C#生成缩略图控件

 实现原理：
1、    我们现在要做的是自定义Web控件，这和平常设计aspx网页或者用户控件有本质区别，自定义控件是一个派生自System.Web.WebControls.WebControl的类，它是一个类，而不是你想象中的HTML代码，甚至在自定义控件中你完全找不到HTMl的任何风格。因此，你必须对类的编写设计非常数量，或者是，跳出设计HTML的圈子，拓展思维！
2、    我们要实现不依靠文件系统、不依靠额外的任何其他东西，仅仅依靠一个类来实现它，这样做好处自然明显——各位只要复制得下面的一堆代码，自己建一个cs文件放进去就可以编译（编译为dll）。所有功能都是自含的，除了位于公共位置的.net类库，其他任何dll我都不需要引用。

然而，要实现如第2条这样的效果，我们得把对缩略图的请求设计成为对包含控件网页本身的请求，因为针对缩略图的这一次请求，本质仍然对本网页的请求，这样，网页中包含的缩略图控件才有机会操纵流。当然，两次请求都针对同一张网页的话，我们要设法区分开，哪一次是真正请求网页的原内容，哪一次是针对请求一个缩略图。
不知上述这段话大家能否理解，
如果不这样做的话，我们就不得不需要额外的控件或网页来实行了。

看懂了上述原理，我现在把流程写在下面，就自然好理解了：

1、    客户请求一张网页，如index.aspx，网页中含有缩略图控件，
2、    Index.aspx编译执行为HTML后被发送到客户端浏览器。这个控件生成了一个img标记，src属性指向一张该网页本身，但是后面附带的参数改变了。浏览器解析了，知道要向src属性获得一张图片。它开始向这个位置请求（也就是重新以新的附带参数请求这个网页）。
3、    ASP.Net获得了这个请求，index.aspx这个页面又开始执行，因为index.aspx中包含这个缩略图控件，控件就有机会识别这段特殊的参数，并且重新改写响应流，它会在文件系统中获得原始图像，然后根据你的要求，使用GDI.NET将原图重新按照新尺寸绘制，得到的新图是一个流对象，我们不存储它，而是将它直接附着在响应流中，发送给客户端。
4、    客户端浏览器得到这张图片。网页加载完成。

好，原理是说了不少，虽然我尽量写得通俗，但难免大家一时半会儿可能不好理解。那我现在就把代码贴出来让大家参考。
这个缩略图控件还没有添加水印等类似版权保护的功能，各位有兴趣的高手不妨完善一下这部分内容。同时与各位朋友相互学习，加强思考，增进思维。我们不管ASP.NET孰优孰劣，对于个人来说，既然追随ASP.NET，就要把ASP.NET用好，随波逐流的程序员永远不会优秀。

 

using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Drawing;
using System.ComponentModel;
using System.Drawing.Design;
using System.Drawing.Drawing2D;
using System.Text;
using System.IO;

namespace BlogLan.Web.Controls
{
    /// <summary>
    /// 缩略图控件。
    /// </summary>
    [DefaultProperty("ImageUrl")]
    [ToolboxData("<{0}:Thumbnail runat=server></{0}:Thumbnail>")]
    [DesignerAttribute(typeof(Designer.ThumbnailDesigner))]
    [Description("缩略图控件。")]
    public class Thumbnail : WebControl
    {
        public Thumbnail()
            : base(HtmlTextWriterTag.Img)
        {
            this.Width = Unit.Parse("100");
            this.Height = Unit.Parse("75");
        }


        //Private Members
        private bool urlResolved;

        /// <summary>
        /// 获取或设置图片路径。
        /// </summary>
        [Editor("System.Web.UI.Design.ImageUrlEditor, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor)), Description("获取或设置图片路径"), Bindable(true), Category("Appearance"), DefaultValue(""), UrlProperty]
        public string ImageUrl
        {
            get
            {
                if (this.ViewState["ImageUrl"] != null)
                {
                    return (string)this.ViewState["ImageUrl"];
                }
                return string.Empty;
            }
            set
            {
                this.ViewState["ImageUrl"] = value;
            }
        }

        /// <summary>
        /// 获取或设置Jpeg缩略图的质量，值范围-100。
        /// </summary>
        [DefaultValue(80)]
        public int JpegQuality
        {
            get
            {
                if (this.ViewState["JpegQuility"] != null)
                {
                    return (int)this.ViewState["JpegQuility"];
                }
                return 80;
            }
            set
            {
                if (value > 100)
                {
                    this.ViewState["JpegQuility"] = 100;
                }
                else if (value < 20)
                {
                    this.ViewState["JpegQuility"] = 20;
                }
                else
                {
                    this.ViewState["JpegQuility"] = value;
                }
            }
        }

