Hi Marc,
If you are searching for features you may be able to use the queryFeatures method FeatureTable Class | ArcGIS for Developers for each of the layers.
This method accepts a QueryParameters Class | ArcGIS for Developers which allows you to set a geometry to query. So, your workflow would be something like:
1 - get the position at which the user clicked
2 - use the buffer method of GeometryEngine Class | ArcGIS for Developers to define your search area
3 - pass the Polygon returned by the buffer operation to a QueryParameters
4 - pass the QueryParameters to start a queryFeatures task for each layer you are interested in
One issue with that approach is that the buffer will be constructed using an absolute distance rather than a number of pixels. If that's an issue you would need to find some way of mapping 200 pixels to a distance for the current map scale etc. You could do something like:
1 - take the users point in screen-space
2 - shift it by 200 along the x-axis
3 - get the map location of that point
4 - then use distance method on GeometryEngine Class | ArcGIS for Developers to work out the distance between the 2 points (this is what the buffer radius should be).
I hope that helps - please let me know how you get on.
Thanks,
Luke