public enum RasterMosaicOperatorType : System.Enum, System.IComparable, System.IConvertible, System.IFormattable  

await QueuedTask.Run(() =>
{
  // Get the image sub-layer from the mosaic layer.
  ImageServiceLayer mosaicImageSublayer = mosaicLayer.GetImageLayer() as ImageServiceLayer;
  // Get the mosaic rule.
  CIMMosaicRule mosaicRule = mosaicImageSublayer.GetMosaicRule();
  // Set the Mosaic Operator to Mean.
  mosaicRule.MosaicOperatorType = RasterMosaicOperatorType.Mean;
  // Update the mosaic with the changed mosaic rule.
  mosaicImageSublayer.SetMosaicRule(mosaicRule);
});

await QueuedTask.Run(() =>
{
  // Get the mosaic rule of the image service.
  CIMMosaicRule mosaicingRule = isLayer.GetMosaicRule();
  // Set the Mosaic Operator to Mean.
  mosaicingRule.MosaicOperatorType = RasterMosaicOperatorType.Mean;
  // Update the image service with the changed mosaic rule.
  isLayer.SetMosaicRule(mosaicingRule);
});