dropdownlist selectedValue problem

<asp:DropDownList ID="dropdownlist_name"
selectedValue='<%# DataBinder.Eval(Container.DataItem,"field_name").ToString() %>' DataTextField='<%# DataBinder.Eval(Container.DataItem,"field_name").ToString() %>' DataValueField='<%# DataBinder.Eval(Container.DataItem,"field_name").ToString() %>'    runat="server">
    <asp:ListItem Value="A">A</asp:ListItem>
    <asp:ListItem Value="B">B</asp:ListItem>
    </asp:DropDownList>

code behind:
string selected= ((DropDownList)RepeaterID.Items[i].FindControl("dropdownlist_name")).SelectedValue;
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why i can get the selectedValue "B"?? i always return value "A"
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