C# grammer[2]

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--Default.aspx--

XML

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>

<!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>无标题页</title>
</head>
<body>
    <form id="form1" runat="server">
    <asp:Button ID="Button1" runat="server" Height="30px"
        Text="Button" Width="69px" onclick="Button1_Click"  />
    <asp:TextBox ID="TextBox1" runat="server" ></asp:TextBox>
    <div>

    </div>
    </form>
</body>
</html>

--Default.aspx.cs--

using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;

public delegate void Y();

public partial class _Default : System.Web.UI.Page 
{ 
    public event Y X;
    protected void Page_Load(object sender, EventArgs e)
    {
        A a = new A();
        X += a.f;
    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        X();
    }
}

public class A:_Default
{
    public void f()
    {
        TextBox1.Text = "success";
    }
}

1)why TextBox1.Text="success" will generate a NullReferenceException in function f() when run Default.aspx and click Button on browser.

Posted 1-Jun-10 20:10pm

shilf

Updated 1-Jun-10 22:03pm

v5

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kornakar · Accepted Answer · 2010-06-01T21:00:00

Solution 1

Because you just create an instance of class _Default but the controls in it don't get instantiated. This is what the ASP.NET engine does when you request a page.

This page may help you: http://www.west-wind.com/weblog/posts/120530.aspx[^]

Posted 1-Jun-10 21:00pm

kornakar

Comments

shilf 2-Jun-10 3:50am

if the function Button1_Click(object sender, EventArgs e) is :
            protected void Button1_Click(object sender, EventArgs e)  
            {        
                           TextBox1.Text="success";    
            }

you said"you just create an instance of class _Default but the controls in it don't get instantiated"
why it is right, when i click the button

kornakar 2-Jun-10 5:27am

You have TWO different instances of the page, the one you're clicking the button on, and the another you create manyally on Page_Load. The one you see on your browser is initialized by ASP.NET engine and that's why the TextBox1 has an instance. However, the class A is just manyally created and doesn't get initialized (TextBox1 is null). I suggest you to read a few articles about classes and references etc.

shilf 2-Jun-10 10:24am

I see,thanks for your help