How to make an event of a gridview column dynamically

Are you using Ajax Control toolkit Combobox? If yes then do it like this Namespace: 

using AjaxControlToolkit;

 C#:

protected void ComboBox_OnSelectedIndexChanged(object sender, EventArgs e)
{
    GridViewRow row = (sender as ComboBox).NamingContainer as GridViewRow;
    ComboBox cb = (sender as ComboBox);
    string item = cb.SelectedItem.Value;
    DataSet ds = new DataSet();
    // Code for searching the item description from SQL and hold it into DataTable or Dataset

    //Set the values in TextBox
    (row.FindControl("TxtName") as TextBox).Text = ds.Tables[0].Rows[0]["Name"].ToString();
}