Public Overrides Sub GetUrl( _
   ByVal properties As DynamicLayer.ImageParameters, _
   ByVal onComplete As DynamicMapServiceLayer.OnUrlComplete _
) 

public override void GetUrl( 
   DynamicLayer.ImageParameters properties,
   DynamicMapServiceLayer.OnUrlComplete onComplete
)

The Map has a private method loadLayerInView which calls Layer.Draw. The DynamicMapServiceLayer abstract class overrides the Draw method and calls DynamicMapServiceLayer.GetUrl which must be implemented in a subclass. The last parameter is the OnUrlComplete delegate, which is used to pass the appropriate values (url, width, height, envelope) to the private DynamicMapServiceLayer.getUrlComplete method.