Bind TextBox based on multi-select DropDownList using C# and VB.Net in ASP.Net

Hi micah,

Refer below sample code.

HTML

<asp:DropDownList ID="ddlCountries" runat="server" multiple="multiple" CssClass="multiselect">
    <asp:ListItem Text="Argentina" Value="Argentina"></asp:ListItem>
    <asp:ListItem Text="Austria" Value="Austria"></asp:ListItem>
    <asp:ListItem Text="Belgium" Value="Belgium"></asp:ListItem>
    <asp:ListItem Text="Brazil" Value="Brazil"></asp:ListItem>
    <asp:ListItem Text="Canada" Value="Canada"></asp:ListItem>
</asp:DropDownList>
<asp:HiddenField ID="hfSelected" runat="server" />
<asp:Button Text="Filter" runat="server" OnClick="Country_Changed" />
<hr />
<asp:TextBox ID="txtDetails" runat="server"></asp:TextBox>
<div>
    <script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
    <link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/css/bootstrap.min.css" />
    <script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/js/bootstrap.min.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-multiselect/0.9.13/js/bootstrap-multiselect.js"></script>
    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-multiselect/0.9.13/css/bootstrap-multiselect.css">
    <script type="text/javascript">
        $(function () {
            $('[id*=ddlCountries]').multiselect({ includeSelectAllOption: true });
            $('#ddlCountries').on('change', function () {
                $('#<%=hfSelected.ClientID%>').val($(this).val());
            });
        });
    </script>
</div>

Namespaces

C#

using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

VB.Net

Imports System.Collections.Generic
Imports System.Configuration
Imports System.Data
Imports System.Data.SqlClient

Code

C#

protected void Country_Changed(object sender, EventArgs e)
{
    string query = "SELECT ContactName FROM Customers";

    string selectedCountries = hfSelected.Value;
    string condition = string.Empty;
    for (int i = 0; i < selectedCountries.Split(',').Length; i++)
    {
        condition += string.Format("'{0}',", selectedCountries.Split(',')[i]);
    }

    if (!string.IsNullOrEmpty(condition))
    {
        condition = string.Format(" WHERE Country IN ({0})", condition.Substring(0, condition.Length - 1));
    }

    DataTable dt = GetDataTable(query + condition);
    List<string> names = new List<string>();
    foreach (DataRow dr in dt.Rows)
    {
        names.Add(dr["ContactName"].ToString());
    }

    txtDetails.Text = string.Join(" ", names.ToArray());
}

private DataTable GetDataTable(string query)
{
    string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(conString))
    {
        using (SqlCommand cmd = new SqlCommand(query))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
            {
                cmd.Connection = con;
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);

                    return dt;
                }
            }
        }
    }
}

VB.Net

    Protected Sub Country_Changed(ByVal sender As Object, ByVal e As EventArgs)
        Dim query As String = "SELECT ContactName FROM Customers"
        Dim selectedCountries As String = hfSelected.Value
        Dim condition As String = String.Empty

        For i As Integer = 0 To selectedCountries.Split(","c).Length - 1
            condition += String.Format("'{0}',", selectedCountries.Split(","c)(i))
        Next

        If Not String.IsNullOrEmpty(condition) Then
            condition = String.Format(" WHERE Country IN ({0})", condition.Substring(0, condition.Length - 1))
        End If

        Dim dt As DataTable = GetDataTable(query & condition)
        Dim names As List(Of String) = New List(Of String)()

        For Each dr As DataRow In dt.Rows
            names.Add(dr("ContactName").ToString())
        Next

        txtDetails.Text = String.Join(" ", names.ToArray())
    End Sub

    Private Function GetDataTable(ByVal query As String) As DataTable
        Dim conString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
        Using con As SqlConnection = New SqlConnection(conString)

            Using cmd As SqlCommand = New SqlCommand(query)
                Using sda As SqlDataAdapter = New SqlDataAdapter(cmd)
                    cmd.Connection = con
                    Using dt As DataTable = New DataTable()
                        sda.Fill(dt)
                        Return dt
                    End Using
                End Using
            End Using
        End Using
    End Function