页回发到服务器时出现"未将对象引用设置到对象的实例"!

[reddust]

我做了个简单的新闻系统，有一个公共类库如下：
public class NewsPublic
{
private string siteName,siteUrl,siteMail,siteLogo,siteAd,newsAd;
public NewsPublic()
{
OleDbDataReader dr=DataAccess.OleDbDr("SELECT * FROM SiteConfig");
dr.Read();
siteName=dr["db_SiteName"].ToString();
siteUrl=dr["db_SiteUrl"].ToString();
siteMail=dr["db_SiteMail"].ToString();
siteLogo=dr["db_SiteLogo"].ToString();
siteAd=dr["db_SiteAd"].ToString();
newsAd=dr["db_NewsAd"].ToString();
}

public string SiteName
{
get
{
return siteName;
}
set
{
siteName=value;
}

}

public string SiteUrl
{
get
{
return siteUrl;
}
set
{
siteUrl=value;
}

}

.....//一些方法


在新闻系统中的.cs文件里创建了一个全局的NewsPublic np; 然后在相应的.aspx页面中用<%=np.SiteName%>调用一直没有问题，但是如果我在页面放一个按钮并引发事件后，页面却提示：未将对象引用设置到对象的实例。<%=np.SiteName%>行有错误，这是怎么回事呢？

[树猫]

页面放一个按钮并引发事件后:页面回发、刷新

我想你的代码可能有问题，所以页面回发后导致np没有被重新实例化

[reddust]

估计是哪部分代码呢？我贴出来！

[CMIC]

同意freecs(^=^)
NewsPublic np=new NewsPublic();相关语句贴出来

[zhongkeruanjian]

知道，你的NewsPublic np=new NewsPublic();是在IS(!POSTBACK)里做的，请确保在回访时也能NewsPublic np=new NewsPublic();

[reddust]

部分代码如下：

public class BigClass : System.Web.UI.Page
{
protected System.Web.UI.WebControls.PlaceHolder PH_ShowSClassNews;
protected System.Web.UI.WebControls.DataGrid DataGrid1;
protected System.Web.UI.WebControls.Button BTN_Search;
protected System.Web.UI.WebControls.TextBox TBX_SearchKeys;
protected System.Web.UI.WebControls.DropDownList DDL_Item;
public NewsPublic np;

private void Page_Load(object sender, System.EventArgs e)
{

if(!Page.IsPostBack)
{

np=new NewsPublic();
np.ShowBClassAllNews(HttpContext.Current.Request.QueryString["BigClassName"],PH_ShowSClassNews);
OleDbDataReader dr= DataAccess.OleDbDr("SELECT db_ClassName FROM SmallClass WHERE db_BigClassName='" + HttpContext.Current.Request.QueryString["BigClassName"]+"'");
dr.Read();
DataGrid1.DataSource=DataAccess.OleDbdbView("SELECT * FROM news WHERE db_NewsSmallClass='"+dr[0].ToString()+"'",dr[0].ToString());
DataGrid1.DataBind();

}
// 在此处放置用户代码以初始化页面
}

[saucer]

np is null when IsPostBack is true

if(!Page.IsPostBack)
{
np=new NewsPublic();
}

try to move it outside the if block, but your code is probably wrong, consider to use a <asp:Label> in your aspx at the place you output np.SiteName:


<asp:Label id=lblSite runat=server />


....

public class BigClass : System.Web.UI.Page
{
protected System.Web.UI.WebControls.PlaceHolder PH_ShowSClassNews;
protected System.Web.UI.WebControls.DataGrid DataGrid1;
protected System.Web.UI.WebControls.Button BTN_Search;
protected System.Web.UI.WebControls.TextBox TBX_SearchKeys;
protected System.Web.UI.WebControls.DropDownList DDL_Item;
protected System.Web.UI.WebControls.Label lblSite;
public NewsPublic np;


private void Page_Load(object sender, System.EventArgs e)
{

if(!Page.IsPostBack)
{

np=new NewsPublic();
np.ShowBClassAllNews(HttpContext.Current.Request.QueryString["BigClassName"],PH_ShowSClassNews);

lblSite.Text =np.SiteName;

OleDbDataReader dr= DataAccess.OleDbDr("SELECT db_ClassName FROM SmallClass WHERE db_BigClassName='" + HttpContext.Current.Request.QueryString["BigClassName"]+"'");
dr.Read();
DataGrid1.DataSource=DataAccess.OleDbdbView("SELECT * FROM news WHERE db_NewsSmallClass='"+dr[0].ToString()+"'",dr[0].ToString());
DataGrid1.DataBind();
dr.Close();

}
}