Change this
graphicsLayerHeat.Graphics.AddRange(buffersList);
to this:
graphicsLayerHeat.Graphics.Add(graphic);
I assume you don't want to add the bufferlist
over and over again (as the other answerer's solution would do).
The newer version of the ESRI silverlight API does have an AddRange
method (time to update?), but I don't think that's what you want to use since you also want to do the contains check.