C#如何从获取到的网页内容中抓取自己想要的部分？
假设抓取到的网页内容如下：
<meta http-equiv="Content-Type" content="text/html; charset=GBK">
<html>
<head>
<title>
研究生院信息系统
->
查看考试成绩
</title>
<link href="/media/default/css/default.css" rel="stylesheet" type="text/css">
<link href="/media/default/css/customize.css" rel="stylesheet" type="text/css">
<script language="JavaScript" src="/js/css_expression.js"></script>
<script language="JavaScript" src="/js/util.js"></SCRIPT>
<script language="JavaScript" src="/js/usecalendar.js"></SCRIPT>
<script language="JavaScript" src="/js/customize.js"></script>
</head>
<body>
<br>
<br>
<table width="98%" border="0" align="center" cellpadding="0" cellspacing="1" class="list_RoundTitle" title="查看考试成绩">
<tr>
<td width="20%">学号：
1111111111
</td>
<td width="15%">姓名：
张三
</td>
<td>课程编号：
0000001
</td>
</tr>
</table>
<table cellpadding="0" cellspacing="0" border="0" width="98%" align="center">
<tr>
<td align="top">
<!-- 显示找到结果 -->
<table width="100%" cellpadding="4" class="list_caption_change_mouse" align="center" cellspacing="1" border="0" id="row">
<thead>
<tr>
<th width="7%" class="sortable" align="left">
<a href="queryDepartmentAction.do?d-16544-s=0&studentid=xdleess20120514sn0727&degreecourseno=0022001&d-16544-o=2&d-16544-p=1">上课学期</a></th>
<th width="16%" class="sortable" align="left">
<a href="queryDepartmentAction.do?d-16544-s=1&studentid=xdleess20120514sn0727&degreecourseno=0022001&d-16544-o=2&d-16544-p=1">课程名称</a></th>
<th width="4%" class="sortable" align="left">
<a href="queryDepartmentAction.do?d-16544-s=2&studentid=xdleess20120514sn0727&degreecourseno=0022001&d-16544-o=2&d-16544-p=1">学分</a></th>
<th width="14%" class="sortable" align="left">
<a href="queryDepartmentAction.do?d-16544-s=3&studentid=xdleess20120514sn0727&degreecourseno=0022001&d-16544-o=2&d-16544-p=1">任课教师</a></th>
<th width="7%" class="sortable" align="left">
<a href="queryDepartmentAction.do?d-16544-s=4&studentid=xdleess20120514sn0727&degreecourseno=0022001&d-16544-o=2&d-16544-p=1">总评成绩</a></th>
<th width="7%" class="sortable" align="left">
<a href="queryDepartmentAction.do?d-16544-s=5&studentid=xdleess20120514sn0727&degreecourseno=0022001&d-16544-o=2&d-16544-p=1">获得学分</a></th>
<th width="14%" class="sortable" align="left">
<a href="queryDepartmentAction.do?d-16544-s=6&studentid=xdleess20120514sn0727&degreecourseno=0022001&d-16544-o=2&d-16544-p=1">备注</a></th></tr></thead>
<tbody>
<tr class="odd">
<td width="7%" align="left">2012秋</td>
<td width="16%" align="left">
体育课
</td>
<td width="4%" align="left">0.5</td>
<td width="14%" align="left">李四</td>
<td width="7%" align="left">
96.0
</td>
<td width="7%" align="left">
0.5
</td>
<td width="14%" align="left">
</td></tr></tbody></table><span class="pagebanner">共找到<font color="#FF0000"> 1 </font>条记录.</span><span class="pagelinks"></span>
</td>
</tr>
</table>
string start = "<tbody>";
int indexStart = str.IndexOf(start);
string end = "</tbody>";
int indexEnd = str.IndexOf(end);
int length = indexEnd - indexStart;
string s2 = str.Substring(indexStart, length);
MessageBox.Show(s2);
