public partial class frmLogin : Form
{
    //声明变量
    OleDbConnection conObj;
    OleDbCommand cmdObj;
    OleDbDataReader readObj;
    string sql;

    //窗体的构造方法
    public frmLogin()
    {
        InitializeComponent();
    }

    //窗体的 Load 事件
    private void frmLogin_Load(object sender, EventArgs e)
    {
        conObj = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0; Data Source=txl.mdb");
        conObj.Open();
    }

    //自定义方法
    private bool Allfill()
    {
        if (txtId.Text == "") return false;
        if (txtPass.Text == "") return false;
        return true;
    }    

    //登录
    private void button1_Click(object sender, EventArgs e)
    {
        if (Allfill())
        {
            sql = "SELECT * FROM MngInfo WHERE "
                + "Id ='" + txtId.Text + "'";
            cmdObj = new OleDbCommand(sql, conObj);
            readObj = cmdObj.ExecuteReader();
            if (readObj.HasRows)
            {
                readObj.Read();
                if (readObj.GetString(1) == txtPass.Text)
                {                     
                    frmMain fMain = new frmMain(this);
                    fMain.ShowDialog();
                }
                else
                {
                    MessageBox.Show("密码出错!");
                }

            }
            else
            {
                MessageBox.Show("用户不存在!");
            }
        }
        else
        {
            MessageBox.Show("字段不能为空");
        }
    }

    //退出
    private void button2_Click(object sender, EventArgs e)
    {
        Application.Exit();
    }
}
上学时候班主任写的代码~虽然简单但是很有用哦~学习c#的朋友可以参考参考