public RasterMosaicMethod MosaicMethod {get; set;}

await QueuedTask.Run(() =>
{
  // Get the image sub-layer from the mosaic layer.
  ImageServiceLayer mosaicImageSubLayer = mosaicLayer.GetImageLayer() as ImageServiceLayer;
  // Get the mosaic rule.
  CIMMosaicRule mosaicingRule = mosaicImageSubLayer.GetMosaicRule();
  // Set the Mosaic Method to Center.
  mosaicingRule.MosaicMethod = RasterMosaicMethod.Center;
  // Update the mosaic with the changed mosaic rule.
  mosaicImageSubLayer.SetMosaicRule(mosaicingRule);
});

await QueuedTask.Run(() =>
{
  // Get the mosaic rule of the image service.
  CIMMosaicRule mosaicRule = isLayer.GetMosaicRule();
  // Set the Mosaic Method to Center.
  mosaicRule.MosaicMethod = RasterMosaicMethod.Center;
  // Update the image service with the changed mosaic rule.
  isLayer.SetMosaicRule(mosaicRule);
});