1,目的:

• 实现多种图片格式的相互转换，图片大小可自定义等。

2，知识点:

• 转换成图标图像(ico)时，需要获取图像句柄，然后根据句柄生成Ico图像，否则生成的图像不能作为应用的图标使用。

                            IntPtr hwd = bitmap.GetHicon();
                            Icon icon = Icon.FromHandle(hwd);
                            icon.Save(fs);

• 利用反射获取系统可支持的图片类型，获取静态属性的值。

 ImageFormat format = typeof(ImageFormat).GetProperty(comboBox1.Text).GetValue(null) as ImageFormat;

3，效果展示:

4,代码：

 public partial class Form1 : Form
    {
        string useExt;
        public Form1()
        {
            InitializeComponent();
        }

        private void btnSelect_Click(object sender, EventArgs e)
        {
            using(OpenFileDialog ofd=new OpenFileDialog())
            {
                ofd.Multiselect = false;
                if (useExt != null)
                {
                    ofd.Filter = useExt;
                }
                if(ofd.ShowDialog()== DialogResult.OK)
                {
                    txtFilePath.Text = ofd.FileName;
                }
            }
        }

        private void btnSave_Click(object sender, EventArgs e)
        {
            using(SaveFileDialog sfd=new SaveFileDialog())
            {
                sfd.CheckPathExists = true;
                string ext=comboBox1.Text;
                if (comboBox1.Text.ToUpper() == "JPEG")
                {
                    ext = "jpg";
                }
                if (comboBox1.Text.ToUpper() == "ICON")
                {
                    ext = "ico";
                }
                sfd.Filter = $"*.{ext}文件|*.{ext}";
                if(sfd.ShowDialog()== DialogResult.OK)
                {
                    txtSavePath.Text = sfd.FileName;
                }
            }
        }

        private void button3_Click(object sender, EventArgs e)
        {
            if(string.IsNullOrEmpty(txtFilePath.Text)|| string.IsNullOrEmpty(txtSavePath.Text))
            {
                MessageBox.Show("请先选择文件路径","提示", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                return;
            }
            string sizeStr = comboBox2.Text;
            string[] wandh = sizeStr.Split('*');
            double width,height;

            if (wandh.Length == 2)
            {
                if(double.TryParse(wandh[0],out width) && double.TryParse(wandh[1],out height))
                {
                    Image img = Image.FromFile(txtFilePath.Text);
                    Size size ;
                    if ((width ==1)&& (height == 1))
                    {
                        size = new Size(img.Width, img.Height);
                    }
                    else
                    {
                        size = new Size((int)width, (int)height);
                    }
                    Bitmap bitmap = new Bitmap(img,size);

                    ImageFormat format = typeof(ImageFormat).GetProperty(comboBox1.Text).GetValue(null) as ImageFormat;

                   using(FileStream fs=new FileStream(txtSavePath.Text, FileMode.Create))
                    {
                        if (format == ImageFormat.Icon)
                        {
                            IntPtr hwd = bitmap.GetHicon();
                            Icon icon = Icon.FromHandle(hwd);
                            icon.Save(fs);
                        }
                        else
                        {
                            bitmap.Save(fs, format);
                        }
                    }
                    MessageBox.Show("已保存至："+txtSavePath.Text,"提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
                else
                {
                    MessageBox.Show("目标尺寸参数格式异常", "提示", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                }
            }
            else
            {
                MessageBox.Show("目标尺寸参数数量异常", "提示", MessageBoxButtons.OK, MessageBoxIcon.Warning);
            }
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

        private void Form1_Load(object sender, EventArgs e)
        {
            List<string> list = new List<string>();
            //获取所有可供转换的类型
            foreach (var item in typeof(ImageFormat).GetProperties())
            {
                comboBox1.Items.Add(item.Name);
                list.Add($"*.{item.Name}文件|*.{item.Name}");
            }
            list.Add("*.jpg文件|*.jpg");
            list.Reverse();
            useExt = string.Join("|", list);
            if (comboBox1.Items.Count > 0)
                comboBox1.SelectedIndex = comboBox1.Items.Count-1;
            comboBox2.SelectedIndex = 0;

        }
    }