The map's InformationLayer does not have a built-in selection support, which mean that the data reader also does not provide a template for a selected point. In order to achieve such functionality you can write custom code that handles this scenario. For example you can attach for the mouse left button down event of the element in the PointTemplate and implement the selection there.
Note that the map control provides two rendering engine packs - the old one consists of the the InformationLayer, VirtualizationLayer and DynamicaLayer. The new one is implemented through the VisualizationLayer
and it has few new features, and also improved performance. One the those features is the built-in selection
which you can use for your requirement. However, you will need to implement the logic for changing the color of all points. What you can do is basically to subscribe for the SelectionChanged event of the visualization layer and inside of its handle to change the colors and manually update your QueryableEntityCollectionView.
For your convenience I prepared a project that includes an example implementation of such approach. You can find it in the attachment.
You can also consider to wrap the data from the reader into view models. This will allow you more flexible control over the layer and its items.
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