我无法理解connectionString命令背后的逻辑。我正在尝试创建用户名注册页面，在提交之前检查用户名的可用性。我已经按照我的ASP.NET释放的教科书来发球，但我仍然得到“对象参考未设置”错误。我必须在此处相应地命名我的数据库和表格，还是我错过了其他的东西？我一直在疯狂地搜索互联网，试图理解这一点，但我只是在困惑自己。任何建议将不胜感激。
以下是我的ShowAjaxValidator.aspx和AjaxValidator.cs页面：
<%@ Page Language="C#" %>
<%@ Register TagPrefix="custom" Namespace="myControls" %>
<%@ Import Namespace="System.Data.SqlClient" %>
<%@ Import Namespace="System.Web.Configuration" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
/// <summary>
/// Validation function that is called on both the client and server
/// </summary>
protected void AjaxValidator1_ServerValidate(object source, ServerValidateEventArgs args)
{
if (UserNameExists(args.Value))
args.IsValid = false;
else
args.IsValid = true;
}
/// <summary>
/// Returns true when user name already exists
/// in Users database table
/// </summary>
private bool UserNameExists(string userName)
{
string conString =
WebConfigurationManager.ConnectionStrings["UsersDB"].ConnectionString;
SqlConnection con = new SqlConnection(conString);
SqlCommand cmd = new SqlCommand("SELECT COUNT(*) FROM Users WHERE UserName=@UserName", con);
cmd.Parameters.AddWithValue("@UserName", userName);
bool result = false;
using (con)
{
con.Open();
int count = (int)cmd.ExecuteScalar();
if (count > 0)
result = true;
}
return result;
}
/// <summary>
/// Insert new user name to Users database table
/// </summary>
protected void btnSubmit_Click(object sender, EventArgs e)
{
string conString =
WebConfigurationManager.ConnectionStrings["UsersDB"].ConnectionString;
SqlConnection con = new SqlConnection(conString);
SqlCommand cmd = new SqlCommand("INSERT Users (UserName,FavoriteColor) VALUES (@UserName,@FavoriteColor)", con);
cmd.Parameters.AddWithValue("@UserName", txtUserName.Text);
cmd.Parameters.AddWithValue("@FavoriteColor", txtFavoriteColor.Text);
using (con)
{
con.Open();
cmd.ExecuteNonQuery();
}
txtUserName.Text = String.Empty;
txtFavoriteColor.Text = String.Empty;
}
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
<title>Show AjaxValidator</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Label
id="lblUserName"
Text="User Name:"
AssociatedControlID="txtUserName"
Runat="server" />
<asp:TextBox
id="txtUserName"
Runat="server" />
<custom:AjaxValidator
id="AjaxValidator1"
ControlToValidate="txtUserName"
Text="User name already taken!"
OnServerValidate="AjaxValidator1_ServerValidate"
Runat="server" />
<br /><br />
<asp:Label
id="lblFavoriteColor"
Text="Favorite Color:"
AssociatedControlID="txtFavoriteColor"
Runat="server" />
<asp:TextBox
id="txtFavoriteColor"
Runat="server" />
<br /><br />
<asp:Button
id="btnSubmit"
Text="Submit"
Runat="server" OnClick="btnSubmit_Click" />
</div>
</form>
</body>
</html>
AjaxValidator.cs
using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace myControls
{
/// <summary>
/// Enables you to perform custom validation on both the client and server
/// </summary>
public class AjaxValidator : BaseValidator, ICallbackEventHandler
{
public event ServerValidateEventHandler ServerValidate;
string _controlToValidateValue;
protected override void OnPreRender(EventArgs e)
{
String eventRef = Page.ClientScript.GetCallbackEventReference
(
this,
"",
"",
""
);
// Register include file
String includeScript =
Page.ResolveClientUrl("~/ClientScripts/AjaxValidator.js");
Page.ClientScript.RegisterClientScriptInclude("AjaxValidator",
includeScript);
// Register startup script
String startupScript =
String.Format("document.getElementById('{0}').evaluationfunction = 'AjaxValidatorEvaluateIsValid';", this.ClientID);
Page.ClientScript.RegisterStartupScript(this.GetType(), "AjaxValidator", startupScript, true);
base.OnPreRender(e);
}
/// <summary>
/// Only do the AJAX on browsers that support it
/// </summary>
protected override bool DetermineRenderUplevel()
{
return Context.Request.Browser.SupportsCallback;
}
/// <summary>
/// Server method called by client AJAX call
/// </summary>
public string GetCallbackResult()
{
return ExecuteValidationFunction(_controlToValidateValue).ToString();
}
/// <summary>
/// Return callback result to client
/// </summary>
public void RaiseCallbackEvent(string eventArgument)
{
_controlToValidateValue = eventArgument;
}
/// <summary>
/// Server-side method for validation
/// </summary>
protected override bool EvaluateIsValid()
{
string controlToValidateValue =
this.GetControlValidationValue(this.ControlToValidate);
return ExecuteValidationFunction(controlToValidateValue);
}
/// <summary>
/// Performs the validation for both server and client
/// </summary>
private bool ExecuteValidationFunction(String controlToValidateValue)
{
ServerValidateEventArgs args = new ServerValidateEventArgs (controlToValidateValue, this.IsValid);
if (ServerValidate != null)
ServerValidate(this, args);
return args.IsValid;
}
}
}
我遇到的错误指向.aspx文件的一部分：
string conString =
WebConfigurationManager.ConnectionStrings["UsersDB"].ConnectionString;
错误表示用户无法处理空异常，并且“对象引用未设置为对象的实例”。
我尝试将connectionStrings添加到web.config文件中，但我不确定它是否使用了正确的语法。你能告诉我这是不是你在说什么：
<?xml version="1.0"?>
<!--
For more information on how to configure your ASP.NET application, please visit
http://go.microsoft.com/fwlink/?LinkId=169433
-->
<configuration>
<connectionStrings>
<add name="UsersDB"
providerName="System.Data.SqlClient"
connectionString="Data Source=.\SQLExpress;
AttachDbFilename=|DataDirectory|UsersDB.mdf;
Integrated Security=True;User Instance=True" />
</connectionStrings>
<system.web>
<compilation debug="true" targetFramework="4.0"/>
</system.web>
</configuration>
我现在收到一个SqlException，说： 尝试为文件C：\ Users \ Mike \ Documents \ Visual Studio 2010 \ WebSites \ Validation_Attempt \ App_Data \ UsersDB.mdf附加自动命名的数据库失败。存在具有相同名称的数据库，或者无法打开指定的文件，或者它位于UNC共享上。
我是ASP和C＃的新手。编辑青铜。
答案 0 :(得分：0)
您似乎正在尝试读取不存在的配置值，然后访问该空对象上的属性。
您的web.config文件中需要一个connectionStrings节点，该节点的密钥为UsersDB，其中包含数据库的连接字符串。没有它，该对象为null并且调用ConnectionString属性会抛出空引用异常。解决该设置后，此属性将正常运行。