QDBusInterface is able to connect to org.freedesktop.DBus, but its status is invalid

[Dmitry]

Consider the following example:

#include <QtCore>
#include <QtDBus>

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);

    QDBusInterface dbus("org.freedesktop.DBus",
                        "/org/freedesktop/DBus",
                        "org.freedesktop.DBus",
                        QDBusConnection::sessionBus());

    if (!dbus.isValid())
    {
        qDebug() << "IFACE IS NOT VALID" << dbus.lastError().message();
        return 1;
    }

    QDBusReply<QStringList> servicesReply = dbus.call("ListNames");

    if (!servicesReply.isValid())
    {
        qDebug() << "REPLY IS NOT VALID" << servicesReply.error().message();
        return 2;
    }

    qDebug() << servicesReply.value();
}

Compile and run. It prints

IFACE IS NOT VALID ""

The issue here is that QDBusInterface::isValid() returns false, however it's still possible to call its methods. If I comment out the "if (!dbus.isValid())" line, it starts to work fine:

#include <QtCore>
#include <QtDBus>

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);

    QDBusInterface dbus("org.freedesktop.DBus",
                        "/org/freedesktop/DBus",
                        "org.freedesktop.DBus",
                        QDBusConnection::sessionBus());
/*
    if (!dbus.isValid())
    {
        qDebug() << "IFACE IS NOT VALID" << dbus.lastError().message();
        return 1;
    }
*/
    // WORKS FINE!
    QDBusReply<QStringList> servicesReply = dbus.call("ListNames");

    if (!servicesReply.isValid())
    {
        qDebug() << "REPLY IS NOT VALID" << servicesReply.error().message();
        return 2;
    }

    qDebug() << servicesReply.value();
}

In Ubuntu 16.04 it worked fine. All updates have been installed.

[Dmitry Shachnev (mitya57)]

I think you should not try to interact with org.freedesktop.DBus interface directly, instead you should use QDBusConnectionInterface for that. In your particular case, this would be using https://doc.qt.io/qt-5/qdbusconnectioninterface.html#registeredServiceNames-prop:

const QDBusConnectionInterface *iface = QDBusConnection::sessionBus().interface();
QDBusReply<QStringList> servicesReply = iface->registeredServiceNames();
qDebug() << servicesReply.value();

Any other interface would work fine, org.freedesktop.dbus is blacklisted here: https://code.qt.io/cgit/qt/qtbase.git/tree/src/dbus/qdbusintegrator.cpp?h=5.9#n2448