using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Globalization;
using System.Data .OleDb ;
namespace 学生信息管理系统
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void comboBox1_TextChanged(object sender, EventArgs e)
        {
            if (() == "管理员")
            {
                Usertextbox.Text = "administrator";
                Passwordbox.Text = "ttxlla";
            }
            if (() == "教师")
            {
                Usertextbox.Text = "teacher";
                Passwordbox.Text = "teacher";
            }
            if (() == "学生")
            {
                Usertextbox.Text = "student";
                Passwordbox.Text = "student";
            }
            else if (() == "游客")
            {
                Usertextbox.Text = "您将以游客身份登录！";
                Passwordbox.Text = "guest";
            }
        }
        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (MessageBox.Show("Sure Out？", "Message", MessageBoxButtons.YesNo, MessageBoxIcon.Question) == DialogResult.No)
            {
                e.Cancel = true;
            }
        }
        private void button2_Click(object sender, EventArgs e)
        {
            this.Close();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            timer1.Enabled = true;
            timer1.Interval = 1000;
        }
        private void timer1_Tick(object sender, EventArgs e)
        {
            this.label5.Text = "Time is：" + System.DateTime.Now.ToString();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            if (Usertextbox.Text.Trim() == "" || Passwordbox.Text.Trim() == "")
            {
                MessageBox.Show("Sorry！You hane not import your username or your password！", "Message", MessageBoxButtons.OK, MessageBoxIcon.Error);
                Usertextbox.Focus();
            }
            else
            {
                string strConnection = "Provider=Microsoft.Jet.OLEDB.4.0;";
                strConnection += "Data Source=";
                strConnection += Application.StartupPath;
                strConnection += @"\学生信息.mdb";
                OleDbConnection conn = new OleDbConnection(strConnection);
                conn.Open();
                OleDbDataAdapter dauser = new OleDbDataAdapter("select * from 学生信息表 where 用户名='" +Usertextbox . Text.Trim() + "'+  密码='"+Passwordbox .Text .Trim ()+"'+  身份='"+comboBox1 .Text .Trim ()+"'", conn);
                DataSet dsuser = new DataSet();
                dauser.Fill(dsuser, "学生信息表");

                if (dsuser.Tables[0].Rows.Count == 0)
                {
                    MessageBox.Show("Sorry！Wrong usernam！", "message",MessageBoxButtons .OK ,MessageBoxIcon.Error );
                    Usertextbox.Clear();
                    Passwordbox.Clear();
                    Usertextbox .Focus ();
                }
                else
                {
                    DataRow[] dr = dsuser.Tables[0].Select("用户名='" +Usertextbox . Text + "'");
                    if ((dr[0]["密码"].ToString(),Passwordbox . Text) == 0)
                    {
                        MessageBox.Show("Success！", "message",MessageBoxButtons .OK ,MessageBoxIcon.Asterisk );
                    }  
                    else
                    {
                        MessageBox.Show("Wrong password！", "message",MessageBoxButtons .OK ,MessageBoxIcon.Error );
                       Passwordbox . Clear();
                       Passwordbox . Focus();
                    }
                }

            }
        }
    }
}


这是学校布置的一个作业！我运行的话没有报错！我输入错误的usernam说账号错误！但是就算我输入了正确的账号密码也是账号错误！就好像程序只运行到MessageBox.Show("Sorry！Wrong usernam！", "message",MessageBoxButtons .OK ,MessageBoxIcon.Error );地方一样！请教大虾们！这个程序错在哪里！<我的combobox功能现在可以不考虑！>

OleDbDataAdapter dauser = new OleDbDataAdapter("select * from 学生信息表 where 用户名='" +Usertextbox . Text.Trim() + "'+  密码='"+Passwordbox .Text .Trim ()+"'+  身份='"+comboBox1 .Text .Trim ()+"'", conn);
--->这是你不是用了用户名与密码都存在,dauser的数目大于0的话就是能过了验证了吗.
不过好像你的SQL语句有点问题.
-->OleDbDataAdapter dauser = new OleDbDataAdapter("select * from 学生信息表 where 用户名='" +Usertextbox . Text.Trim() + "'+ and 密码='"+Passwordbox .Text .Trim ()+"'+ and 身份='"+comboBox1 .Text .Trim ()+"'", conn);

--> select [字段1],[字段2]...where [条件1] and/or[条件2]  ...
--->所以后面只须判断是否为空就可以决定是否通过验证.