Add items to existing ASP.Net DropDownList on Button Click using C# and VB.Net

Hi landomarossi,

Please refer below sample.

HTML

<asp:TextBox ID="txtFruits" runat="server"></asp:TextBox>
<asp:Button ID="btnAdd" runat="server" Text="Add Item" OnClick="AddItem" /><br />
<asp:DropDownList ID="ddlFruits" runat="server">
</asp:DropDownList>

Namespace

C#

using System.Data;

VB.Net

Imports System.Data

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[1] { new DataColumn("Items") });
        dt.Rows.Add("Orange");
        dt.Rows.Add("Banana");
        dt.Rows.Add("Apple");
        ddlFruits.DataTextField = "Items";
        ddlFruits.DataSource = dt;
        ddlFruits.DataBind();
    }
}

protected void AddItem(object sender, EventArgs e)
{
    string fruit = txtFruits.Text.Trim();
    txtFruits.Text = string.Empty;
    if (!string.IsNullOrEmpty(fruit))
    {
        ddlFruits.Items.Add(new ListItem(fruit, fruit));
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim dt As DataTable = New DataTable()
        dt.Columns.AddRange(New DataColumn(0) {New DataColumn("Items")})
        dt.Rows.Add("Orange")
        dt.Rows.Add("Banana")
        dt.Rows.Add("Apple")
        ddlFruits.DataTextField = "Items"
        ddlFruits.DataSource = dt
        ddlFruits.DataBind()
    End If
End Sub

Protected Sub AddItem(ByVal sender As Object, ByVal e As EventArgs)
    Dim fruit As String = txtFruits.Text.Trim()
    txtFruits.Text = String.Empty
    If Not String.IsNullOrEmpty(fruit) Then
        ddlFruits.Items.Add(New ListItem(fruit, fruit))
    End If
End Sub

Screenshot