在Xamarin Forms Android中循环出arraylist时，无法获取KML文件地标

Question

I have been working with a project that uses google maps and kml-routes. This project worked successfully a few months ago but now when I debug it the app crashes with the following errormessage:

System.ArgumentNullException

If I comment away...

foreach (var data in lineString.ToEnumerable())
{

}

...then the app successfully runs meaning that it can't get the values inside ”Placemarks”.

This is the code:

public async void OnMapReady(GoogleMap googleMap)
{
     InvokeOnMapReadyBaseClassHack(googleMap);
     map = googleMap;
     layer = new KmlLayer(map, Resource.Raw.mykmlfile, Android.App.Application.Context); 
}

async void MoveCameraToKmlTwo(KmlLayer kmlLayer)
{
        var list = new List<object>();

        if (kmlLayer.HasContainers)
        {
            void IterateProperties(KmlContainer containers)
            {
                foreach (var property in containers.Properties.ToEnumerable())
                {

                }
            }
            void IterateLineString(ArrayList lineString)
            {
                //when i try to get out the data in here i get the crash
                foreach (var data in lineString.ToEnumerable())
                {

                }
            }

            void IteratePlaceMarks(KmlContainer container)
            {
                foreach (KmlPlacemark placemark in container.Placemarks.ToEnumerable())
                {
                    IterateProperties(container);
                    if (placemark.HasGeometry & placemark.Geometry is KmlLineString)
                    {
                        IterateLineString(placemark.Geometry.GeometryJavaObject() as Java.Util.ArrayList);

                    }
                }
            }

            void IterateSubContainers(KmlContainer container)
            {
                IterateProperties(container);
                IteratePlaceMarks(container);
                if (container.HasContainers)
                {
                    foreach (KmlContainer subContainer in container.Containers.ToEnumerable())
                        IterateSubContainers(subContainer);
                }
            }

            foreach (KmlContainer container in kmlLayer.Containers.ToEnumerable())
                IterateSubContainers(container);

        }
}

My kml file looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://earth.google.com/kml/2.1">

<Document>
<name>Routes</name>
<description></description>
    <visibility>1</visibility>
    <open>1</open>

    <Style id="dark_green">
        <LineStyle>
        <color>C8008C14</color>
        <width>4</width>
        </LineStyle>
    </Style>

    <Style id="route_dark_blue">
        <LineStyle>
        <color>96F01414</color>
        <width>4</width>
        </LineStyle>
    </Style>


  <Folder>
        <name>Tracks</name>
        <description>A list of tracks</description>
        <visibility>1</visibility>            
        <open>0</open>                                      
            <Placemark>
                <visibility>0</visibility>            
                <open>0</open> 
                <styleUrl>#red</styleUrl>
                <name>Around the lake</name>
                <description>Track no. 1</description>
                <LineString>
                    <extrude>true</extrude>
                    <tessellate>true</tessellate>
                    <altitudeMode>clampToGround</altitudeMode> 
                    <coordinates>
                        7.366653,42.281982,106.075562 7.366759,42.282024,99.504028 7.366846,42.282043,95.945312 7.366909,42.282085,94.900269 7.366982,42.282116,94.186218 7.367039,42.282150,90.530640 7.367124,42.282135,87.749268 7.367111,42.282192,89.794800 7.367088,42.282242,90.636597 7.367079,42.282299,89.214539                             
                    </coordinates>
                </LineString>
            </Placemark>                          
    </Folder>
  </Document>
 </kml>

Answer 1

I think the problem is this cast

placemark.Geometry.GeometryJavaObject() as Java.Util.ArrayList

Instead you should use

placemark.Geometry.GeometryJavaObject().JavaCast<Java.Util.ArrayList>()

That is if you are 100% certain what you get back is a ArrayList .

You can always figure out whether you need to use JavaCast<T>() by trying to cast explicitly. Why JavaCast works while a normal cast doesn't can be read about in the Xamarin Documentation about JavaCast<T>() . In short terms: it is a hack...