How Can I Retrieve Corresponding 'Name' Of Id In Text-Box

In the Event "ComboBox1_SelectedValueChanged", Use the SelectedValue and SelectedIndex property,

if(ComboBox1.SelectedIndex == id)
{
    textBox1.Text = ComboBox1.SelectedValue.ToString();
}

You can try the following approach assuming you have loaded the data from the SQL query into a DataTable.
(If not see Populating a DataSet from a DataAdapter[^])

// Dummy table created as an example
DataTable dt = new DataTable();
dt.Columns.Add("Id", typeof(int));
dt.Columns.Add("Name", typeof(string));

dt.Rows.Add(1, "Donald Duck");
dt.Rows.Add(2, "Donald Trump");
dt.Rows.Add(3, "Duffy Duck");

comboBox1.DataSource = dt;
comboBox1.ValueMember = "Id";
comboBox1.DisplayMember = "Name";

This way the Name will be shown in the combo box, and the Id is accessible as well.

For example:

void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    DataRowView drv = (comboBox1.SelectedItem as DataRowView);
    if (drv != null)
    {
        DataRow dr = drv.Row;
        int id = (int)dr["Id"];
        string name = dr["Name"].ToString();
        // If you really want to use a text box, then set 
        textBox1.Text = dr["Name"].ToString();    }
}