How to use VARIANT* with dynamicCall?

问题

I'm trying to use a COM object and i'm having problem with the parameter type VARIANT*. I can use the functions of the COM object just fine, except when they have a parameter of this type.

The doc generated by generateDocumentation is :

QVariantList params = ...

object->dynamicCall("GetRanges(int,int,int&, QVariant&)", params);

According to the doc provided with the COM object, the parameters should be of type LONG, LONG, LONG* and VARIANT*, and it is precised that the VARIANT* is a pointer to a VARIANT containing an array of BSTR.

I should normally be able to retrieve the third and fourth parameter (of type LONG* and VARIANT*), and their values are not used by the function.

Here is my code (a and b are int previously initialized):

QStringList sl;
QVariantList params;
int i = -1;
params << QVariant (a);
params << QVariant (b);
params << QVariant (i);
params << QVariant (sl);

comobject->dynamicCall("GetRanges(int,int,int&,QVariant&)",params);
sl = params[3].toStringList();
i = param[2].toInt();

Now with that code, all i get is an error QAxBase: Error calling IDispatch member GetRanges: Unknown error, which is not very helpful.

I tried to change some things and I managed to progress (sort of) by using this code :

QStringList sl;
QVariant v = qVariantFromValue(sl);
QVariantList params;
int i = -1;
params << QVariant (a);
params << QVariant (b);
params << QVariant (i);
params << qVariantFromValue((void*)&v);

comobject->dynamicCall("GetRanges(int,int,int&,QVariant&)",params);
sl = params[3].toStringList();
i = param[2].toInt();

It gets rid of the error, and the value of i is correct at the end, but sl is still empty. And I know it should not be, because I have a sample demo in C# that works correctly.

So if anyone has an idea on how to make it works...

Otherwise I looked around a bit and saw that it was also possible to query the interface ans use it directly, but I didn't understand much, and I'm not sure it will solve my problems.

I'm on a Windows7 64 bits platform, and I'm using msvc2012 as compiler. I'm using Qt 5.1.0 right now, but it didn't work in the 5.0.2 either.

回答1:

I guess you really can't do it with dynamicCall. I finally found how to do it. It was easier than I'd thought. With the installation of Qt comes a tool called dumpcpp. Its full path for me was C:\Qt\Qt5.1.0x86\5.1.0\msvc2012\bin\dumpcpp.exe (obviously depends on settings). You can just add the bin folder to your path to make it easier to use. Then I went into my project folder and executed this command :

dumpcpp -nometaobject {00062FFF-0000-0000-C000-000000000046} (the CLSID is just for the example, not the one I used)

It creates a header file, you can include it in the file where you're trying to use the COM Object.

In this file in my case there was two classes (IClassMeasurement and ClassMeasurement) in a namespace (MeasurementLib). Again, the names are not the real ones.

In your initial project file, you can call the desired function like this :

MeasurementLib::ClassMeasurement test; //Do not use IClassMeasurement, you only get write access violations
QVariant rangesVar;
int p1 = 0;
int p2 = 0;
int p3 = 0;
test.getRanges(p1,p2,p3,ranges);
QStringList ranges = ranges.toStringList();

Hopes that it helps someone !

来源：https://stackoverflow.com/questions/17595606/how-to-use-variant-with-dynamiccall

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