C# Windows Formsでお絵描きアプリを作る｜MouseイベントとPaint処理をやさしく解説

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using System.Drawing;
using System.Drawing.Drawing2D;
using System.Windows.Forms;

namespace DrawApp
{
    public partial class Form1 : Form
    {
        private Point startPos;
        private Point endPos;
        private readonly Pallet pallet;

        public Form1()
        {
            InitializeComponent();

            pallet = new Pallet();
            pallet.Show();
        }

        private void DrawFigures(object sender, PaintEventArgs e)
        {
            e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;

            int type = pallet.GetFigureType();
            Color color = pallet.GetColor();
            int penSize = pallet.GetPenSize();

            int width = endPos.X - startPos.X;
            int height = endPos.Y - startPos.Y;

            if (type == 1)
            {
                using (SolidBrush brush = new SolidBrush(color))
                {
                    e.Graphics.FillEllipse(
                        brush, startPos.X, startPos.Y, width, height);
                }
            }
            else if (type == 2)
            {
                using (SolidBrush brush = new SolidBrush(color))
                {
                    e.Graphics.FillRectangle(
                        brush, startPos.X, startPos.Y, width, height);
                }
            }
            else if (type == 3)
            {
                using (Pen pen = new Pen(color, penSize))
                {
                    e.Graphics.DrawLine(pen, startPos, endPos);
                }
            }
        }

        private void MousePressed(object sender, MouseEventArgs e)
        {
            startPos = new Point(e.X, e.Y);
            endPos = startPos;
        }

        private void MouseDragged(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                endPos = new Point(e.X, e.Y);
                Invalidate();
            }
        }
    }
}

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using System;
using System.Drawing;
using System.Windows.Forms;

namespace DrawApp
{
    public partial class Pallet : Form
    {
        private int figureType;

        public Pallet()
        {
            InitializeComponent();
            figureType = 1;
        }

        public int GetFigureType()
        {
            return figureType;
        }

        public int GetPenSize()
        {
            if (int.TryParse(penSizeBox.Text, out int size) && size > 0)
            {
                return size;
            }

            return 1;
        }

        public Color GetColor()
        {
            return colorButton.BackColor;
        }

        private void CircleButtonClicked(object sender, EventArgs e)
        {
            figureType = 1;
        }

        private void RectButtonClicked(object sender, EventArgs e)
        {
            figureType = 2;
        }

        private void LineButtonClicked(object sender, EventArgs e)
        {
            figureType = 3;
        }

        private void ColorButtonClicked(object sender, EventArgs e)
        {
            using (ColorDialog colorDialog = new ColorDialog())
            {
                if (colorDialog.ShowDialog() == DialogResult.OK)
                {
                    colorButton.BackColor = colorDialog.Color;
                }
            }
        }
    }
}

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private void MousePressed(object sender, MouseEventArgs e)
{
    startPos = new Point(e.X, e.Y);
    endPos = startPos;
}

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private void MouseDragged(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        endPos = new Point(e.X, e.Y);
        Invalidate();
    }
}

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endPos = new Point(e.X, e.Y);
Invalidate();

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e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;

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int width = endPos.X - startPos.X;
int height = endPos.Y - startPos.Y;

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e.Graphics.FillEllipse(brush, startPos.X, startPos.Y, width, height);
e.Graphics.FillRectangle(brush, startPos.X, startPos.Y, width, height);

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using (Pen pen = new Pen(color, penSize))
{
    e.Graphics.DrawLine(pen, startPos, endPos);
}

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using (ColorDialog colorDialog = new ColorDialog())
{
    if (colorDialog.ShowDialog() == DialogResult.OK)
    {
        colorButton.BackColor = colorDialog.Color;
    }
}

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if (int.TryParse(penSizeBox.Text, out int size) && size > 0)
{
    return size;
}

return 1;

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using (SolidBrush brush = new SolidBrush(color))
{
    e.Graphics.FillEllipse(brush, startPos.X, startPos.Y, width, height);
}

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private Rectangle GetDrawingRectangle()
{
    int x = Math.Min(startPos.X, endPos.X);
    int y = Math.Min(startPos.Y, endPos.Y);
    int width = Math.Abs(endPos.X - startPos.X);
    int height = Math.Abs(endPos.Y - startPos.Y);

    return new Rectangle(x, y, width, height);
}

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Rectangle area = GetDrawingRectangle();

e.Graphics.FillEllipse(brush, area);
e.Graphics.FillRectangle(brush, area);

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public enum FigureType
{
    Circle,
    Rectangle,
    Line
}