Public Overrides Sub GetUrl( _
   ByVal extent As Envelope, _
   ByVal width As Integer, _
   ByVal height As Integer, _
   ByVal onComplete As DynamicMapServiceLayer.OnUrlComplete _
) 

public override void GetUrl( 
   Envelope extent,
   int width,
   int height,
   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.