PaintEvent not overpainting

Question

I have a Windows Forms application in C# with drawing panel and a button - for drawing a line.

When you click the button, you can draw a line for 2 random points.

Answer 1

You have to call the Invalidate method on the panel:

private void drawPanel_MouseUp(object sender, MouseEventArgs e)
{
  onMouseUpFlag = true;
  drawPanel.Invalidate();
}

Also, use the Graphic object from the PaintEvent:

private void drawPanel_Paint(object sender, PaintEventArgs e)
{
  if (onMouseUpFlag)
  {
    e.Graphics.DrawLine(p, ps, pe); 
  }
} 

For multiple lines, you would have to hold the points in a collection object.

Per your updated code, here is a working example of what I think you are trying to do:

private class Line {
  public Point Starting { get; set; }
  public Point Ending { get; set; }

  public Line(Point starting, Point ending) {
    this.Starting = starting;
    this.Ending = ending;
  }
}
List lines = new List();

private Point downPoint = Point.Empty;
private Point movePoint = Point.Empty;
private bool movingLine = false;

public Form1() {
  InitializeComponent();

  panel1.Paint += panel1_Paint;
  panel1.MouseDown += panel1_MouseDown;
  panel1.MouseMove += panel1_MouseMove;
  panel1.MouseUp += panel1_MouseUp;
}

void panel1_MouseDown(object sender, MouseEventArgs e) {
  if (e.Button == MouseButtons.Left) {
    downPoint = e.Location;
  }
}

void panel1_MouseMove(object sender, MouseEventArgs e) {
  if (e.Button == MouseButtons.Left) {
    movingLine = true;
    movePoint = e.Location;
    panel1.Invalidate();
  }
}

void panel1_MouseUp(object sender, MouseEventArgs e) {
  if (e.Button == MouseButtons.Left) {
    movingLine = false;
    lines.Add(new Line(downPoint, e.Location));
    panel1.Invalidate();
  }
}

void panel1_Paint(object sender, PaintEventArgs e) {
  e.Graphics.Clear(Color.White);
  foreach (Line l in lines) {
    e.Graphics.DrawLine(Pens.Black, l.Starting, l.Ending);
  }

  if (movingLine) {
    e.Graphics.DrawLine(Pens.Black, downPoint, movePoint);
  }

}

Use an inherited panel to turn on the DoubleBuffer property to avoid flickering.