How do I draw an ellipse with mouse down, move, and up actions in a Windows Forms application?

Question

How do I draw an ellipse with mouse down, move, and up actions in a Windows Forms application?

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Answer 1

Using the following code snippet, we can draw an ellipse in a Windows Forms application, just as we do in the Windows Paint application.

public partial class Form1 : Form
    {
        // Define variables to determine a ellipse
        struct Ellipse
        {
            // The point user press Mouse Down
            public Point start;
            // The point user let Mouse Up
            public Point end;
            // Direction comparing the end point and the start point
            public Direction direction;
        }

        // Four quadrants in coordinate
        enum Direction
        {
            One,
            Two,
            Three,
            Four
        }

        // Store all the ellipses
        List<Ellipse> mEllipses = new List<Ellipse>();
        // The current drawing ellipse
        Ellipse mEllipse;
        // Whether Mouse is Down to draw ellipse
        bool mDrawing;

        public Form1()
        {
            InitializeComponent();
        }

        // Draw ellipse by its start, end points and direction
        private void DrawEllipse(Ellipse ellipse, PaintEventArgs e)
        {
            switch (ellipse.direction)
            {
                case Direction.One:
                    e.Graphics.DrawEllipse(Pens.Black, new Rectangle(new Point(ellipse.start.X,
                        ellipse.end.Y), new Size(ellipse.end.X - ellipse.start.X,
                            ellipse.start.Y - ellipse.end.Y)));
                    break;
                case Direction.Two:
                    e.Graphics.DrawEllipse(Pens.Black, new Rectangle(ellipse.end,
                        new Size(ellipse.start.X - ellipse.end.X,
                            ellipse.start.Y - ellipse.end.Y)));
                    break;
                case Direction.Three:
                    e.Graphics.DrawEllipse(Pens.Black, new Rectangle(new Point(ellipse.end.X,
                        ellipse.start.Y), new Size(ellipse.start.X - ellipse.end.X,
                            ellipse.end.Y - ellipse.start.Y)));
                    break;
                case Direction.Four:
                    e.Graphics.DrawEllipse(Pens.Black, new Rectangle(ellipse.start,
                        new Size(ellipse.end.X - ellipse.start.X,
                            ellipse.end.Y - ellipse.start.Y)));
                    break;
                default:
                    MessageBox.Show("Error");
                    break;
            }
        }

        // Determine the direction of ellipse by start and end points
        private void DetermineDirection(MouseEventArgs e)
        {
            if (e.Y < 0 && e.X < 0)
            {
                mEllipse.end.X = 0;
                mEllipse.end.Y = 0;
            }
            else if (e.X < 0)
            {
                mEllipse.end.X = 0;
                mEllipse.end.Y = e.Y;
            }
            else if (e.Y < 0)
            {
                mEllipse.end.X = e.X;
                mEllipse.end.Y = 0;
            }
            else
                mEllipse.end = e.Location;

            if (mEllipse.end.X > mEllipse.start.X && mEllipse.end.Y <= mEllipse.start.Y)
                mEllipse.direction = Direction.One;
            else if (mEllipse.end.X <= mEllipse.start.X && mEllipse.end.Y < mEllipse.start.Y)
                mEllipse.direction = Direction.Two;
            else if (mEllipse.end.X < mEllipse.start.X && mEllipse.end.Y >= mEllipse.start.Y)
                mEllipse.direction = Direction.Three;
            else if (mEllipse.end.X >= mEllipse.start.X && mEllipse.end.Y > mEllipse.start.Y)
                mEllipse.direction = Direction.Four;
        }

        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            foreach (Ellipse ellipse in mEllipses)
                this.DrawEllipse(ellipse, e);

            if (mDrawing)
                this.DrawEllipse(mEllipse, e);
        }

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            // Mouse Down to determine start point of an ellipse
            mEllipse.start = e.Location;
            if (e.Button == MouseButtons.Left) 
                mDrawing = true;
            else
                this.Invalidate();
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            if (!mDrawing) return;
            mDrawing = false;
            this.DetermineDirection(e);
            // Add the newly created ellipse into ellipse list
            mEllipses.Add(mEllipse);
            this.Invalidate();
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (!mDrawing) return;
            // When Mouse Move, determine the direction of an ellipse 
            // constantly
            this.DetermineDirection(e);
            this.Invalidate();
        }
    }

 

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