首先建一个类：
代码如下:

using System; 
using System.Collections.Generic; 
using System.Linq; 
using System.Web; 
using System.Drawing; 
/// <summary> 
///Class1 的摘要说明 
/// </summary> 
public class Class1:IHttpHandler //调用接口 
{ 
public Class1() 
{ 
// 
//TODO: 在此处添加构造函数逻辑 
// 
} 
public bool IsReusable 
{ 
get { return true; } 
} 
public void ProcessRequest(HttpContext context) 
{ 
HttpRequest req = context.Request; 
if (req.UrlReferrer != null && req.UrlReferrer.Host.Length > 0) //反盗链代码判断 
{ 
System.Drawing.Image img = System.Drawing.Image.FromFile(context.Request.PhysicalPath); 
System.Drawing.Graphics g = Graphics.FromImage(img); 
g.DrawString("三国演义", new Font("宋体", 20, FontStyle.Bold), Brushes.White, 10, 10); 
img.Save(context.Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg); 
context.Response.Flush(); 
context.Response.End(); 
} 
else 
{ 
context.Response.Write("您不能盗链本站图片"); 
} 
} 
} 
 
在web.config中注册接口： 
 代码如下:

<httpHandlers> 
<add verb="*" path="images/*.jpg" type="Class1,App_Code"/> 
</httpHandlers> 
 
url:http://greatverve.cnblogs.com/archive/2011/12/20/asp-net-hotlinking.html 
参考： 
1.修改web.config 
 代码如下:

<system.web> 
<httpHandlers> 
<remove verb="*" path="*.asmx"/> 
<!--解决图片防盗链问题--> 
<add verb="*" path="*.jpg" type="MyHttpHandler.Watermark"/> 
<add verb="*" path="*.gif" type="MyHttpHandler.Watermark"/> 
<add verb="*" path="*.png" type="MyHttpHandler.Watermark"/> 
</httpHandlers> 
</system.web> 
 
2.添加一个一般执行文件Watermark.ashx，代码如下： 
 代码如下:

