Unknown error loading plugin in Design Mode on Windows

[RobertoD]

I built the qmlextensionplugins example using Qt 11.1, QtCreator 4.7.1 and MSVC2017 on Windows. I'm loading the generated plugin in a simple QtQuick project displaying the clock in a window.

If I build and run the example it works well.

But if I opend the qml in Design Mode I have an error near the Window component and the plugin components are not loaded.

The error is the following:
found not working imports: file:///C:/XXX/main.qml:3 plugin cannot be loaded for module "TimeExample": cannot load library C:\XXX\TimeExample\qmlqtimeexampleplugin.dll: Unknown error 0x000000c1

I tried with other plugins with the same result. Instead the same code works well also in Design mode on Linux and macOs.

The project files are the following:

main.qml

import QtQuick 2.11
import QtQuick.Window 2.11
import TimeExample 1.0

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
    Clock { // this class is defined in QML (imports/TimeExample/Clock.qml)

        Time { // this class is defined in C++ (plugin.cpp)
            id: time
        }

        hours: time.hour
        minutes: time.minute

    }
}

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QDebug>
int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    QList<QQmlError> errors;
    engine.addImportPath("C:\\XXX\\imports");
    for(auto e: errors)
        qDebug() << e.toString();
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

example.pro:

QT += quick
CONFIG += c++11

# The following define makes your compiler emit warnings if you use
# any feature of Qt which as been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS QT_DEBUG_PLUGINS QML_IMPORT_TRACE

# You can also make your code fail to compile if you use deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
        main.cpp

RESOURCES += qml.qrc

# Additional import path used to resolve QML modules in Qt Creator's code model
QML_IMPORT_PATH = C:\XXX\imports
QML2_IMPORT_PATH = C:\XXX\imports
QT_PLUGIN_PATH = C:\XXX\imports

# Additional import path used to resolve QML modules just for Qt Quick Designer
QML_DESIGNER_IMPORT_PATH = C:\XXX\imports

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

How can I fix this problem?

[christian.gudrian]

Hi!

On Windows QtCreator is a 32 bit application and can hence only load 32 bit DLLs. I suspect your plugin is a 64 bit DLL.

Christian