        [DefaultValue(typeof(Unit), "100px")]
        public override Unit Width
        {
            get
            {
                return base.Width;
            }
            set
            {
                base.Width = value;
            }
        }

        [DefaultValue(typeof(Unit), "75px")]
        public override Unit Height
        {
            get
            {
                return base.Height;
            }
            set
            {
                base.Height = value;
            }
        }

        internal bool UrlResolved
        {
            get
            {
                return this.urlResolved;
            }
            set
            {
                this.urlResolved = value;
            }
        }

        /// <summary>
        /// 获取或设置控件相对于网页上其他元素的对齐方式。
        /// </summary>
        [DefaultValue(ImageAlign.NotSet), Description("获取或设置控件相对于网页上其他元素的对齐方式。")]
        public virtual ImageAlign ImageAlign
        {
            get
            {
                object obj2 = this.ViewState["ImageAlign"];
                if (obj2 != null)
                {
                    return (ImageAlign)obj2;
                }
                return ImageAlign.NotSet;
            }
            set
            {
                if ((value < ImageAlign.NotSet) || (value > ImageAlign.TextTop))
                {
                    throw new ArgumentOutOfRangeException("value");
                }
                this.ViewState["ImageAlign"] = value;
            }
        }

        /// <summary>
        /// [禁用Enabled属性在任何位置编辑。]
        /// </summary>
        [Browsable(false), EditorBrowsable(EditorBrowsableState.Never)]
        public override bool Enabled
        {
            get
            {
                return base.Enabled;
            }
            set
            {
                base.Enabled = value;
            }
        }

        /// <summary>
        /// 如无法显示图片时显示的替代文本。
        /// </summary>
        [DefaultValue(""), Bindable(true), Localizable(true), Description("如无法显示图片时显示的替代文本。")]
        public virtual string AlternateText
        {
            get
            {
                if (this.ViewState["AlternateText"] != null)
                {
                    return (string)this.ViewState["AlternateText"];
                }
                return string.Empty;
            }
            set
            {
                this.ViewState["AlternateText"] = value;
            }
        }

        /// <summary>
        /// 获取或设置一个值，该值指示控件是否生成空字符串值的替换文字属性。
        /// </summary>
        [DefaultValue(true), Description("获取或设置一个值，该值指示控件是否生成空字符串值的替换文字属性。")]
        public virtual bool GenerateEmptyAlternateText
        {
            get
            {
                if (this.ViewState["GenerateEmptyAlternateText"] != null)
                {
                    return (bool)this.ViewState["GenerateEmptyAlternateText"];
                }
                return true;
            }
            set
            {
                this.ViewState["GenerateEmptyAlternateText"] = value;
            }
        }

        /// <summary>
        /// 获取或设置图像详细说明的位置。
        /// </summary>
        [Editor("System.Web.UI.Design.UrlEditor, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor)), UrlProperty, DefaultValue(""), Description("获取或设置图像详细说明的位置。")]
        public virtual string DescriptionUrl
        {
            get
            {
                string str = (string)this.ViewState["DescriptionUrl"];
                if (str != null)
                {
                    return str;
                }
                return string.Empty;
            }
            set
            {
                this.ViewState["DescriptionUrl"] = value;
            }
        }


        //Methods

        /// <summary>
        /// 重写AddAttributesToRender方法。
        /// </summary>
        /// <param name="writer"></param>
        protected override void AddAttributesToRender(HtmlTextWriter writer)
        {
            if (string.IsNullOrEmpty(this.Page.Request["thumbnail"]))
            {
                this.AddThumbnailAttributesToRender(writer, string.Empty);
            }
        }

        protected virtual internal void AddThumbnailAttributesToRender(HtmlTextWriter writer, string ForcedImageUrl)
        {
            base.AddAttributesToRender(writer);
            string imageUrl = this.ImageUrl;
            

            if (!this.UrlResolved)
            {
                imageUrl = base.ResolveClientUrl(imageUrl);
            }

            //在设计时强制赋予图形路径。
            if (!string.IsNullOrEmpty(ForcedImageUrl))
            {
                imageUrl = ForcedImageUrl;
                writer.AddAttribute(HtmlTextWriterAttribute.Src, imageUrl);
            }
            else
            {
                writer.AddAttribute(HtmlTextWriterAttribute.Src, this.Page.Request.Url.AbsolutePath + "?" + this.GetQueryString(), false);
            }

