<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <configSections>
  </configSections>
  <connectionStrings>
    <add name="【自定义连接名称 如：SQLDBConnectionString(数据库名称+ConnectionString)】"
        connectionString="Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\【数据库文件如：SqlDB.mdf】;Integrated Security=True;User Instance=True;Asynchronous Processing=true"
        providerName="System.Data.SqlClient" />
  </connectionStrings>
</configuration>

------解决方案--------------------

登录按钮：
C# code

private void button1_Click(object sender, EventArgs e)
        {
            if ( textBox2.Text.Trim() == "" || textBox3.Text.Trim() == "")
            {
               MessageBox.Show("提示：请输入登录用户名和密码！", "警告");
            }
            else 
            {

              
                SqlConnection connect = DB.DB_Login();
                string strSQL = "select * from 表名 where Login='" + textBox2.Text.Trim() + "'and Password='" + textBox3.Text.Trim() + "'";
                SqlCommand cmd = new SqlCommand(strSQL, connect);
                SqlDataReader asd = cmd.ExecuteReader();
                asd.Read();
                if (asd.HasRows)
                {
                   this.Hide();
                   MainForm formchild = new MainForm();
                   formchild.users = textBox2.Text.Trim();
                   formchild.Show();
                }
                else
                {
                        MessageBox.Show("提示：学生用户名或密码错误！", "警告");
                }
                   connect.Close();
                }               
            }
        }