VS2017中GDI+绘图进阶教程（四）

在前几篇文章中，我们已经介绍了如何在VS2017中使用GDI+进行基本的图形绘制。然而，细心的读者可能会发现在尝试覆盖保存现有图片时遇到‘GDI+一般性错误’。这一错误主要是因为微软为了保护原始图像文件，不允许直接对原文件进行修改。为了避免这种错误，我们可以通过创建一个新的Bitmap对象来间接保存修改后的图像。

以下是具体的实现代码，包括了创建新Bitmap对象、绘制图形、保存图像等步骤，并附有详细注释以帮助理解：

```csharp
using System;
using System.Drawing;
using System.Windows.Forms;
using System.IO;

namespace GdiPlusDrawing
{
public partial class MainForm : Form
{
private Graphics graphics;
private Bitmap originalBitmap, workingBitmap, watermarkBitmap;
private string imagePath;

public MainForm()
{
InitializeComponent();
watermarkBitmap = new Bitmap("watermark.png");
}

private void MainForm_Paint(object sender, PaintEventArgs e)
{
// 绘制方法
}

private void GeneratePicture()
{
if (File.Exists(imagePath))
originalBitmap = new Bitmap(imagePath);
else
originalBitmap = new Bitmap(pictureBox1.Width, pictureBox1.Height);

workingBitmap = new Bitmap(originalBitmap);
originalBitmap.Dispose();
graphics = Graphics.FromImage(workingBitmap);
DrawGraphics(graphics);
pictureBox1.Image = workingBitmap;
}

private void LoadPictureButton_Click(object sender, EventArgs e)
{
using (OpenFileDialog dialog = new OpenFileDialog())
{
if (dialog.ShowDialog() == DialogResult.OK)
{
imagePath = dialog.FileName;
GeneratePicture();
}
}
}

private void SavePictureButton_Click(object sender, EventArgs e)
{
if (!File.Exists(imagePath))
{
using (SaveFileDialog dialog = new SaveFileDialog())
{
dialog.Filter = "JPEG Images (*.jpg)|*.jpg|BMP Images (*.bmp)|*.bmp";
if (dialog.ShowDialog() == DialogResult.OK)
imagePath = dialog.FileName;
}
}

if (!string.IsNullOrEmpty(imagePath))
workingBitmap.Save(imagePath);
}

private void DrawButton_Click(object sender, EventArgs e)
{
GeneratePicture();
}

private void DrawGraphics(Graphics g)
{
Pen greenPen = new Pen(Color.Green, 2);
g.DrawLine(greenPen, pictureBox1.Width / 4, pictureBox1.Height / 4, pictureBox1.Width * 3 / 4, pictureBox1.Height * 3 / 4);
g.DrawRectangle(greenPen, pictureBox1.Width / 4, pictureBox1.Height / 4, pictureBox1.Width / 2, pictureBox1.Height / 2);
Pen bluePen = new Pen(Color.Blue, 1);
g.DrawEllipse(bluePen, pictureBox1.Width / 4, pictureBox1.Height / 4, 160, 160);
g.DrawString("Drawing is here!", new Font("SimSun", 14), new SolidBrush(Color.Red), new PointF(160, 160));
g.DrawImage(watermarkBitmap, new PointF(workingBitmap.Width * 3 / 4, workingBitmap.Height * 3 / 4));
}
}
}
```

上述代码展示了如何通过GDI+在C#中实现图像的加载、绘制和保存功能。对于希望进一步扩展功能的开发者，可以在`DrawGraphics`方法中添加更多的绘图逻辑。此外，本文还提供了源代码下载链接，供读者参考学习。

源代码下载：[点击下载](#)