            

            imageUrl = this.DescriptionUrl;
            if (imageUrl.Length != 0)
            {
                writer.AddAttribute(HtmlTextWriterAttribute.Longdesc, base.ResolveClientUrl(imageUrl));
            }
            imageUrl = this.AlternateText;
            if ((imageUrl.Length > 0) || this.GenerateEmptyAlternateText)
            {
                writer.AddAttribute(HtmlTextWriterAttribute.Alt, imageUrl);
            }
            switch (this.ImageAlign)
            {
                case ImageAlign.Left:
                    writer.AddAttribute(HtmlTextWriterAttribute.Align, "left");
                    break;

                case ImageAlign.Right:
                    writer.AddAttribute(HtmlTextWriterAttribute.Align, "right");
                    break;

                case ImageAlign.Baseline:
                    writer.AddAttribute(HtmlTextWriterAttribute.Align, "baseline");
                    break;

                case ImageAlign.Top:
                    writer.AddAttribute(HtmlTextWriterAttribute.Align, "top");
                    break;

                case ImageAlign.Middle:
                    writer.AddAttribute(HtmlTextWriterAttribute.Align, "middle");
                    break;

                case ImageAlign.Bottom:
                    writer.AddAttribute(HtmlTextWriterAttribute.Align, "bottom");
                    break;

                case ImageAlign.AbsBottom:
                    writer.AddAttribute(HtmlTextWriterAttribute.Align, "absbottom");
                    break;

                case ImageAlign.AbsMiddle:
                    writer.AddAttribute(HtmlTextWriterAttribute.Align, "absmiddle");
                    break;

                case ImageAlign.NotSet:
                    break;

                default:
                    writer.AddAttribute(HtmlTextWriterAttribute.Align, "texttop");
                    break;
            }

            if (this.BorderWidth.IsEmpty)
            {
                writer.AddStyleAttribute(HtmlTextWriterStyle.BorderWidth, "0px");
            }
        }

        private string GetQueryString()
        {
            StringBuilder sb = new StringBuilder();
            sb.Append("thumbnail=" + this.Page.Server.HtmlEncode(this.ImageUrl));
            sb.Append("&w=" + this.Width.Value.ToString());
            sb.Append("&h=" + this.Height.Value.ToString());
            sb.Append("&bc=" + this.BackColor.ToArgb().ToString());
            sb.Append("&jq=" + this.JpegQuality.ToString());

            return sb.ToString();
        }

        protected override void RenderContents(HtmlTextWriter writer)
        {
        }

        protected override ControlCollection CreateControlCollection()
        {
            return new EmptyControlCollection(this);
        }

        protected override void OnInit(EventArgs e)
        {
            base.OnInit(e);
            this.Init += new EventHandler(Thumbnail_Init);
            this.PreRender += new EventHandler(Thumbnail_PreRender);
        }

        void Thumbnail_PreRender(object sender, EventArgs e)
        {
            if (HttpContext.Current != null)
            {
                RenderThumbnailImage(this.Page.Request, this.Page.Response);
            }
        }

        void Thumbnail_Init(object sender, EventArgs e)
        {
            if (HttpContext.Current != null)
            {
                RenderThumbnailImage(this.Page.Request, this.Page.Response);
            }
        }

        static void RenderThumbnailImage(HttpRequest request, HttpResponse response)
        {
            if (string.IsNullOrEmpty(request["thumbnail"]))
            {
                return;
            }

            Thumbnail thumbnail = new Thumbnail();
            

            Bitmap tmb;
            try
            {
                thumbnail.ImageUrl = request["thumbnail"];

                try
                {
                    thumbnail.Width = Unit.Parse(request["w"]);
                }
                catch
                {
                    thumbnail.Width = Unit.Pixel(100);
                }

                try
                {
                    thumbnail.Height = Unit.Parse(request["h"]);
                }
                catch
                {
                    thumbnail.Height = Unit.Pixel(75);
                }
                try
                {
                    thumbnail.BackColor = Color.FromArgb(Convert.ToInt32(request["bc"]));
                }
                catch
                {
                    thumbnail.BackColor = Color.Black;
                }
                try
                {
                    thumbnail.JpegQuality = Convert.ToInt32(request["jq"]);
                }
                catch
                {
                    thumbnail.JpegQuality = 80;
                }

                tmb = thumbnail.GetThumbnail();
            }
            catch
            {
                tmb = Properties.Resources.GenerateImageError;
            }

            response.Clear();
            string[] ImageFilePart = thumbnail.ImageUrl.ToLower().Split('.');
            string Suffix = "bmp";
            if (ImageFilePart.Length > 1)
            {
                Suffix = ImageFilePart[ImageFilePart.Length - 1];
            }

            if (Suffix == "gif")
            {
                response.ContentType = "image/gif";
                tmb.Save(response.OutputStream, System.Drawing.Imaging.ImageFormat.Gif);
            }
            else if (Suffix == "jpg" || Suffix == "jpeg")
            {
                response.ContentType = "image/jpeg";

