public void SetSnappable( 
   bool isSnappable
)

// is the layer snappable?
bool isSnappable = fLayer.IsSnappable;

// set snappability for a specific layer - needs to run on the MCT
await QueuedTask.Run(() =>
{
  // use an extension method
  fLayer.SetSnappable(true);

  // or use the CIM directly
  //var layerDef = fLayer.GetDefinition() as ArcGIS.Core.CIM.CIMGeoFeatureLayerBase;
  //layerDef.Snappable = true;
  //fLayer.SetDefinition(layerDef);
});


// turn all layers snappability off
layerList = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>();
await QueuedTask.Run(() =>
{
  foreach (var layer in layerList)
  {
    layer.SetSnappable(false);
  }
});

// interested in only snapping to the vertices of a specific layer of interest and not the vertices of other layers
//  all other snapModes should be off.

// snapping must be on
ArcGIS.Desktop.Mapping.Snapping.IsEnabled = true;

// turn all application snapModes off
//At 2.x - ArcGIS.Desktop.Mapping.Snapping.SetSnapModes();
ArcGIS.Desktop.Mapping.Snapping.SetSnapModes(null);

// set application snapMode vertex on 
ArcGIS.Desktop.Mapping.Snapping.SetSnapMode(SnapMode.Vertex, true);

// ensure layer snapping is on
await QueuedTask.Run(() =>
{
  fLayer.SetSnappable(true);
});

// set vertex snapping only
var vertexOnly = new LayerSnapModes(false) { Vertex = true };

// set vertex only for the specific layer, clearing all others
var dict = new Dictionary<Layer, LayerSnapModes>();
dict.Add(fLayer, vertexOnly);
ArcGIS.Desktop.Mapping.Snapping.SetLayerSnapModes(dict, true);  // true = reset other layers

if (!layer.IsVisible)
  layer.SetVisibility(true);

if (layer is FeatureLayer featureLayer)
{
  if (!featureLayer.IsEditable)
    featureLayer.SetEditable(true);

  if (!featureLayer.IsSnappable)
    featureLayer.SetSnappable(true);
}