how to filter records in gridview by selecting column name from dropdown list and search term from textbox in asp.net

<asp:DropDownList ID="ddlSearchItems" runat="server">
       
       <asp:TextBox ID="txtSearch" runat="server">
       <br />
       <br />
       <asp:Button ID="btnSearch" runat="server" Text="Search" OnClick="Search" />
       <asp:GridView ID="gvEmployees" runat="server" AutoGenerateColumns="false">
           <columns>
               <asp:BoundField DataField="ID" HeaderText="ID"/>
               <asp:BoundField DataField="FirstName" HeaderText="FirstName"/>
               <asp:BoundField DataField="LastName" HeaderText="LastName"/>
               <asp:BoundField DataField="Sources" HeaderText="Sources" />               
           </columns>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;

public partial class Report2 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
            this.BindSearchTerm();
        }
    }
    string query = "SELECT ID,FirstName,LastName FROM tblPersonalDetails";
    protected void Search(object sender, EventArgs e)
    {
        string constr = @"Data Source=Vrushali-pc;Initial Catalog=HRRecruitment;Persist Security Info=True;User ID=sa;Password=hrrecruitment";
        query += string.Format(" WHERE {0} = '{1}'", this.ddlSearchItems.SelectedItem.Text.Trim(), this.txtSearch.Text.Trim());
        this.gvEmployees.DataSource = this.GetData(constr, query);
        this.gvEmployees.DataBind();
    }

    private void BindSearchTerm()
    {
        DataTable dt = new DataTable();
        string constr = @"Data Source=Vrushali-pc;Initial Catalog=HRRecruitment;Persist Security Info=True;User ID=sa;Password=hrrecruitment";
        this.ddlSearchItems.DataSource = this.GetData(constr, "SELECT ID, FirstName+','+LastName+','+Sources AS Name FROM tblPersonalDetails");
        //ddlSearchItems.DataSource = ds.Tables[0].Columns;
        for (int i = 0; i < dt.Columns.Count; i++)
        {
            ddlSearchItems.Items.Add(new ListItem(dt.Columns[i].ColumnName));
        }
        this.ddlSearchItems.DataSource = this.GetData(constr, "SELECT ID, FirstName+','+LastName+','+Sources AS Name FROM tblPersonalDetails");
        this.ddlSearchItems.DataTextField = "Name";
        this.ddlSearchItems.DataValueField = "ID";
        this.ddlSearchItems.DataBind();
    }

    private DataTable GetData(string constr, string query)
    {
        DataTable dt = new DataTable();
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand(query))
            {
                using (SqlDataAdapter sda = new SqlDataAdapter())
                {
                    cmd.CommandType = CommandType.Text;
                    cmd.Connection = con;
                    sda.SelectCommand = cmd;
                    sda.Fill(dt);
                }
            }
            return dt;
        }
    }
}