实现步骤：

1.实现整个鼠标框选的几个事件(down、move、up)，当鼠标点下记录鼠标框选的起点，鼠标抬起结束操作。

2.以鼠标框选过程中获取的鼠标坐标为基点计算框选的矩形的4点坐标,4点坐标以顺时针方向布点。

3.通过Shape.Path类实现在类上画出此矩形。

代码如下：
复制代码 代码如下:
namespace HostDemo {
public class HostCanvas : Canvas {
public HostCanvas() {
InitializeComponent();
}

private void InitializeComponent() {
this.Loaded += OnLoad;
this.MouseDown += OnMouseDown;
this.MouseMove += OnMouseMove;
this.MouseUp += OnMouseUp;
locus = new Path();
locus.Fill = new SolidColorBrush(Color.FromArgb(1, 255, 255, 255));
locus.Stroke = Brushes.Red;
locus.StrokeThickness = 1;
locus.IsManipulationEnabled = true;
}

void OnMouseUp(object sender, System.Windows.Input.MouseButtonEventArgs e) {
ispath = false;
}

void OnMouseMove(object sender, System.Windows.Input.MouseEventArgs e) {
if(ispath){
endpoint = e.GetPosition(this);
locus.Data = DrawingRect(startpoint,endpoint);
}
}

void OnMouseDown(object sender, System.Windows.Input.MouseButtonEventArgs e) {
if(!this.Children.Contains(locus)) this.Children.Add(locus);
if (locus.Data != null) locus.Data = null;
startpoint = e.GetPosition(this);
ispath = true;
}

void OnLoad(object sender, System.Windows.RoutedEventArgs e) {
this.Background = new SolidColorBrush(Color.FromArgb(35, 255, 255, 255));
}

private PathGeometry DrawingRect(Point beginpoint, Point closepoint) {
PathGeometry result = new PathGeometry();
PathFigure figure = new PathFigure();
figure.IsClosed = true;
figure.StartPoint = beginpoint;
PathSegmentCollection pathSegmentCollection = new PathSegmentCollection();
PathFigureCollection pathFigureCollection = new PathFigureCollection();
LineSegment m1 = new LineSegment();
m1.Point = new Point(closepoint.X, beginpoint.Y);
LineSegment m2 = new LineSegment();
m2.Point = closepoint;
LineSegment m3 = new LineSegment();
m3.Point = new Point(beginpoint.X, closepoint.Y);
pathSegmentCollection.Add(m1);
pathSegmentCollection.Add(m2);
pathSegmentCollection.Add(m3);
figure.Segments = pathSegmentCollection;
pathFigureCollection.Add(figure);
result.Figures = pathFigureCollection;

return result();
}

private Path locus;
private bool ispath = false;
private Point startpoint;
private Point endpoint;
}
}