asp.net 未将对象引用设置到对象的实例。

marcus1982 · 发表于 2022-8-11 20:34

本帖最后由 marcus1982 于 2022-8-11 22:14 编辑

前台：

[HTML] 纯文本查看 复制代码

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<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="StudentEdit.aspx.cs" Inherits="考试系统.teacher.StudentEdit" %>
 
<!DOCTYPE html>
 
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
        <div>
            学号：<input type="text" name="txtStudentId" value="<%=EditStudentInfo.StudentId %>" /><br />
            姓名：<input type="text" name="txtStudentname" value="<%=EditStudentInfo.StudentName %>" /><br />
 
            <input type="submit" value="编辑确认" />
        </div>
    </form>
</body>
</html>

后台：

[C#] 纯文本查看 复制代码

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using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
 
namespace 考试系统.teacher
{
    public partial class StudentEdit : System.Web.UI.Page
    {
        kaoshidbEntities db = new kaoshidbEntities();
        public StudentInfo EditStudentInfo { set; get; }
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                ShowEditStundenInfo();
            }
            else
            {
                UpdateStudentInfo();
            }
        }
        protected void ShowEditStundenInfo()
        {
            string studentId = Request.QueryString["studentid"].ToString();
 
            StudentInfo studentInfo = db.StudentInfo.SingleOrDefault(S => S.StudentId == studentId);
            if (studentInfo != null)
            {
                EditStudentInfo = studentInfo;
            }
            else
            {
                Response.Redirect("/MyErrorPage.aspx");
            }
        }
        protected void UpdateStudentInfo()
        {
            string studentId = Request.QueryString["studentid"].ToString();
            StudentInfo studentInfo = db.StudentInfo.SingleOrDefault(S => S.StudentId == studentId);
            if (studentInfo != null)
            {
                studentInfo.StudentId = Request.Form["txtStudentId"].ToString();
                studentInfo.StudentName = Request.Form["txtStudentname"].ToString();
            }
            int r = db.SaveChanges();
            if (r > 0)
            {
                Response.Write("<script languge='javascript'>alert('添加成功！');window.location.href='StudentManage.aspx'</script>");
            }
            else
            {
                Response.Redirect("/MyErrorPage.aspx");
            }
        }
    }
}

提交后，出现以下错误，但是查看数据库后，修改也能成功。新手求救。
“/”应用程序中的服务器错误。
未将对象引用设置到对象的实例。说明: 执行当前 Web 请求期间，出现未经处理的异常。请检查堆栈跟踪信息，以了解有关该错误以及代码中导致错误的出处的详细信息。

异常详细信息: System.NullReferenceException: 未将对象引用设置到对象的实例。

源错误:

行 11: <form id="form1" runat="server">行 12: <div>行 13: 学号：<input type="text" name="txtStudentId" value="<%=EditStudentInfo.StudentId %>" /><br />行 14: 姓名：<input type="text" name="txtStudentname" value="<%=EditStudentInfo.StudentName %>" /><br />行 15:

源文件: c:\kaoshi2022\考试系统\考试系统\teacher\StudentEdit.aspx 行: 13

堆栈跟踪:

[NullReferenceException: 未将对象引用设置到对象的实例。] ASP.teacher_studentedit_aspx.__Renderform1(HtmlTextWriter __w, Control parameterContainer) in c:\kaoshi2022\考试系统\考试系统\teacher\StudentEdit.aspx:13 System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +103 System.Web.UI.HtmlControls.HtmlForm.RenderChildren(HtmlTextWriter writer) +177 System.Web.UI.HtmlControls.HtmlContainerControl.Render(HtmlTextWriter writer) +32 System.Web.UI.HtmlControls.HtmlForm.Render(HtmlTextWriter output) +360 System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +128 System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +287 System.Web.UI.HtmlControls.HtmlForm.RenderControl(HtmlTextWriter writer) +53 System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +197 System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) +9 System.Web.UI.Page.Render(HtmlTextWriter writer) +30 System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +128 System.Web.UI.Control.RenderControl(HtmlTextWriter writer, ControlAdapter adapter) +287 System.Web.UI.Control.RenderControl(HtmlTextWriter writer) +27 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5628

版本信息: Microsoft .NET Framework 版本:4.0.30319; ASP.NET 版本:4.8.4494.0

不忘初心哟 · 发表于 2022-8-11 20:54

public StudentInfo EditStudentInfo { set; get; } 这个默认是空相当于是null null.StudentId 相当于空指针就会报未将对象引用到实例

李玉风我爱你 · 发表于 2022-8-11 20:56

断点看看 29行的 studentInfo是什么？

marcus1982 · 发表于 2022-8-11 21:27

不忘初心哟发表于 2022-8-11 20:54
public StudentInfo EditStudentInfo { set; get; } 这个默认是空相当于是null null.StudentId 相当于空指 ...

好像明白了点，请问如何避免这个情况

marcus1982 · 发表于 2022-8-11 21:30

李玉风我爱你发表于 2022-8-11 20:56
断点看看 29行的 studentInfo是什么？

第一次载入页面显示：
{System.Data.Entity.DynamicProxies.StudentInfo_603A87C76AC597BEDC599A5AC96267CAF6D0DC292EBAC8AE44321A5D7DA71E2E}
[System.Data.Entity.DynamicProxies.StudentInfo_603A87C76AC597BEDC599A5AC96267CAF6D0DC292EBAC8AE44321A5D7DA71E2E]: {System.Data.Entity.DynamicProxies.StudentInfo_603A87C76AC597BEDC599A5AC96267CAF6D0DC292EBAC8AE44321A5D7DA71E2E}
ExamStudent: Count = 5
StudentClass: {System.Data.Entity.DynamicProxies.StudentClass_7A9923E5D75ABD30E8E924195BB9FB298B712AF8102178D26296EAF8EE7D713A}
StudentClassId: 1
StudentGender: "女"
StudentId: "1001"
StudentJieDa: Count = 1
StudentName: "张11112"
StudentPwd: "111"
当修改后，点击确认：
就直接报错了

gongjing457 · 发表于 2022-8-11 21:59

第29行 StudentInfo studentInfo = db.StudentInfo.SingleOrDefault(S => S.StudentId == studentId);
从数据库查到的有内容吗？
空指针说明studentInfo是空的

marcus1982 · 发表于 2022-8-11 22:13

gongjing457 发表于 2022-8-11 21:59
第29行 StudentInfo studentInfo = db.StudentInfo.SingleOrDefault(S => S.StudentId == studentId);
从 ...

能查到内容，我把Response.Write("<script languge='javascript'>alert('添加成功！');window.location.href='StudentManage.aspx'</script>");改成Response.Redirect("StudentManage.aspx");就没问题了。

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