【发布时间】：2011-11-27 21:20:43
【问题描述】：
我有一个简单的网页。其内容：
<%@ Page Language="C#"
AutoEventWireup="true"
CodeFile="default.aspx.cs"
Inherits="_Default" %>
<!DOCTYPE HTML PUBLIC
"-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-type"
content="text/html; charset=utf-8" />
<title>Test Page</title>
</head>
<body>
<form id="form1"
runat="server">
<div id="header" >
<asp:Button
ID="previous_BUT"
runat="server"
Height="25px"
Text="Previous"
Visible="false"
Width="100px" />
<asp:Label
ID="txtNewsItem"
runat="server"
Height="20px"
Visible="false"
Width="550px" />
<asp:Button
ID="next_BUT"
runat="server"
Height="25px"
Text="Next"
Visible="false"
Width="100px" />
</div>
</form>
</body>
</html>
页面背后的代码（不包括 Session 变量和 Web.config 值检索）是
using ...
// **************************************************** class _Default
public partial class _Default : Page
{
DataTable news_items = null;
// ********************************************* Connection_String
/// <summary>
/// get the value of the database connection string from
/// the Web.Config file ConnectionString element
/// </summary>
public string Connection_String ...
// ********************************************* Current_News_Item
/// <summary>
/// set or get the value of the Current_News_Item Session
/// variable
/// </summary>
private int Current_News_Item ...
// ********************************************* Number_News_Items
/// <summary>
/// set or get the value of the Number_News_Items Session
/// variable
/// </summary>
private int Number_News_Items ...
// ************************************************ get_news_items
/// <summary>
/// retrieve the news items from the database
/// </summary>
/// <returns></returns>
private DataTable get_news_items ( )
{
DataTable data_table = null;
SqlConnection SQL_connection = null;
try
{
DataSet data_set = new DataSet ( );
DateTime now = DateTime.Now;
StringBuilder select_statement;
SqlDataAdapter SQL_data_adapter;
DateTime today;
today = new DateTime ( now.Year, now.Month, now.Day );
select_statement = new StringBuilder ( ).AppendFormat (
"SELECT hpn.description " +
"FROM tbl_homePageNews hpn " +
"WHERE effectiveDate <= GETDATE() and status = 1 " +
"ORDER BY sortOrder ASC",
today.ToShortDateString ( ) );
SQL_connection = new SqlConnection ( Connection_String );
SQL_connection.Open ( );
SQL_data_adapter = new SqlDataAdapter (
select_statement.ToString ( ),
SQL_connection );
SQL_data_adapter.Fill ( data_set, "tbl_homePageNews" );
data_table = data_set.Tables [ "tbl_homePageNews" ].
Copy ( );
}
catch
{
data_table = null;
}
finally
{
if ( SQL_connection != null )
{
SQL_connection.Close ( );
}
}
return ( data_table );
}
// ***************************************************** Page_Load
protected void Page_Load ( object sender, EventArgs e )
{
next_BUT.Click += new EventHandler( next_BUT_Click );
previous_BUT.Click += new EventHandler( previous_BUT_Click );
Current_News_Item = 0;
Number_News_Items = 0;
if ( !Page.IsPostBack )
{
news_items = get_news_items ( );
if ( news_items != null )
{
Number_News_Items = news_items.Rows.Count;
previous_BUT.Visible = false;
txtNewsItem.Visible = false;
next_BUT.Visible = false;
if ( Number_News_Items > 0 )
{
txtNewsItem.Visible = true;
txtNewsItem.Text =
" " +
news_items.Rows [ Current_News_Item ]
[ "description" ].
ToString ( );
if ( Number_News_Items > 1 )
{
previous_BUT.Visible = true;
next_BUT.Visible = true;
}
}
}
}
}
// ******************************************** previous_BUT_Click
/// <summary>
/// handle the Previous button click
/// </summary>
public void previous_BUT_Click ( Object sender,
EventArgs e )
{
Current_News_Item--;
if ( Current_News_Item < 0 )
{
Current_News_Item = Number_News_Items - 1;
}
txtNewsItem.Text =
" " + news_items.Rows [ Current_News_Item ]
[ "description" ].ToString ( );
}
// ************************************************ next_BUT_Click
/// <summary>
/// handle the Next button click
/// </summary>
public void next_BUT_Click ( Object sender,
EventArgs e )
{
Current_News_Item++;
if ( Current_News_Item >= Number_News_Items )
{
Current_News_Item = 0;
}
txtNewsItem.Text =
" " + news_items.Rows [ Current_News_Item ]
[ "description" ].ToString ( );
}
} // class _Default
在后面的代码中，在 Page_Load 中，是语句
txtNewsItem.Visible = true;
txtNewsItem.Text =
" " +
news_items.Rows [ Current_News_Item ]
[ "description" ].
ToString ( );
两者都按预期执行 - 显示第一个新闻项目。用户可以点击“上一个”按钮或“下一个”按钮来移动新闻项目。但是这里出现了问题。下面的代码是不好的地方
// ************************************************ next_BUT_Click
/// <summary>
/// handle the Next button click
/// </summary>
public void next_BUT_Click ( Object sender,
EventArgs e )
{
Current_News_Item++;
if ( Current_News_Item >= Number_News_Items )
{
Current_News_Item = 0;
}
txtNewsItem.Text =
" " + news_items.Rows [ Current_News_Item ]
[ "description" ].ToString ( );
}
声明
txtNewsItem.Text =
" " + news_items.Rows [ Current_News_Item ]
[ "description" ].ToString ( );
导致 txtNewsItem 出现红色错误。 previous_BUT_Click 也出现同样的错误。
由于Page_Load中使用了相同的语句，我无法理解错误的原因。
【问题讨论】：
-
@Henk 代码已发布到原始帖子。 TIA。
-
您只是第一次加载数据。以下两个答案当场。
标签： c# .net html visual-studio-2008 nullreferenceexception