Databinding in dropdownlist from database in asp.net with C#.NET

Hi,ash

First Enable AutoPostBack of DropDownList to true after that in Selected_index changed event

using System.Data.SqlClient;
SqlConnection con=new SqlConnection("connection string");
    SqlCommand com;
    string str;

void Page_Load()

{
if(!IsPostBack)
{
      con.Open();
      str = "select * from student";
      com = new SqlCommand(str, con);
      SqlDataReader reader = com.ExecuteReader();
    while (reader.Read())
    {
    dropdownlist.Items.Add(reader["name"]);
    }
      reader.Close();
      con.Close();
}

}

void DropDownList1_SelectedIndexChanged()
{

    con.Open();
    str = "select * from student where name='"  + DropDownList1.SelectedValue + "'";
    com = new SqlCommand(str, con);
    SqlDataReader reader=com.ExecuteReader();
    if( reader.Read())
    {
    textBox1.Text = reader["id"].ToString();
    textBox2.Text = reader["age"].ToString();
    textBox3.Text = reader["marks"].ToString();
    textBox4.Text = reader["address"].ToString();
   
    }
               
    con.Close();
    reader.Close();
      
        }

}

If this help you mark it as Answer.

Hello
Here is a little help for you, may be it work.
Creating an editable GridView whose data comes from a SqlDataSource is amazingly easy. To start, the GridView's SqlDataSource must contain an UpdateCommand with appropriate parameters. Creating such a SqlDataSource is as simple as checking the Generate Insert, Update, and Delete Statements from the dialog box(Recall that this dialog box is accessed through the second step of the SqlDataSource wizard, by clicking the Advanced button.)

 You are doing an exciting practice, let me know what you get out of it.

Welcome to CsharpCorner.
For your requirment first bind the Dropdownlist with values while page is loading and next in dropdown selected index changed event write the code to selected value and get the values for other textboxes.