将下拉列表中的结果绑定到ASP.NET Web表单中的文本框

将DropDownList和TextBox添加到Default.aspx。在下拉列表中，我们将使用DropDownList控件中的DataTextField和DataValueField属性存储学生姓名和学生ID。当我们绑定数据时，它将被映射。

    <div class="jumbotron">
        <h1>Student Report Card Application</h1>
        <p>&nbsp;</p>
        <p>
           Select a Student:
           <asp:DropDownList ID="ddl_StudentName" AutoPostBack="true" DataTextField="student_name" DataValueField="student_id" runat="server">

           </asp:DropDownList>
           &nbsp;&nbsp;&nbsp; <asp:TextBox ID="Student_ID" AutoPostBack="true" runat="server" MaxLength="40"></asp:TextBox>

        </p>
    </div>
    <div class="row">
    </div>

</asp:Content>

接下来，我们需要在后面添加一些代码来处理数据的检索。通过Default.aspx.cs后面的代码。本示例将连接到SqlServer数据库。还要注意，每当我们更改学生的下拉值时，它将通过名为SelectedIndexChanged的onchange事件绑定学生的ID。也可以查看评论。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.Sql;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;

namespace WebApplication_Test1
{
    public partial class _Default : Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            //connect to the database now 
            if (Page.IsPostBack == false)
            {
                //we store the database connect information in Web.Config
                //so we retrieve the connection string from the Web.Config
                String mydatabaseconnection = ConfigurationManager.ConnectionStrings["DBConnection"].ToString();
                SqlConnection con = new SqlConnection(mydatabaseconnection);
                //select all records from the grades table via
                //here you can replace this table 'Grades' with your table's schema  
                String myquery = "Select * From Grades";
                SqlCommand command = new SqlCommand(myquery);
                command.CommandType = System.Data.CommandType.Text;
                command.Connection = con;
                try
                {
                    //open the connection to the database 
                    con.Open();
                    SqlDataAdapter adapter = new SqlDataAdapter(command);

                    DataSet ds = new DataSet("Grades");

                    //populate the data into a DataSet 
                    adapter.Fill(ds);

                    //ddl_StudentName.DataSource = ds.Tables[0];
                    ddl_StudentName.DataSource = ds;
                    ddl_StudentName.DataBind(); // bind the data from the table now 
                                                // this is were DataTextField and DataValueField will get mapped
                                                // to database fields student_name and student_id

                    //to handle the drop down change use event SelectedIndexChanged
                    ddl_StudentName.SelectedIndexChanged += Ddl_StudentName_SelectedIndexChanged;

                    //gets the first student from the database and populate the textbox
                    Student_ID.Text = ds.Tables[0].Rows[0]["student_id"].ToString();

                    //close connection to database 
                    con.Close();
                }
                catch (Exception ex)
                {

                }
            }else
            {
                ddl_StudentName.SelectedIndexChanged += Ddl_StudentName_SelectedIndexChanged
            }
        }

        private void Ddl_StudentName_SelectedIndexChanged(object sender, EventArgs e)
        {
            //when we change the dropdownlist we need to get the student id and set it to the textbox
            DropDownList mydropdownlist = sender as DropDownList;
            Student_ID.Text = mydropdownlist.SelectedValue;

        }
    }
}

示例Web.config代码段。在这里，您可以看到连接字符串参数server = DESKTOP-CPJ3R2K23 \ SQLEXPRESS，数据库= UniversityDB，用户名= sa，密码= test1，提供程序是SqlClient。在此示例中，连接到SqlServer Express数据库是必需的。

 <connectionStrings>
  <add name="DBConnection" connectionString="server=DESKTOP-CPJ3R2K23\SQLEXPRESS;database=UniversityDB;Integrated Security=True;uid=sa;pwd=test1"  providerName="System.Data.SqlClient"/> 
 </connectionStrings>

来自UniversityDB数据库的样本表方案。

USE [UniversityDB]
GO

CREATE TABLE [dbo].[Grades](
    [grade] [varchar](10) NULL,
    [student_id] [int] NULL,
    [student_name] [varchar](40) NULL
) ON [PRIMARY]

希望这可以帮助。