自定义控件继承的事件未触发

Question

I have a user control which inherits from Control and is drawn by myself using Graphics Object.

public class Line : Control
{
    public Point start { get; set; }
    public Point end { get; set; }
    public Pen pen = new Pen(Color.Red);
}

This is main Form

public partial class Form1 : Form
{
    public Line line = new Line() { start = new Point(50, 50), end = new Point(100, 100) };
    public Form1()
    {
        InitializeComponent();
        Controls.Add(line);
        line.MouseEnter += new EventHandler(line_MouseEnter);
    }

    void line_MouseEnter(object sender, EventArgs e)
    {
        MessageBox.Show("Hello World");
    }

    private void Form1_Paint(object sender, PaintEventArgs e)
    {
        using (Graphics g = this.CreateGraphics())
        {
            g.DrawLine(line.pen, line.start, line.end);
        }
    }
}

Now whenever the mouse rolls over the control the message box should pop up, but it doesn't. I have tried debugging and it seems that the event is never getting invoked. What's wrong here

Answer 1

In fact your Control is visible but has Size of Empty . The line you see is not the line drawn on your Control, it's drawn on your form and that's why you thought your line is visible. You want to create your line which has an arbitrary direction. I think that's a little hard to do, you have to calculate the Rectangle of your line (one of the sides is your line thickness) depending on the angle made by your line direction and the horizontal line. If you can calculate that Rectangle , you have to use the Region property to make your line control exactly fit its Rectangle . Something like yourLine.Region = new Region(calculatedRectangle); . For that complexity , many UI library just supports Horizontal line and Vertical line because we can calculate their Rectangle easily and they are used more frequently than other kind of lines. Here is the code for a Horizontal line, I don't have time to dig into coding a full-feature line as you want, but you may want to try yourself:

public class Line : Control
{    
  Color lineColor = Color.Red;
  public Color LineColor {
      get { return lineColor;}
      set {
          lineColor = value;
          BackColor = value;
      }
  }
  public Line(){
     Height = 1;//Default thickness
     Width = 100;//Default length
     BackColor = lineColor;
  }
  //for the case you want to draw yourself, add code in this method
  protected override void OnPaint(PaintEventArgs e){
       base.OnPaint(e);
       //your code
  }
}
//use it
Line line = new Line(){Left = 50, Top = 50};
line.MouseEnter += new EventHandler(line_MouseEnter);

If you want to draw the line on your form, I think your line is in fact a structure storing information of a line (thickness, length, start point, end point). So you don't need it to inherit Control , and to register MouseEnter , you have to add MouseMove event handler for your form, not your line. However your line Rectangle will help. Again, you still have to calculate your line Rectangle , which is not easy as I said before. To demonstrate it, I just calculate the Rectangle of your line in a simple way (increase X of both start and end point by 1):

//Your control has Size of Empty in this example, it just stores information of your line and is used to register the MouseMove event handler with the Form when its Parent is changed (to Form).
public class Line : Control
{
  public Point start { get; set; }
  public Point end { get; set; }
  public Pen pen = new Pen(Color.Red);
  protected override void OnParentChanged(EventArgs e){
    if(Parent != null){
        Parent.MouseMove -= MouseMoveParent;
        Parent.MouseMove += MouseMoveParent;
    }
  }
  private void MouseMoveParent(object sender, MouseEventArgs e){
     //the line Rectangle is specified by 4 points
     //I said that this is just for demonstrative purpose because to calculate
     //the exact 4 points (of the line Rectangle), you have to add much more code.
     Point p1 = start;
     Point p2 = new Point(p1.X + 1, p1.Y);
     Point p3 = new Point(end.X + 1, end.Y);
     Point p4 = end;
     System.Drawing.Drawing2D.GraphicsPath gp = new System.Drawing.Drawing2D.GraphicsPath();
     gp.AddPolygon(new Point[]{p1,p2,p3,p4});
     Region lineRegion = new Region(gp);
     if(lineRegion.IsVisible(e.Location)){
         MessageBox.Show("Hello World");
     }
  }
}
public partial class Form1 : Form
{
  public Line line = new Line() { start = new Point(50, 50), end = new Point(100, 100) };
  public Form1()
  {
    InitializeComponent();
    Controls.Add(line);
  }     
  private void Form1_Paint(object sender, PaintEventArgs e)
  {        
    e.Graphics.DrawLine(line.pen, line.start, line.end);        
  }
}