Calling a function that involves PaintEventArgs when a button is clicked

Question

Sorry if this has been asked before.

I was working on a simple connect 4 game in C# windows forms as I haven't done any work involving graphics before. To do this I need the program to draw circles when a button is pressed however I don't know how to call the function to do so.

public void printToken(PaintEventArgs pe, int x)
    {
        Graphics g = pe.Graphics;
        Pen blue = new Pen(Color.Blue);
        Pen red = new Pen(Color.Red);

        Rectangle rect = new Rectangle(50 + ((x-1) * 100), 50, 50, 50);
        g.DrawEllipse(blue, rect);
    }

    private void button1_Click(object sender, EventArgs e)
    {
        printToken(null, 1);
    }

The null in place is just a placeholder as obviously that will not work.

Any and all help is appreciated.

Answer 1

Typically in a Windows Forms application where you want to do custom drawing, you either draw directly on a Form or PictureBox in the Paint event handler or create a subclass of Control in which you override the OnPaint method. In the Paint event handler or OnPaint, you draw everything (i.e. not just one circle, but all the circles). Then when your underlying data structure changes, indicating that you need a repaint, you call Invalidate() on the control, which marks it as needing redraw, and on the next pass through the event loop, your Paint event handler will run or your OnPaint method will be called. Within that method, you'll have the PaintEventArgs object you need to get a Graphics object with which to do your drawing. You don't want to "draw once and forget" (e.g. when a button is clicked) because there are all sorts of things that can cause your control to need to repaint itself. See this question and answer for more details on that.

Edit: here's some hand-holding in response to your comment. (It was going to be a comment but it got too long.)

If I assume for the moment that you're starting with a blank Form in Visual Studio's Windows Forms Designer, the quickest way to get going would just be to select the Form, and in VS's Properties pane, click the lightning bolt toolbar button to view the form's events. Scroll down to the Paint event and double-click anywhere in the blank space to the right of the label. That will wire up a Paint event handler for the form and take you to the newly added method in the form's code file. Use the PaintEventArgs object called e to do your drawing. Then, if you need to change what gets drawn upon some button click, in your click handler change the data that determine what gets drawn (e.g. positions and colors of of the playing pieces) and, when you're done, call Invalidate().

Answer 2

If you add a UserControl representing a single chip to your form, and override the OnPaint method and put the coloring code into that event, then the Graphics object will not be null when the Paint event fires.

To kick off the paint of the UserControl, either call the Refresh method or InvalidateRectangle method - either will fire the OnPaint method.

Make sure to call up the class hierarchy to the OnPaint first, to make sure that the background of the object is drawn first:

protected override void OnPaint(PaintEventArgs e) {
    base.OnPaint(e);
    Graphics g = e.Graphics;
        Pen blue = new Pen(Color.Blue);
        Pen red = new Pen(Color.Red);
        Rectangle rect = new Rectangle(50 + ((x-1) * 100), 50, 50, 50);
        g.DrawEllipse(blue, rect);

}