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[Dmitry Shachnev <mitya57@xxxxxxxxx>]

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

** Changed in: qtbase-opensource-src (Ubuntu)
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1776446

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

Status in qtbase-opensource-src package in Ubuntu:
  Invalid

Bug description:
  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.

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