Sunday, April 28, 2013

Traversing JavaScript objects in .NET

I’ve been playing around with hosting a Google Map in a .NET WinForms control, for no good reason other than to see if it can be done. The key part of this integration is calling JavaScript on a web page from .NET. This appears reasonably straightforward, the WebBrowser control has a Document.InvokeScript method that does the trick. If your JavaScript function returns a value, you can pick this up from the return value of InvokeScript.

This works great for simple types, but what if your JavaScript function returns a more complex type? This is where things get a bit more tricky. The returned object has a type of System.__ComObject and initially it looks like this has no useful methods on it. This is the object that is used when calling any COM object, but generally you’ll be able to import a type library to create a .NET friendly wrapper around the raw object. This obviously isn’t the case here.

So my first thought was to find the real underlying type of the __ComObject. This piece of code helped out here. It turns out the actual underlying type was a JScriptTypeInfo but there’s very little information out there about what this type does or how to use it.

But it turns out there’s a much simpler way to access the returned object, cast the returned object as IReflect and use its methods to get properties etc. So the code for my .NET property looks like this

    public LatLng Centre
        object centre = webBrowser.Document.InvokeScript("getCentre");
        if (centre == null)
          return new LatLng(0, 0);

        IReflect reflect = centre as IReflect;
        double lat = (double)reflect.InvokeMember("lat", BindingFlags.InvokeMethod, null, 
          centre, null, null, null, null);
        double lng = (double)reflect.InvokeMember("lng", BindingFlags.InvokeMethod, null, 
          centre, null, null, null, null);

        return new LatLng(lat, lng);
          new object[] { value.Latitude, value.Longitude });

Another approach to this would be to have two JavaScript functions, getCentreLat and getCentreLng, which just return simple types, but that could get cumbersome with really complex types.

I’m not sure if it’s possible to pass complex objects into JavaScript but I haven’t needed that yet. Again, for really complex types, passing in each part of them could get messy.


Anonymous said...

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I was wondering if you know where I can get updated data on please?

Chris Bell said...

The March data has only just been released, so it's as up to date as it can be

Anonymous said...

Thanks! I'll keep checking back.