Two way databinding in ASP.NET

Sadly to say that, but ASP.NET does not support two-way binding to .net objects... instead you can use something like "manual binding" on every post back (here AddIncomeSources is RepeaterControl)

public List<Income> AdditionalIncomeList 
{
    get { return ViewState["AdditionalIncome"] as List<Income>; }
    set { ViewState["AdditionalIncome"] = value; }
} 

foreach (RepeaterItem item in AddIncomeSources.Items)
{
    var amount = (TextBox)item.Controls.Cast<Control>().First(c => c.ID == "Amount");
    var document = (DropDownList)item.Controls.Cast<Control>().First(c => c.ID == "Document");
    AdditionalIncomeList[item.ItemIndex].Amount = amount.Text.ToDouble();
    AdditionalIncomeList[item.ItemIndex].IncomeDocument = document.SelectedValue;
}

AddIncomeSources.DataSource = AdditionalIncomeList;
AddIncomeSources.DataBind();