C# 未将对象引用设置为对象的实例

Question

我正在从网站上执行预约功能。它可以在创建新约会之前比较日期和时间，如果数据库中存在用户键入相同的日期和时间，则会弹出消息框。当我尝试插入与数据库不同的不同日期和时间时，它给我错误。 Error

我在这一行遇到错误:

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

我不知道我的代码有什么问题，有人可以指出我吗？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.");
        }
}
}

Answer 1

ExcuteScalar(); 似乎有问题，因为当查询不存在记录时它返回 null。

你可以这样使用

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

详情

ExecuteScalar throws NullReferenceException