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ASP.NET gridview dropdown with related records in textbox

Here we add the Grid View control to the web page, then add four template columns: one for the Dropdown List and three for the Textbox's. Set the AutoPostBack property to "True" for the Dropdown List, and create the SelectedIndexChanged event.
 
Now we have to handle the SelectedIndexChanged event of the DropDownList. Here we have to detect the exact Row of the GridView from where the SelectedIndexChanged event is fired, so we have to compare the "ClientID" of the DropDownList with the dropdowns available in all the rows of the GridView. Once the CliendID is matched, we can bind the TextBoxs with the data of that Row.So here we bind the dropdownlist with one column data and when user select any value its corresponding records are automatic bind with the TextBoxs present inside the gridview.
 
Table creation with data :

tables in database
 
The Default.aspx code is given below :
 
<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
  
 <html xmlns="http://www.w3.org/1999/xhtml">
 <head runat="server">
     <title>Untitled Page</title>
 </head>
 <body>
     <form id="form1" runat="server">
     <div>
     <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false"
    OnRowDataBound="GridView1_RowDataBound">
  
 <Columns>
 <asp:TemplateField HeaderText="EmployeeID">
 <ItemTemplate>
 <asp:DropDownList Width="100" runat="server"
    id="ddlTest" AutoPostBack="true"
    OnSelectedIndexChanged="ddlTest_SelectedIndexChanged">
 </asp:DropDownList>
 </ItemTemplate>
 </asp:TemplateField>
 <asp:TemplateField HeaderText="FirstName">
 <ItemTemplate>
 <asp:TextBox ID="txt1" runat="server"></asp:TextBox>
 </ItemTemplate>
 </asp:TemplateField>
 <asp:TemplateField HeaderText="LastName">
 <ItemTemplate>
 <asp:TextBox ID="txt2" runat="server"></asp:TextBox>
 </ItemTemplate>
 </asp:TemplateField>
 <asp:TemplateField HeaderText="Address">
 <ItemTemplate>
 <asp:TextBox ID="txt3" runat="server"></asp:TextBox>
 </ItemTemplate>
 </asp:TemplateField>
 </Columns>
 </asp:GridView>
     </div>
     </form>
 </body>
 </html>
 
The Default. aspx.cs code is given below:
 
 using System;
 using System.Configuration;
 using System.Data;
 using System.Linq;
 using System.Web;
 using System.Web.Security;
 using System.Web.UI;
 using System.Web.UI.HtmlControls;
 using System.Web.UI.WebControls;
 using System.Web.UI.WebControls.WebParts;
 using System.Xml.Linq;
 using System.Data.SqlClient;
 public partial class _Default : System.Web.UI.Page
 {
     string connStr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
     SqlDataAdapter sqlda = new SqlDataAdapter();
     SqlCommand com = new SqlCommand();
     DataTable dt;
     string str;
     protected void Page_Load(object sender, EventArgs e)
     {
         if (!IsPostBack)
         {
             bindgrid();
         }
     }
     private void bindgrid()
     {
         SqlConnection conn = new SqlConnection(connStr);
         dt = new DataTable();
         com.Connection = conn;
         com.CommandText = "SELECT * FROM Employees";
         sqlda = new SqlDataAdapter(com);
         sqlda.Fill(dt);
         GridView1.DataSource = dt;
         GridView1.DataBind();
  
     }
     protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
     {
         if (e.Row.RowType == DataControlRowType.DataRow)
         {
             Control ctrl = e.Row.FindControl("ddlTest");
             if (ctrl != null)
             {
                 DropDownList dd = ctrl as DropDownList;
            
                 SqlConnection conn = new SqlConnection(connStr);
                 dt = new DataTable();
                 com.Connection = conn;
                 com.CommandText = "SELECT * FROM Employees";
                 sqlda = new SqlDataAdapter(com);
                 sqlda.Fill(dt);
                 dd.DataTextField = "EmployeeID";
                 dd.DataValueField = "EmployeeID";
                 dd.DataSource = dt;
                 dd.DataBind();
             }
         }
  
     }
     protected void ddlTest_SelectedIndexChanged(object sender, EventArgs e)
     {
        
         DropDownList ddl = sender as DropDownList;
       
         foreach (GridViewRow row in GridView1.Rows)
         {
            
         Control ctrl = row.FindControl("ddlTest") as DropDownList;
         if (ctrl != null)
          {
           DropDownList ddl1 = (DropDownList)ctrl;
  
           if (ddl.ClientID == ddl1.ClientID)
           {
              TextBox txt1 = row.FindControl("txt1") as TextBox;
              TextBox txt2 = row.FindControl("txt2") as TextBox;
              TextBox txt3 = row.FindControl("txt3") as TextBox;
              SqlConnection conn = new SqlConnection(connStr);
              conn.Open();
              str = "select * from Employees where EmployeeID='" + ddl1.SelectedItem.Text + "'";
              com = new SqlCommand(str, conn);
              SqlDataReader reader = com.ExecuteReader();
              while (reader.Read())
              {
                 txt1.Text = reader["FirstName"].ToString();
                 txt2.Text = reader["LastName"].ToString();
                 txt3.Text = reader["Address"].ToString(); 
              }
              reader.Close();
              conn.Close();
          }      
      }
    }
 }
 
Output

GridView dropdown in ASP.NET
 
Thanks for reading