帮帮我！你调用的对象是空的 [英] help me out! Object reference not set to an instance of an object

问题描述

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;

namespace Datile
{
    public partial class login : System.Web.UI.Page
    {
     
      public  static string s = System.Configuration.ConfigurationManager.ConnectionStrings["ApplicationDbContext"].ToString();
        SqlConnection con = new SqlConnection(s);
        protected void Page_Load(object sender, EventArgs e)
        {

        }
        protected void Login1_Authenticate(object sender, AuthenticateEventArgs e)
        {
        }
        protected void Button1_Click(object sender, EventArgs e)
        {
            if ((TextUname.Text != "") && (TextPassword.Text != ""))
            {
                con.Open();

                SqlCommand cmd = new SqlCommand("select * from login where username='" + TextUname.Text + "'", con);
                SqlDataReader dr = cmd.ExecuteReader();
                if (dr.Read())
                {
                    if (TextUname.Text == dr["username"].ToString())
                    {
                        if (TextPassword.Text == dr["password"].ToString())
                        {

                            Session["name"] = dr["username"].ToString();
                            Session["uid"] = dr["uid"].ToString();
                         

                        }
                        else
                        {

                            // if (TextBox2.Text != dr["pwd"].ToString())
                            Label3.Text = "Incorrect Password";
                        }
                    }
                    else if (TextUname.Text != dr["username"].ToString() && TextPassword.Text != dr["password"].ToString())
                    {
                        Label3.Text = "both UserName and Password are wrong";
                    }
                    else
                    {
                        Label3.Text = "Incorrect UserName ";
                    }
                }
                else
                {
                    Label1.Text = "both UserName and Password are wrong";
                }
                dr.Close();
                con.Close();
            }
        }

    }
}

这里我得到了声明字符串's'时出现此错误

对象引用未设置为对象实例。

here im getting this error at declaration of string ''s''
Object reference not set to an instance of an object.

推荐答案

你没有显示带有对象引用未设置为对象实例消息的异常的位置，但这是最容易检测和修复的情况之一。它只是意味着某些引用类型的某个成员/变量通过使用和它的实例（非静态）成员解除引用，这要求此成员/变量为非null，但实际上它似乎为null。只需在调试器下执行它，它将停止抛出异常的执行。在该行上设置一个断点，重新启动应用程序并再次到达这一点。评估下一行中涉及的所有引用，并查看哪一个为null，而不需要为null。解决这个问题之后，修复代码：确保将成员/变量正确初始化为非空引用，或者将其检查为null，如果为null，则执行其他操作。

-SA

You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown, but this is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: wither make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

—SA

这意味着System.Configuration.ConfigurationManager.ConnectionStrings [ApplicationDbContext]返回null。如果在null上调用任何属性或方法，则抛出NullReferenceException。



检查配置文件中是否有相应的连接字符串名称。

This means that System.Configuration.ConfigurationManager.ConnectionStrings["ApplicationDbContext"] is returning null. If you call any property or method on a null then NullReferenceException is thrown.

Check your config file for the appropriate connection string name.

请检查web.config中的连接字符串，或者没有正确拼写ApplicationDbContext。

please check you connection string in web.config or you have not spelled "ApplicationDbContext" correctly.

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