searching from database by giving 2 textbox and 2 dropdownlist input

public partial class search : System.Web.UI.Page
{
   
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["SQLDbConnection"].ConnectionString);

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            fill_ddlPhone();
            fill_ddlEmail();
        }
        
    }


    private void fill_ddlPhone()
    {
        try
        {
            string sql = "SELECT * FROM registration";

            ddlPhone.Items.Clear();
            ddlPhone.AppendDataBoundItems = true;
            ddlPhone.Items.Add(new ListItem("Phone", "-1"));
            ddlPhone.DataSource = cmscon.getRows(sql);
            ddlPhone.DataTextField = "Phone";
            ddlPhone.DataValueField = "Phone";
            ddlPhone.DataBind();
            ddlPhone.SelectedValue = "-1";
        }
        catch (Exception ex)
        {

        }
    }
    private void fill_ddlEmail()
    {
        try
        {
            string sql = "SELECT * FROM registration";

            ddlEmail.Items.Clear();
            ddlEmail.AppendDataBoundItems = true;
            ddlEmail.Items.Add(new ListItem("E-Mail", "-1"));
            ddlEmail.DataSource = cmscon.getRows(sql);
            ddlEmail.DataTextField = "Email";
            ddlEmail.DataValueField = "Email";
            ddlEmail.DataBind();
            ddlEmail.SelectedValue = "-1";
        }
        catch (Exception ex)
        {

        }
    }
    protected void btnSearch_Click(object sender, EventArgs e)
    {

        String str = "select * from registration where(FirstName like '%' + @search + '%'AND StudentID LIKE '%' + @StId + '%' )";
        

        SqlCommand cmdSearch = new SqlCommand(str, con);
        cmdSearch.Parameters.Add("@search", SqlDbType.VarChar).Value = txtContactFirstName.Text;
        cmdSearch.Parameters.Add("@StId", SqlDbType.VarChar).Value = txtContactID.Text;

        con.Open();
        cmdSearch.ExecuteNonQuery();
        SqlDataAdapter da = new SqlDataAdapter();
        da.SelectCommand = cmdSearch;
        DataSet ds = new DataSet();
        DataTable dt = new DataTable();

        da.Fill(ds, "FirstName");
        da.Fill(ds, "StudentID");
        gvSearch.DataSource = ds;
        gvSearch.DataBind();
        con.Close();

        

           

    }
}