Access DropDownList inside class file using C# and VB.Net in ASP.Net

Hi AliYilmaz,

Create method and pass the control to the class file to access all properties.

Refer below sample code.

HTML 

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
    <asp:Panel ID="Panel1" runat="server" class="sayfaBaslik">
        <div class="col-md-4">
            Ders :
            <asp:DropDownList ID="ddlDers" runat="Server" OnSelectedIndexChanged="ddlDers_SelectedIndexChanged"
                AutoPostBack="true">
                <asp:ListItem Text="Select" Value="0"></asp:ListItem>
                <asp:ListItem Text="Der 1" Value="1"></asp:ListItem>
                <asp:ListItem Text="Der 2" Value="2"></asp:ListItem>
                <asp:ListItem Text="Der 3" Value="3"></asp:ListItem>
                <asp:ListItem Text="Der 4" Value="4"></asp:ListItem>
            </asp:DropDownList>
        </div>
    </asp:Panel>
</asp:Content>

Code

C#

protected void ddlDers_SelectedIndexChanged(object sender, EventArgs e)
{
    Data data = new Data();
    string selectedValue = data.GetSelectedValue(ddlDers);
    ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('Selected Value : " + selectedValue + "')", true);
}

VB.Net

Protected Sub ddlDers_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim data As Data = New Data()
    Dim selectedValue As String = data.GetSelectedValue(ddlDers)
    ClientScript.RegisterClientScriptBlock(Me.GetType(), "", "alert('Selected Value : " & selectedValue & "')", True)
End Sub

Data Class

C#

using System.Web.UI;
using System.Web.UI.WebControls;

/// <summary>
/// Summary description for Data
/// </summary>
public class Data
{
    public Data()
    {
        //
        // TODO: Add constructor logic here
        //
    }

    public string GetSelectedValue(Control ctrl)
    {
        DropDownList ddl = ctrl as DropDownList;
        string ders = ddl.SelectedItem.Value;
        return ders;
    }
}

VB.Net

Imports Microsoft.VisualBasic

Public Class Data
    Public Function GetSelectedValue(ByVal ctrl As Control) As String
        Dim ddl As DropDownList = TryCast(ctrl, DropDownList)
        Dim ders As String = ddl.SelectedItem.Value
        Return ders
    End Function
End Class

Screenshot

Note: Rename name Data class with different name as .Net we have already Data class present in System namespace. So it might be conflict while calling on page.