ASP.NET 怎么动态修改 Header 属性如添加 Meta 标签 keywords description_ASP.NET

ASP.NET 如何动态修改 Header 属性如添加 Meta 标签 keywords description!
我现在做了一个类里面写的是
public class initfunc : System.Web.UI.Page
{
public initfunc()
{
//
// TODO: 在此处添加构造函数逻辑
//
}
public void setMeta(string key, string des)
{
HtmlMeta keywords = new HtmlMeta(), description = new HtmlMeta(); // keywords, description
keywords.Name = "keywords";
keywords.Content = key;

description.Name = "description";
description.Content = des;

Page.Header.Controls.Add(keywords);
Page.Header.Controls.Add(description);
}
}

然后我在test.aspx.cs代码中这么写
initfunc ini = new initfunc();
protected void Page_Load(object sender, EventArgs e)
{
ini.setMeta("1111", "2222");
}
程序出现错误：System.NullReferenceException: 未将对象引用设置到对象的实例。
我就是想在编写每个页面代码的时候都能调用这个类进行修改 keywords description。希望帮我解答或者帮我想个更好的办法！

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C# code

// Render: <meta name="keywords" content="Some words listed here" />HtmlMeta meta = new HtmlMeta();meta.Name = "keywords";meta.Content = "Some words listed here";this.Header.Controls.Add(meta);// Render: <meta name="robots" content="noindex" />meta = new HtmlMeta();meta.Name = "robots";meta.Content = "noindex";this.Header.Controls.Add(meta);// Render: <meta name="date" content="2006-03-25" scheme="YYYY-MM-DD" />meta = new HtmlMeta();meta.Name = "date";meta.Content = DateTime.Now.ToString("yyyy-MM-dd");meta.Scheme = "YYYY-MM-DD";this.Header.Controls.Add(meta);
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我是写在masterpage里，不同的页，赋值Session("currentpage"),根据库里的不同页不同关键词填充。
  Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
       makemeta(Session("currentpage"))
   End Sub
   Sub makemeta(ByVal item As String)
       Dim ds As DataSet = toolClass.getmeta(item)
       Page.Title = ds.Tables(0).Rows(0)("title")
       Dim meta As New HtmlMeta
       meta.Name = "keywords"
       meta.Content = ds.Tables(0).Rows(0)("keywords")
       Page.Header.Controls.Add(meta)
       meta.Name = "description"
       meta.Content = ds.Tables(0).Rows(0)("description")
       Page.Header.Controls.Add(meta)
   End Sub
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C# codepublic class initfunc : System.Web.UI.Page{public initfunc(){//// TODO: 在此处添加构造函数逻辑//}     public void setMeta(System.Web.UI.Control page,string key, string des)     {        HtmlMeta keywords = new HtmlMeta(), description = new HtmlMeta(); // keywords, description        keywords.Name = "keywords";        keywords.Content = key;        description.Name = "description";        description.Content = des;        page.Header.Controls.Add(keywords);        page.Header.Controls.Add(description);    }} //加个参数，使用的时候initfunc ini = new initfunc();protected void Page_Load(object sender, EventArgs e){    ini.setMeta(this,"1111", "2222");}
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C# code    public class AA    {         public static void setMeta(string keyWords)        {            System.Web.UI.HtmlControls.HtmlMeta m = new HtmlMeta();            m.Name = "keywords";            m.Content = keyWords;            System.Web.UI.Page p = (System.Web.UI.Page)System.Web.HttpContext.Current.Handler;            p.Header.Controls.Add(m);        }    }调用:protected void Page_Load(object sender, EventArgs e)        {            AA.setMeta("关键词AAA");                        }
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幕白兄来了！！