ASP.NET

namespace mountainbrook.UI
{

    partial class VendorSelection : BaseApplicationPage
    {

        public VendorSelection()
        {
            AddSpecialEventHookUps();
            this.Init += new System.EventHandler(My_Init);
        }
        private void My_Init(object sender, System.EventArgs e)
        {
        }
        public string GetVendorSelection()
        {
            System.Web.UI.WebControls.DropDownList VendorSelectionDropDownList = new System.Web.UI.WebControls.DropDownList();
            VendorSelectionDropDownList = this.Page.FindControl("VendorSelectionDropDownList") as System.Web.UI.WebControls.DropDownList;

            return VendorSelectionDropDownList.Text;
        }
    }
}

public class SomeStuff
{
    public static void DoSomethingWithTheVendor()
    {
      ... get the currently selected vendor from the drop down list...
      ... do something with it...
    }
}

public class SomeStuff
{
    public static void DoSomethingWithTheVendor(string vendorId)
    {
      ... do something with the vendor, given the passed-in vendorId...
    }
}

void MyButton_ClickEventHandler(object o, EventArgs e)
{
    // get the currently selected vendor
    string vendorId = myVendorDropDownList.SelectedValue();

    // call an App_Code function to operate on the selected vendor
    SomeStuff.DoSomethingWithTheVendor(vendorId);
}