Remove DropDownList selected item on Button Click using C# and VB.Net in ASP.Net

Hi KatieNgoc,

Refer below sample.

HTML

<asp:DropDownList ID="DropDownList1" runat="server" >
</asp:DropDownList>
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />

Namespaces

C#

using System.Data;

VB.Net

Imports System.Data

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        if (Session["ListItems"] == null)
        {
            DataTable dt = DropDownItems();
            DropDownList1.DataSource = dt;
            DropDownList1.DataTextField = "Text";
            DropDownList1.DataValueField = "Value";
            DropDownList1.DataBind();
            Session["ListItems"] = dt;
        }
    }
}

private DataTable DropDownItems()
{
    DataTable dt = new DataTable();
    dt.Columns.Add("Text");
    dt.Columns.Add("Value");
    dt.Rows.Add("Item1", "1");
    dt.Rows.Add("Item2", "2");
    dt.Rows.Add("Item3", "3");
    dt.Rows.Add("Item4", "4");
    return dt;
}

protected void Button1_Click(object sender, EventArgs e)
{
    DataTable dt = Session["ListItems"] as DataTable;
    ListItemCollection liCol = DropDownList1.Items;
    for (int i = 0; i < dt.Rows.Count; i++)
    {
        ListItem li = liCol[i];
        if (li.Selected)
        {
            dt.Rows[i].Delete();
        }
    }
    Session["ListItems"] = dt;
    DropDownList1.DataSource = dt;
    DropDownList1.DataTextField = "Text";
    DropDownList1.DataValueField = "Value";
    DropDownList1.DataBind();
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then

        If Session("ListItems") Is Nothing Then
            Dim dt As DataTable = DropDownItems()
            DropDownList1.DataSource = dt
            DropDownList1.DataTextField = "Text"
            DropDownList1.DataValueField = "Value"
            DropDownList1.DataBind()
            Session("ListItems") = dt
        End If
    End If
End Sub

Private Function DropDownItems() As DataTable
    Dim dt As DataTable = New DataTable()
    dt.Columns.Add("Text")
    dt.Columns.Add("Value")
    dt.Rows.Add("Item1", "1")
    dt.Rows.Add("Item2", "2")
    dt.Rows.Add("Item3", "3")
    dt.Rows.Add("Item4", "4")
    Return dt
End Function

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)
    Dim dt As DataTable = TryCast(Session("ListItems"), DataTable)
    Dim liCol As ListItemCollection = DropDownList1.Items

    For i As Integer = 0 To dt.Rows.Count - 1
        Dim li As ListItem = liCol(i)

        If li.Selected Then
            dt.Rows(i).Delete()
        End If
    Next

    Session("ListItems") = dt
    DropDownList1.DataSource = dt
    DropDownList1.DataTextField = "Text"
    DropDownList1.DataValueField = "Value"
    DropDownList1.DataBind()
End Sub

Sreenshot