOpenFileWith(string sFilename)
    {
        if (Environment.OSVersion.Version.Major > 5)
        {
        IntPtr hwndParent = IntPtr.Zero;
        if (null != MyAppForm) hwndParent = MyAppForm.Handle;

        tagOPENASINFO oOAI = new tagOPENASINFO();
        oOAI.cszFile = sFilename;
        oOAI.cszClass = String.Empty;
        oOAI.oaifInFlags = tagOPEN_AS_INFO_FLAGS.OAIF_ALLOW_REGISTRATION | tagOPEN_AS_INFO_FLAGS.OAIF_EXEC;
        SHOpenWithDialog(hwndParent, ref oOAI);
        }
        else
        {
        using (Process.Start("rundll32", "shell32.dll,OpenAs_RunDLL " + sFilename)) { }
        }
    }

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