在新表格上绘制图形

Question

I am facing a problem on drawing graphics, pardon me as I am totally new to it. What I have now, is a form named FormDraw. In FormDraw, I have a button. what the button does is

private void button1_Click(object sender, EventArgs e)
{
    using (Form form = new Form())
    {
        form.Text = "About Us";
        System.Drawing.Graphics graphics = this.CreateGraphics();
        System.Drawing.Rectangle rectangle = new System.Drawing.Rectangle(100, 100, 200, 200);
        graphics.DrawEllipse(System.Drawing.Pens.Black, rectangle);
        graphics.DrawRectangle(System.Drawing.Pens.Red, rectangle);
        // form.Controls.Add(...);

        form.ShowDialog();
    }
}

What I am trying to achieve is draw the graphics on the new form, however upon clicking on the button it creates on the old form (FormDraw). Any idea what am I doing wrong?

Answer 1

The main problem here seems to be that you haven't researched how drawing in the Winforms API works and so you're trying to use the API without understanding it. :(

The code you posted will attempt to draw just once into the newly created form object. This drawing is unlikely to work at all, since the rest of the form initialization hasn't completed yet, but even if something did draw to the screen, it would immediately be lost as new things are later drawn to the screen (like the form itself when it's finally shown).

Winforms is, like most mainstream GUI APIs (including Mac OS, Java's SWT, AWT, Swing, etc. and of course the native Windows API) an "immediate mode" API. That is, your code is expected to draw on demand, and to draw what needs to be presented at that moment. The API does not remember anything you've drawn; any time something happens (to your data or on the screen itself, such as window overlap changes) that would invalidate what you've already drawn, you have to draw it again.

The only appropriate place to draw into a Winforms control (including a form) is when handling the Paint event, either in an actual handler or by overriding OnPaint() . If you want to execute drawing statements once, then you have to draw into a bitmap object (effectively caching them), and then draw the bitmap itself during the Paint event.

There's not enough context in your question to really be able to understand exactly what you're trying to do. But the code you posted can be fixed to work as (I think) you expect, by modifying it to look like this:

private void button1_Click(object sender, EventArgs e) {
    using (Form form = new Form())
    {
        form.Text = "About Us";
        form.Paint += (sender, e) =>
        {
            System.Drawing.Rectangle rectangle = new System.Drawing.Rectangle(100, 100, 200, 200);
            e.Graphics.DrawEllipse(System.Drawing.Pens.Black, rectangle);
            e.Graphics.DrawRectangle(System.Drawing.Pens.Red, rectangle);
        }

        // form.Controls.Add(...);

        form.ShowDialog();
    }
}

The PaintEventArgs class, passed to the handler of a Paint event, includes a Graphics property that is the Graphics instance that you are to use when drawing during the Paint event.

The above subscribes an anonymous event handler method to the new form's Paint event. In that handler, your desired drawing is done using the Graphics instance provided to you via the PaintEventArgs passed to the handler.

(Note that the above only fixes the Paint handling. Of course, you will need to otherwise correctly initialize the form object, such as setting its width and height, actually adding desired controls etc.)

Answer 2

You can use 'PictureBox' to show created image on new form.

private void Button1_Click(object sender, EventArgs e)
    {
        // create Form instance
        Form form = new Form
        {
            Text = "About Us",
            Width = 440,
            Height = 460,
        };

        // create bmp image
        Bitmap bmp = new Bitmap(400, 400);
        // draw on bmp image
        using (Graphics graphics = Graphics.FromImage(bmp))
        {
            graphics.Clear(Color.Transparent);
            Rectangle rectangle = new Rectangle(100, 100, 200, 200);
            graphics.DrawEllipse(Pens.Black, rectangle);
            graphics.DrawRectangle(Pens.Red, rectangle);

        }
        // create PictureBox instance
        PictureBox pictureBox = new PictureBox
        {
            Image = bmp,
            //BorderStyle = BorderStyle.FixedSingle,
            //Dock = DockStyle.Fill,
            /// To center
            Size = new Size(400,400),
            Location = new Point(10,10)
        };
        // add pictureBox control to form
        form.Controls.Add(pictureBox);
        // show form in dialog box
        form.ShowDialog();
    }

Answer 3

Instead of using the above method i have posted in the question, i manage to do it using

Form form2 = new Form();
form2.Show();

var graphics = form2.CreateGraphics();
var rectangle = new System.Drawing.Rectangle(100, 100, 200, 200);

graphics.DrawEllipse(System.Drawing.Pens.Black, rectangle);
graphics.DrawRectangle(System.Drawing.Pens.Red, rectangle);

however, i will do more research on the drawing class. thank you Peter Duniho