                System.Drawing.Imaging.EncoderParameters encoderParams = new System.Drawing.Imaging.EncoderParameters();
                long[] quality = new long[] { Convert.ToInt64(thumbnail.JpegQuality) };
                System.Drawing.Imaging.EncoderParameter encoderParam = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);
                encoderParams.Param[0] = encoderParam;
                System.Drawing.Imaging.ImageCodecInfo[] arrayICI = System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders();
                System.Drawing.Imaging.ImageCodecInfo jpegICI = null;
                for (int fwd = 0; fwd < arrayICI.Length - 1; fwd++)
                {
                    if (arrayICI[fwd].FormatDescription.Equals("JPEG"))
                    {
                        jpegICI = arrayICI[fwd];
                        break;
                    }
                }

                if (jpegICI != null)
                {
                    tmb.Save(response.OutputStream, jpegICI, encoderParams);
                }
                else
                {
                    tmb.Save(response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
                }


            }
            else
            {
                response.ContentType = "image/bmp";
                tmb.Save(response.OutputStream, System.Drawing.Imaging.ImageFormat.Bmp);
            }

            tmb.Dispose();

            response.Flush();
            
        }

        //

        public Bitmap GetThumbnail()
        {
            return this.GetThumbnail(this);
        }

        private Bitmap GetThumbnail(Thumbnail thumbnail)
        {
            if (thumbnail == null)
                return Properties.Resources.GenerateImageError;

            System.Drawing.Image OriginalImage;
            try
            {
                if (thumbnail.DesignMode)
                {
                    //string pagepath = thumbnail.MapPathSecure("/");
                    OriginalImage = (System.Drawing.Image)Properties.Resources.ThumbnailDesign;
                    
                }
                else
                {
                    OriginalImage = System.Drawing.Image.FromFile(thumbnail.Context.Server.MapPath(thumbnail.ImageUrl));
                }
            }
            catch
            {
                return Properties.Resources.GenerateImageError;
            }

            Size thumbnailNewSize = this.GetNewSize(Convert.ToInt32(thumbnail.Width.Value), Convert.ToInt32(thumbnail.Height.Value), OriginalImage.Width, OriginalImage.Height);

            Bitmap outbmp = new Bitmap(Convert.ToInt32(thumbnail.Width.Value), Convert.ToInt32(thumbnail.Height.Value));
            Graphics g = Graphics.FromImage(outbmp);
            try
            {
                g.CompositingQuality = CompositingQuality.HighQuality;
                g.SmoothingMode = SmoothingMode.HighQuality;
                g.InterpolationMode = InterpolationMode.HighQualityBicubic;

                g.Clear(thumbnail.BackColor);

                Rectangle _Vb_t_record_0 = new Rectangle((int)Math.Round((double)(((double)(thumbnail.Width.Value - thumbnailNewSize.Width)) / 2.0)), (int)Math.Round((double)(((double)(thumbnail.Height.Value - thumbnailNewSize.Height)) / 2.0)), thumbnailNewSize.Width, thumbnailNewSize.Height);
                g.DrawImage(OriginalImage, _Vb_t_record_0, 0, 0, OriginalImage.Width, OriginalImage.Height, GraphicsUnit.Pixel);
                g.Dispose();
            }
            catch
            {
                return Properties.Resources.GenerateImageError;
            }
            finally
            {
                g.Dispose();
                OriginalImage.Dispose();
            }

            return outbmp;

        }

        private Size GetNewSize(int maxWidth, int maxHeight, int width, int height)
        {
            double w = 0.0;
            double h = 0.0;
            double sw = Convert.ToDouble(width);
            double sh = Convert.ToDouble(height);
            double mw = Convert.ToDouble(maxWidth);
            double mh = Convert.ToDouble(maxHeight);
            if ((sw < mw) & (sh < mh))
            {
                w = sw;
                h = sh;
            }
            else if ((sw / sh) > (mw / mh))
            {
                w = maxWidth;
                h = (w * sh) / sw;
            }
            else
            {
                h = maxHeight;
                w = (h * sw) / sh;
            }
            return new Size(Convert.ToInt32(w), Convert.ToInt32(h));
        }



    }
}