ArcGIS Pro secondary development - drawing a line on the map

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using ArcGIS.Core.CIM;
using ArcGIS.Core.Data;
using ArcGIS.Core.Geometry;
using ArcGIS.Desktop.Catalog;
using ArcGIS.Desktop.Core;
using ArcGIS.Desktop.Editing;
using ArcGIS.Desktop.Extensions;
using ArcGIS.Desktop.Framework;
using ArcGIS.Desktop.Framework.Contracts;
using ArcGIS.Desktop.Framework.Dialogs;
using ArcGIS.Desktop.Framework.Threading.Tasks;
using ArcGIS.Desktop.Mapping;
using System.Windows.Input;





namespace ProAppModule1
{
    internal class MapToolSelect : MapTool
    {
        private IDisposable _graphic = null;

        private CIMLineSymbol _lineSymbol = null;

        public MapToolSelect()
        {

            IsSketchTool = true;

            SketchType = SketchGeometryType.Line; //Sketch a line geometry

            SketchOutputMode = SketchOutputMode.Map;

        }



        protected override Task OnToolDeactivateAsync(bool hasMapViewChanged)
        {

            if (_graphic != null)

                _graphic.Dispose();//Clear out the old overlay

            _graphic = null;

            return base.OnToolDeactivateAsync(hasMapViewChanged);

        }



        /// <summary>

        /// Occurs when the tool is activated.

        /// </summary>

        /// <param name="hasMapViewChanged">A value indicating if the active <see cref="T:ArcGIS.Desktop.Mapping.MapView"/> has changed.</param>

        /// <returns>

        /// A Task that represents a tool activation event.

        /// </returns>

        protected async override Task OnToolActivateAsync(bool hasMapViewChanged)
        {

            if (_lineSymbol == null)
            {

                _lineSymbol = await Module1.CreateLineSymbolAsync();

            }

            this.SketchSymbol = _lineSymbol.MakeSymbolReference();

        }



        protected override void OnToolMouseDown(MapViewMouseButtonEventArgs e)
        {

            if (_graphic != null)
            {

                _graphic.Dispose();//Clear out the old overlay

                _graphic = null;

            }

            base.OnToolMouseDown(e);

        }



        protected async override Task<bool> OnSketchCompleteAsync(Geometry geometry)
        {

            _graphic = await this.AddOverlayAsync(geometry, _lineSymbol.MakeSymbolReference());

            return true;

        }



    }

}
==========
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Input;
using System.Threading.Tasks;
using ArcGIS.Core.CIM;
using ArcGIS.Core.Data;
using ArcGIS.Core.Geometry;
using ArcGIS.Desktop.Catalog;
using ArcGIS.Desktop.Core;
using ArcGIS.Desktop.Editing;
using ArcGIS.Desktop.Extensions;
using ArcGIS.Desktop.Framework;
using ArcGIS.Desktop.Framework.Contracts;
using ArcGIS.Desktop.Framework.Dialogs;
using ArcGIS.Desktop.Framework.Threading.Tasks;
using ArcGIS.Desktop.Mapping;

namespace ProAppModule1
{
    internal class Module1 : Module
    {
        private static Module1 _this = null;

        /// <summary>
        /// Retrieve the singleton instance to this module here
        /// </summary>
        public static Module1 Current
        {
            get
            {
                return _this ?? (_this = (Module1)FrameworkApplication.FindModule("ProAppModule1_Module"));
            }
        }

        #region Overrides
        /// <summary>
        /// Called by Framework when ArcGIS Pro is closing
        /// </summary>
        /// <returns>False to prevent Pro from closing, otherwise True</returns>
        protected override bool CanUnload()
        {
            //TODO - add your business logic
            //return false to ~cancel~ Application close
            return true;
        }
        internal static Task<CIMLineSymbol> CreateLineSymbolAsync()
        {

            return QueuedTask.Run(() => {

                var lineStroke = SymbolFactory.Instance.ConstructStroke(ColorFactory.Instance.RedRGB, 4.0);

                var marker = SymbolFactory.Instance.ConstructMarker(ColorFactory.Instance.RedRGB, 12, SimpleMarkerStyle.Circle);

                marker.MarkerPlacement = new CIMMarkerPlacementOnVertices()
                {

                    AngleToLine = true,

                    PlaceOnEndPoints = true,

                    Offset = 0

                };

                return new CIMLineSymbol()
                {

                    SymbolLayers = new CIMSymbolLayer[2] { marker, lineStroke }

                };

            });

        }



        #endregion Overrides

    }
}