Please refer this code
<form id="form1" runat="server">
<div>
<asp:DropDownList ID="ddlEmployees" runat="server" OnSelectedIndexChanged="ddlEmployees_OnSelectedIndexChanged"
AutoPostBack="true">
</asp:DropDownList>
<br />
<hr />
<br />
<asp:DetailsView ID="DetailsView1" runat="server" AutoGenerateRows="false">
<Fields>
<asp:TemplateField HeaderText="EmployeeId">
<ItemTemplate>
<asp:TextBox ID="txtEmployeeId" runat="server" Text='<%# Eval("EmployeeId") %>' />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="FirstName">
<ItemTemplate>
<asp:TextBox ID="txtName" runat="server" Text='<%# Eval("FirstName") %>' />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Country">
<ItemTemplate>
<asp:TextBox ID="txtCity" runat="server" Text='<%# Eval("Country") %>' />
</ItemTemplate>
</asp:TemplateField>
</Fields>
</asp:DetailsView>
</div>
</form>
Namespaces
using System;
using System.Configuration;
using System.Data.SqlClient;
protected void Page_Load(object sender, EventArgs e)
{
if (!this.IsPostBack)
{
PopulateDropDownList();
}
}
private void PopulateDropDownList()
{
string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
using (SqlConnection con = new SqlConnection(constr))
{
using (SqlCommand cmd = new SqlCommand("SELECT EmployeeId, FirstName From Employees ", con))
{
con.Open();
this.ddlEmployees.DataSource = cmd.ExecuteReader();
this.ddlEmployees.DataTextField = "FirstName";
this.ddlEmployees.DataValueField = "EmployeeId";
this.ddlEmployees.DataBind();
con.Close();
}
}
}
protected void ddlEmployees_OnSelectedIndexChanged(object sender, EventArgs e)
{
int employeeId;
int.TryParse(this.ddlEmployees.SelectedItem.Value, out employeeId);
string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
using (SqlConnection con = new SqlConnection(constr))
{
using (SqlCommand cmd = new SqlCommand("SELECT EmployeeId, FirstName, Country From Employees WHERE EmployeeId = @EmployeeId", con))
{
cmd.Parameters.AddWithValue("@EmployeeId", employeeId);
cmd.Connection = con;
con.Open();
this.DetailsView1.DataSource = cmd.ExecuteReader();
this.DetailsView1.DataBind();
con.Close();
}
}
}
}
Note: This is just a sample for reference
Screenshot