using System; 
using System.Data; 
using System.Web; 
using System.Collections; 
using System.Web.Services; 
using System.Web.Services.Protocols; 
using System.Drawing.Imaging; 
using System.Drawing; 
using System.IO; 
namespace MyHttpHandler 
{ 
/// <summary> 
/// Summary description for $codebehindclassname$ 
/// </summary> 
[WebService(Namespace = "http://tempuri.org/")] 
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)] 
public class Watermark : IHttpHandler 
{ 
public bool IsReusable 
{ 
get 
{ 
return false; 
} 
} 
public void ProcessRequest(HttpContext context) 
{ 
//设置客户端缓冲时间过期时间为0，即立即过期 
//context.Response.Expires = 0; 
//清空服务器端为此会话开启的
输出缓存 
//context.Response.Clear(); 
//设置
输出文件类型 
context.Response.ContentType = "image/jpg"; 
//将请求文件写入到
输出缓存中 
#region 获取XML配置信息 
DataSet dsConfing = new DataSet(); 
string watermarkConfigPath = context.Server.MapPath("~/Config/WaterMarkConfig.xml"); 
if (System.IO.File.Exists(watermarkConfigPath)) 
dsConfing.ReadXml(watermarkConfigPath); 
else 
{ 
//添加默认的水印配置 
} 
DataRow drConfing = dsConfing.Tables[0].Rows[0]; 
#endregion 
string currentHost = drConfing["allowhost"].ToString(); 
//判断是否是本地网站引用图片，如果是则返回正确的图片 
if (context.Request.Url.Authority.Equals(currentHost, StringComparison.InvariantCultureIgnoreCase)) 
{ 
string localPath = context.Request.Url.LocalPath; 
localPath = localPath.Remove(localPath.LastIndexOf('/')).ToLower();// "/images/userphoto" 
if (drConfing["isflag"].Equals("true") && drConfing["files"].ToString().ToLower().IndexOf(localPath) > 0) 
{ 
#region 水印代码 
string sImgStartPhysicalPath = context.Request.PhysicalPath; 
System.Drawing.Image imgStart = System.Drawing.Image.FromFile(sImgStartPhysicalPath); 
//备份原图片 
//int indexOf = sImgStartPhysicalPath.LastIndexOf("."); 
//string bakPath = sImgStartPhysicalPath.Remove(indexOf) + "_bak" + sImgStartPhysicalPath.Substring(indexOf); 
//imgStart.Save(bakPath); 
Graphics gh = System.Drawing.Graphics.FromImage(imgStart); 
if (drConfing["type"].Equals("img")) 
{ 
System.Drawing.Image imgWatermark = System.Drawing.Image.FromFile(context.Server.MapPath(drConfing["img-path"].ToString())); 
Rectangle rg = SetImgPosition(drConfing["position"].ToString(), imgStart.Width, imgStart.Height, imgWatermark.Width, imgWatermark.Height); 
gh.DrawImage(imgWatermark, rg, 0, 0, imgWatermark.Width, imgWatermark.Height, GraphicsUnit.Pixel); 
gh.Save(); 
gh.Dispose(); 
imgWatermark.Dispose(); 
} 
else if (drConfing["type"].Equals("font")) 
{ 
//文字水印 
string content = drConfing["font-content"].ToString(); 
float Size = (float)Convert.ToDouble(drConfing["font-size"].ToString()); 
FontStyle fontStyle = (FontStyle)int.Parse(drConfing["font-style"].ToString()); 
System.Drawing.Font f = new System.Drawing.Font("Arial", Size, fontStyle); 
Color G_Color = Color.FromName(drConfing["font-color"].ToString()); 
System.Drawing.Brush b = new System.Drawing.SolidBrush(G_Color); 
SizeF sizeF = gh.MeasureString(content, f); 
gh.DrawString(content, f, b, SetFontPosition(drConfing["position"].ToString(), imgStart.Width, imgStart.Height, (int)sizeF.Width, (int)sizeF.Height)); 
gh.Save(); 
gh.Dispose(); 
} 
//将请求文件写入到

