关键点：
- jquery.ajax 方法 和 HTML5 中的 FileReader对象
- ashx 处理程序
- asp.net 的 ScriptManager.RegisterStartupScript 调用客户端js脚本
一、ASP.NET 前台源码：
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="load_pic.aspx.cs" Inherits="Default2" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript" src="Scripts/jquery-1.4.1.min.js"></script>
<script type="text/javascript" src="Scripts/jquery-extend.js"></script>
</head>
<body>
<form id="form1" runat="server">
<div>
<!-- 把浏览图片解析为 base64 字符串 ，通过 jQuery.ajax + ashx 异步保存到数据库中 -->
<input type='file' οnchange="readURL(this);" />
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="C#后台读取显示" />
<!-- 存储 base64 字符串 ，方便在后台处理 -->
<asp:HiddenField ID="hf_base64" runat="server" />
</div>
<div>
<img id="img_prev" src="#" alt="your image" /></div>
</form>
</body>
</html>/* 客户端按比例预览、上传图片 要求：jquery-1.4.1.min.js */
//================================================================================================
function readURL(input) {
if (input.files && input.files[0]) {
var reader = new FileReader();
var maxWidth = 200, maxHeight = 200;
reader.onload = function (e) {
var base64 = e.target.result;
loadbase64(base64, maxWidth, maxHeight);
$("#hf_base64").val(base64);
};
reader.readAsDataURL(input.files[0]);
}
}
// imgobj : img 标签对象,根据容器自动调整宽、高
function load_obj_base64(imgobj, base64) {
maxWidth = $(imgobj).width();
maxHeight = $(imgobj).height();
$("<img/>") // 在内存中创建一个img标记
.attr("src", base64)
.load(function () {
var w, h;
w = this.width;
h = this.height;
if (w < maxWidth && h < maxHeight) { }
else
{ w / h > maxWidth / maxHeight ? this.width = maxWidth : this.height = maxHeight; }
$(imgobj).replaceWith(this);
});
}
function loadbase64(base64, maxWidth, maxHeight) {
$("<img/>") // 在内存中创建一个img标记
.attr("src", base64).attr("id", "img_prev")
.load(function () {
var w, h;
w = this.width;
h = this.height;
// 当图片比预览区域小时不做任何改变
if (w < maxWidth && h < maxHeight) { }
else
// 当实际图片比例大于预览区域宽高比例时
// 缩放图片宽度，反之缩放图片宽度
{ w / h > maxWidth / maxHeight ? this.width = maxWidth : this.height = maxHeight; }
$('#img_prev').replaceWith(this);
$.ajax({
async: false,
type: 'POST',
url: "uploadhandler.ashx",
data: {
file: base64
},
success: function (data) {
//$j('#img_wait').css('display', 'none');
},
beforeSend: function (xhr) {
//$j('#img_wait').css('display', 'inline');
}
});
});
}
三、uploadhandler.ashx
<%@ WebHandler Language="C#" Class="uploadhandler" %>
using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Net;
using System.Web;
using System.Web.Services;
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class uploadhandler : IHttpHandler
{
public void ProcessRequest(HttpContext context)
{
context.Response.ContentType = "text/plain";
context.Response.Charset = "utf-8";
try
{
HttpPostedFile file = context.Request.Files["Filedata"];
DBUtility.IDBHelper dbhelper = new DBUtility.SqlHelper();
string strSql = "UPDATE [LDZ] set Photo = @imagedata WHERE [ID] = 13;";
byte[] buffer = null;
if (file != null)
{
buffer = getByte(file);
}
else
{
string img = context.Request.Params["file"];
img = img.Substring(img.IndexOf("base64,") + 7);
buffer = Convert.FromBase64String(img);
}
int r = dbhelper.ExecuteNonQuery(WebConfig.DB_CONN, System.Data.CommandType.Text,
strSql,new System.Data.SqlClient.SqlParameter("@imagedata", buffer));
context.Response.Write("1");
}
catch (Exception ex)
{
context.Response.Write("0");
}
}
private byte[] getByte(HttpPostedFile file)
{
byte[] buffer = new byte[file.ContentLength];
file.InputStream.Read(buffer, 0, buffer.Length);
return buffer;
}
public bool IsReusable
{
get
{
return false;
}
}
//图片转为base64编码的字符串
protected string ImgToBase64String(string Imagefilename)
{
try
{
System.Drawing.Bitmap bmp = new System.Drawing.Bitmap(Imagefilename);
MemoryStream ms = new MemoryStream();
bmp.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
byte[] arr = new byte[ms.Length];
ms.Position = 0;
ms.Read(arr, 0, (int)ms.Length);
ms.Close();
return Convert.ToBase64String(arr);
}
catch (Exception ex)
{
return null;
}
}
//base64编码的字符串转为图片
protected System.Drawing.Bitmap Base64StringToImage(string strbase64)
{
try
{
byte[] arr = Convert.FromBase64String(strbase64);
MemoryStream ms = new MemoryStream(arr);
System.Drawing.Bitmap bmp = new System.Drawing.Bitmap(ms);
//bmp.Save("test.jpg", System.Drawing.Imaging.ImageFormat.Jpeg);
ms.Close();
return bmp;
}
catch (Exception ex)
{
return null;
}
}
}四、ASP.NET 后台代码
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
public partial class Default2 : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void Button1_Click(object sender, EventArgs e)
{
DBUtility.IDBHelper dbhelper = new DBUtility.SqlHelper();
string strSql = "select Photo from [LDZ] WHERE [ID] = 13;";
DataSet r = dbhelper.ExecuteQuery(WebConfig.DB_CONN, System.Data.CommandType.Text, strSql);
object imgObj = r.Tables[0].Rows[0]["Photo"];
string base64 = "data:image/jpeg;base64,";
if (imgObj != System.DBNull.Value)
{
byte[] buffer = (byte[])imgObj;
base64 += Convert.ToBase64String(buffer);
}
string hf_base64 = this.hf_base64.Value;
ScriptManager.RegisterStartupScript(this, this.GetType(), "",
"loadbase64('" + base64 + "', 500, 500)", true);
}
}注：ajax 方式异步读取数据库显示图片的方法同上传一致，使用 ashx 返回base64字符串在客户端处理即可。
记录一个让我纠结良久的问题：
在Page_Load 函数中，只有第一个用 ScriptManager.RegisterStartupScript 注册的脚本才有效
/// <summary>
/// 此函数范围内只有第一个用 ScriptManager.RegisterStartupScript 注册的脚本才有效
/// </summary>
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
// 首次加载的处理方法
if (this.Request.Params["lid"] != null)
{
// 照片处理，从数据获取过程省略......
object imgObj = row["Photo"];
string base64 = "data:image/jpeg;base64,";
if (imgObj != System.DBNull.Value)
{
byte[] buffer = (byte[])imgObj;
base64 += Convert.ToBase64String(buffer);
this.hf_base64.Value = base64;
// js脚本 load_obj_base64 是用来在客户端渲染图片的
string script = "$('#lz_caption strong').text('修改楼长信息');"
+"load_obj_base64($('#photo_lz').parent().next().children('img'),'"+ base64 + "')";
ScriptManager.RegisterStartupScript(this, this.GetType(), "",script, true);
}
}
此示例的前提是浏览器必须支持 HTML5 ，但现在的IE对 HTML5支持太烂，如需在 IE中使用，请参考下一篇：IE 中上传前按比例预览图片