public static void SetLayerSnapModes( 
   Layer layer,
   bool setAll
)

var featureLayerList = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>();
// configure layer snap modes - must run on the MCT
await QueuedTask.Run(() =>
{
  // configure by layer
  foreach (var layer in featureLayerList)
  {
    // find the state of the snapModes for the layer
    var lsm = Snapping.GetLayerSnapModes(layer);
    bool vertexOn = lsm.Vertex;
    // or use 
    vertexOn = lsm.GetSnapMode(SnapMode.Vertex);
    bool edgeOn = lsm.Edge;
    // or use 
    edgeOn = lsm.GetSnapMode(SnapMode.Edge);
    bool endOn = lsm.End;
    // or use 
    endOn = lsm.GetSnapMode(SnapMode.End);
    // update a few snapModes 
    //   turn Vertex off
    lsm.SetSnapMode(SnapMode.Vertex, false);
    // intersections on
    lsm.SetSnapMode(SnapMode.Intersection, true);
    // and set back to the layer
    Snapping.SetLayerSnapModes(layer, lsm);
    // assign a single snap mode at once
    Snapping.SetLayerSnapModes(layer, SnapMode.Vertex, false);
    // turn ALL snapModes on
    Snapping.SetLayerSnapModes(layer, true);
    // turn ALL snapModes off
    Snapping.SetLayerSnapModes(layer, false);
  }
  // configure for a set of layers
  // set Vertex, edge, end on for a set of layers, other snapModes false
  var vee = new LayerSnapModes(false)
  {
    Vertex = true,
    Edge = true,
    End = true
  };
  Snapping.SetLayerSnapModes(featureLayerList, vee);
  // ensure intersection is on for a set of layers without changing any other snapModes
  // get the layer snapModes for the set of layers
  var dictLSM = Snapping.GetLayerSnapModes(featureLayerList);
  foreach (var layer in dictLSM.Keys)
  {
    var lsm = dictLSM[layer];
    lsm.Intersection = true;
  }
  Snapping.SetLayerSnapModes(dictLSM);
  // set all snapModes off for a list of layers
  Snapping.SetLayerSnapModes(featureLayerList, false);
});