<UserControl x:Class="DrawLine.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480">
    
    <Canvas x:Name="panel" Background="Black" MouseLeftButtonDown="panel_MouseLeftButtonDown">
        
    </Canvas>
</UserControl>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

namespace DrawLine
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
        }
        private bool IsStart = true;//是否是起点。
        private double pX1 = 0, pY1 = 0;//起点坐标
        private double pX2 = 0, pY2 = 0;//终点坐标
        Ellipse ellipse = new Ellipse();//画起点

private void panel_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            Point p = e.GetPosition(sender as FrameworkElement);//取得鼠标点下的位置
            Canvas pnl = sender as Canvas;
            if (IsStart)
            {
                pX1 = p.X;//设置起点坐标
                pY1 = p.Y;
            }
            else
            {
                pX2 = p.X;//设置终点坐标
                pY2 = p.Y;
            }
            if (IsStart)//如果是起点，则画起点。
            {
                DrawPoint(pnl);
            }
            else
            {
                DrawOneLine(pnl);
            }
            IsStart = !IsStart;
        }
        //画点
        private void DrawPoint(Canvas pnl)
        {
            ellipse.Stroke = new SolidColorBrush(Color.FromArgb(255, 255, 255, 255));//动态设置Stroke属性的方法。
            ellipse.StrokeThickness = 2;
            ellipse.Width = 4;
            ellipse.Height = 4;
            Canvas.SetLeft(ellipse, pX1);//动态设置Ellipse的Canvas.Top与Canvas.Left
            Canvas.SetTop(ellipse,pY1);

pnl.Children.Add(ellipse);
        }
        //画直线
        private void DrawOneLine(Canvas pnl)
        {
            Line line = new Line();
            line.X1 = pX1;
            line.Y1 = pY1;
            line.X2 = pX2;
            line.Y2 = pY2;

line.Stroke = new SolidColorBrush(Color.FromArgb(255, 255, 255, 255));
            line.StrokeThickness = 4;

pnl.Children.Add(line);
            pnl.Children.Remove(ellipse);//删除起点。
        }
        /*参考代码，感谢小刚http://www.cnblogs.com/xiaogangqq123/
        public MainPage()
        {
            InitializeComponent();

this.MouseLeftButtonDown += new MouseButtonEventHandler(MainPage_MouseLeftButtonDown);
            this.MouseLeftButtonUp += new MouseButtonEventHandler(MainPage_MouseLeftButtonUp);
        }

private Point pointStart;
        private Path path;
        private LineGeometry lg;
        //private Point pointEnd;
        private bool IsFist = true;//记录是否第一次按下鼠标

void MainPage_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            //第二次点下鼠标
            if (!IsFist)
            {
                path = new Path();
                path.Stroke = new SolidColorBrush(Colors.Red);
                path.StrokeThickness = 5;
                lg = new LineGeometry();
                lg.StartPoint = pointStart;
                lg.StartPoint = pointStart;
                path.Data = lg;
                lg.EndPoint = e.GetPosition(LayoutRoot);

LayoutRoot.Children.Add(path);
            }
            pointStart = e.GetPosition(LayoutRoot);
            IsFist = true;
        }

void MainPage_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
        {
            IsFist = false;

}
        */
    }
}