输出缓存中
imgStart.Save(context.Response.OutputStream, ImageFormat.Jpeg);
imgStart.Dispose();
#endregion
}
else
{
#region 输出原图
//将请求文件写入到输出缓存中
context.Response.WriteFile(context.Request.Url.AbsolutePath);
#endregion
}
}
//如果不是本地引用，则是盗链本站图片
else
{
//将请求文件写入到输出缓存中
context.Response.WriteFile(context.Request.PhysicalApplicationPath + drConfing["errimgpath"].ToString());
}
//将输出缓存中的信息传送到客户端
context.Response.End();
}
/// <summary>
/// 图片绘画水印的位置
/// </summary>
/// <param name="positionConfig">位置类型</param>
/// <param name="width">原图片宽</param>
/// <param name="height"></param>
/// <param name="watermarkWidth">水印图宽</param>
/// <param name="watermarkHeight"></param>
/// <returns></returns>
private Rectangle SetImgPosition(string positionConfig,int width,int height,int watermarkWidth,int watermarkHeight)
{
int xpos = 0;
int ypos = 0;
int margin = 10;
int width_margin = width - margin;
int height_margin = height - margin;
double proportion = 1d;//水印图片缩放比例
//int
if ((width_margin > watermarkWidth * proportion) && (height_margin > watermarkHeight * proportion))
{
}
else if ((width_margin > watermarkWidth * proportion) && (height_margin < watermarkHeight * proportion))
{
proportion = Convert.ToDouble( height_margin) / Convert.ToDouble( watermarkHeight);
}
else if ((width_margin < watermarkWidth * proportion) && (height_margin > watermarkHeight * proportion))
{
proportion = Convert.ToDouble(width_margin) / Convert.ToDouble(watermarkWidth);
}
else
{
double proportionW = Convert.ToDouble(width_margin) / Convert.ToDouble(watermarkWidth);
double proportionH = Convert.ToDouble(height_margin) / Convert.ToDouble(watermarkHeight);
proportion = proportionW >= proportionH ? proportionH : proportionW;
}
watermarkWidth = Convert.ToInt32(watermarkWidth * proportion);
watermarkHeight = Convert.ToInt32(watermarkHeight * proportion);
switch (positionConfig)
{
case "top-left":
xpos = margin;
ypos = margin;
break;
case "top-right":
xpos = width_margin - watermarkWidth;
ypos = margin;
break;
case "bottom-left":
xpos = margin;
ypos = height_margin - watermarkHeight;
break;
case "bottom-right":
xpos = width_margin - watermarkWidth ;
ypos = height_margin - watermarkHeight ;
break;
default:
xpos = width_margin - watermarkWidth ;
ypos = height_margin - watermarkHeight;
break;
}
return new Rectangle(xpos,ypos,watermarkWidth,watermarkHeight);
}
/// <summary>
/// 图片绘画文字位置
/// </summary>
/// <param name="positionConfig">位置类型</param>
/// <param name="width">原图片宽</param>
/// <param name="height"></param>
/// <param name="fontWidth">文字长度</param>
/// <param name="fontHeight"></param>
/// <returns></returns>
private Point SetFontPosition(string positionConfig, int width, int height, int fontWidth, int fontHeight)
{
int xpos = 0;
int ypos = 0;
int margin = 10;
int width_margin = width - margin;
int height_margin = height - margin;
double proportion = 1d;//水印图片缩放比例
//int
if ((width_margin > fontWidth * proportion) && (height_margin > fontHeight * proportion))
{
}
else if ((width_margin > fontWidth * proportion) && (height_margin < fontHeight * proportion))
{
proportion = Convert.ToDouble(height_margin) / Convert.ToDouble(fontHeight);
}
else if ((width_margin < fontWidth * proportion) && (height_margin > fontHeight * proportion))
{
proportion = Convert.ToDouble(width_margin) / Convert.ToDouble(fontWidth);
}
else
{
double proportionH = Convert.ToDouble(height_margin) / Convert.ToDouble(fontHeight);
double proportionW = Convert.ToDouble(width_margin) / Convert.ToDouble(fontWidth);
proportion = proportionW >= proportionH ? proportionH : proportionW;
}
fontWidth = Convert.ToInt32(fontWidth * proportion);
fontHeight = Convert.ToInt32(fontHeight * proportion);
switch (positionConfig)
{
case "top-left":
xpos = margin;
ypos = margin;
break;
case "top-right":
xpos = width_margin - fontWidth;
ypos = margin;
break;
case "bottom-left":
xpos = margin;
ypos = height_margin - fontHeight;
break;
case "bottom-right":
xpos = width_margin - fontWidth;
ypos = height_margin - fontHeight;
break;
default:
xpos = width_margin - fontWidth;
ypos = height_margin - fontHeight;
break;
}
return new Point(xpos, ypos);
}
}
}

3.配置文件的WaterMarkConfig.xml，内容如下：
代码如下:

<?xml version="1.0" encoding="utf-8" ?> 
<watermark> 
<allowhost>localhost:6219</allowhost><!--允许访问的域名--> 
<isflag>true</isflag><!-- true、false--> 
<type>font</type><!-- img、font--> 
<files>/config|/upfiles/ab</files><!--需要加水印的文件夹--> 
<position>bottom-right</position><!-- top-left、top-right、bottom-left、bottom-right--> 
<img-path>~/UpFiles/Watermark.png</img-path><!-- 水印位置 --> 
<font-style>1</font-style><!--普通文本 0, 加粗文本 1, 倾斜文本 2, 带下划线的文本 4, 中间有直线通过的文本 8--> 
<font-size>60</font-size> 
<font-color>red</font-color> 
<font-content>￥：8000元</font-content> 
<errimgpath>images/error.jpg</errimgpath><!-- 盗图片的请求返回的跟目录下的某图片 --> 
</watermark> 

 
 




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