DropDownList in fixed

Question

I have some dropdownlists in a particular screen and whenever I get the value of the same, always comes the first value. What do I do to solve this, the dropdownlist in other pages work correctly and can select the value of them, this can not.

I'm setting the values ​​of this dynamically, every time I go up a file in the server folder, I upgraded, filling it with a vector that has the name (in string) of all items in the directory. When I select a different value than what comes standard on it, quyando'll get to play her value in the database, is as if I had not selected anything. I've tried to solve autoPorsback true and false, and nothing in the page_load. How do I fix this? I have no errors in the code.

NOTE: The drop down list is loaded EVERY TIME I climb a new file, as I do not have it in the IsPostBack. Every time I climb a new file, I have a method MostraImagensCarrefadas() that updates and populates the dropdownlist. This method gets the event of a button. I am not using update panel and nothing of the sort.

all my code for the component:

 //aspx header
<%@ Page Title="" Language="C#" MasterPageFile="~/administrativo/MasterPage.master"    EnableViewState="false" AutoEventWireup="true" CodeFile="cadastroImovel.aspx.cs" Inherits="administrativo_cadastroImovel" %>

//dropdownlist in aspx
<div class="span2">
<div>
<asp:DropDownList ID="ddlImgPrincipal" runat="server" Width="160px" AutoPostBack="false" OnSelectedIndexChanged="ddlImgPrincipal_SelectedIndexChanged">
</asp:DropDownList>
 </div>
</div>

// Method page_load
protected void Page_Load(object sender, EventArgs e)
{

    if (!IsPostBack)
    {

    }
}


//where I try to get the value of the dropdownlist
protected void btSalvar_Click(object sender, EventArgs e)
{
    eImoveis imovel = new eImoveis();

    imovel.area_total = txtArea.Text;
    imovel.bairro = txtBairro.Text;
    imovel.banheiro = txtBanheiro.Text;
    imovel.cep = txtCep.Text;
    imovel.cidade = ((System.Web.UI.WebControls.ListControl)     (ddlCidade)).SelectedValue.ToString();
    imovel.codcliente = ((System.Web.UI.WebControls.ListControl)    (ddlProprietario)).SelectedValue.ToString();
    imovel.codtipoimovel = ((System.Web.UI.WebControls.ListControl)    (ddlCategoriaImovel)).SelectedValue.ToString();
    imovel.complemento = txtComplemento.Text;
    imovel.descricao = txtDescricao.Text;
    imovel.garagem = txtVagasGaragem.Text;
    imovel.nome = txtNome.Text;
    imovel.numero = txtNúmero.Text;
    imovel.quarto = txtQuartos.Text;
    imovel.referencia = txtReferencia.Text;
    imovel.rua = txtRua.Text;
    //imovel.suite = sui
    imovel.valor = txtValor.Text;

    if (manImovel.Insere(imovel))
    {
        eImagens imagem = new eImagens();
        manImagem manImg = new manImagem();

        List<String> files = new manFile().getListFilesForDirectory(this, MAE.DIRETORIO_TEMP_IMG);

        DataTable dt = manImovel.getLastID(imovel);

        DataRow row = dt.NewRow();

        String codigo = dt.Rows[0].ItemArray[0].ToString();

        int tamanho = 0;            

        if (files != null)
        {
            foreach (String str in files)
            {
                imagem.cdImovel = codigo;
                imagem.imagem = str;


                if (ddlImgPrincipal.SelectedItem != null)
                {
                    if (ddlImgPrincipal.SelectedItem.ToString() == str)
                    {
                        imagem.imagemPrincipal = "SIM";

                    }
                }


                if (manImg.Insere(imagem))
                {
                    tamanho++;
                }

                //tamanho++;
            }
        }

        if (tamanho == files.Count)
        {
            if (new manFile().CopiaTodosOsArquivos(this, MAE.DIRETORIO_TEMP_IMG,     MAE.DIRETORIO_IMG))
            {
                if (new manFile().DeletaTodosArquivos(this, MAE.DIRETORIO_TEMP_IMG))
                {
                    MontaLiteralCarrossel();

                }
            }
        }
    }
}
 //Here I fill the dropdownlist
public void MostraImagensCarrefadas()
{
    List<String> files = new manFile().getListFilesForDirectory(this, MAE.DIRETORIO_TEMP_IMG);

    ddlImgPrincipal.Items.Clear();

    if (files != null)
    {
        foreach (String item in files)
        {
            ddlImgPrincipal.Items.Add(new ListItem(item));
        }
    }
}