c#对象引用没有设置为对象的实例

本文关键字：对象 实例 设置 对象引用 | 更新日期: 2023-09-27 17:50:46

我正在做一个来自网站的约会功能。它可以比较日期&创建新约会前的时间，如果用户输入了相同的日期& &;如果数据库中存在时间，则会弹出消息框。当我尝试插入不同的日期&时间和db不一样，给我误差。误差

我在这行得到错误:

string dtime = time.ExecuteScalar().ToString();

我不知道我的代码有什么问题，有人能指出我吗?谢谢。这是我的代码:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Windows.Forms;
public partial class MakeAppointment : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void Button1_Click(object sender, EventArgs e)
{
    string appointmentdate = Convert.ToString(DropDownListDay.Text + "-" + DropDownListMonth.Text + "-" + DropDownListYear.Text);
    string appointmenttime = Convert.ToString(DropDownListHour.Text + ":" + DropDownListMinute.Text + ":" + DropDownListSecond.Text + " " + DropDownListSession.Text);
    SqlConnection con = new SqlConnection("Data Source=USER-PC;Initial Catalog=webservice_database;Integrated Security=True");
    con.Open();
    SqlCommand date = new SqlCommand("Select adate from customer_registration where adate='"+ appointmentdate +"'",con);
    string ddate = date.ExecuteScalar().ToString();
    con.Close();
    if (ddate == appointmentdate)
    {
        con.Open();
        SqlCommand time = new SqlCommand("Select atime from customer_registration where atime='"+ appointmenttime +"'", con);
        string dtime = time.ExecuteScalar().ToString();
        con.Close();
        if (dtime == appointmenttime)
        {
            MessageBox.Show("This appointment is not available. Please choose other date & time.");
        }
}
}

c#对象引用没有设置为对象的实例

似乎ExcuteScalar();有问题，因为当查询没有记录存在时，它返回null。

你可以这样写

   var data= date.ExecuteScalar();
   if(data!=null)
         ddate =data.ToString();
详细

ExecuteScalar抛出NullReferenceException

试试这个

using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Windows.Forms;
using System;
public partial class MakeAppointment : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string appointmentdate = Convert.ToString(DropDownListDay.Text + "-" + DropDownListMonth.Text + "-" + DropDownListYear.Text);
        string appointmenttime = Convert.ToString(DropDownListHour.Text + ":" + DropDownListMinute.Text + ":" + DropDownListSecond.Text + " " + DropDownListSession.Text);
        using (SqlConnection con = new SqlConnection("Data Source=USER-PC;Initial Catalog=webservice_database;Integrated Security=True"))
        {
            con.Open();
            SqlCommand date = new SqlCommand("Select adate from customer_registration where adate='" + appointmentdate + "'", con);
            string ddate = date.ExecuteScalar().ToString();
            if (ddate == appointmentdate)
            {
                SqlCommand time = new SqlCommand("Select atime from customer_registration where atime='" + appointmenttime + "'", con);
                var rtime = time.ExecuteScalar();
                if (rtime != null)
                {
                    string dtime = rtime.ToString();
                    if (dtime == appointmenttime)
                    {
                        MessageBox.Show("This appointment is not available. Please choose other date & time.");
                    }
                }
                else
                {
                    MessageBox.Show("Failed to fetch time from database");
                }
            }
            con.Close();
        }
    }
}

看起来您的问题是，当您调用ExecuteScalar不返回任何值时，时间变量为空。只需检查时间变量是否有效(!)= null)，然后再尝试调用它的ExecuteScalar成员

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