C# 使加载屏幕与选择索引更改同时出现

我想做一个加载屏幕，但是当我触发selectedindexchanged时，代码同时执行，所以它只是加载了很长一段时间，我想做的是选择下拉菜单的选项，然后在页面加载时通过使封面屏幕可见来显示加载屏幕，我尝试了一些不同的东西，包括javascript和System.Threading.Thread.Sleep22；但是运气不好，提前谢谢你抽出时间

<div id="loading">
        <div id="coverScreen" class="LockOn">
        </div>
    </div>
<asp:DropDownList ID="DropDown" DataValueField="IDCoContratante" DataTextField="Designation" runat="server" Enabled="true" CssClass="dropdownCocont" Style="border-radius: 7px; border-style: none;" AutoPostBack="true" OnSelectedIndexChanged="DropDown_ListarPropostas_SelectedIndexChanged"></asp:DropDownList>



protected void DropDown_ListarPropostas_SelectedIndexChanged(object sender, EventArgs e)
        {

            //Response.Write("<script>document.getElementById('coverScreen').style.visibility = 'visible';</script>");
            coverScreen.Visible = true;




            System.Threading.Thread.Sleep(22);

            ClassConn c = new ClassConn();
            c.OpenConection();

            DataTable dt = new DataTable();


            //SqlDataAdapter AtualizaDropLotes = new SqlDataAdapter(" SELECT dbo.TblLoteCoContratanteAdjudicado.IDLote, CAST(dbo.TblLote.NrLote AS varchar) + '  ' + dbo.TblLote.DescricaoLote AS DescricaoLote, dbo.TblLote.IDAcordoQuadro " +
            //                                                      " FROM dbo.TblLoteCoContratanteAdjudicado INNER JOIN dbo.TblLote ON dbo.TblLoteCoContratanteAdjudicado.IDLote = dbo.TblLote.IDLote AND dbo.TblLoteCoContratanteAdjudicado.IDLote = dbo.TblLote.IDLote " +
            //                                                      " WHERE (dbo.TblLote.IDAcordoQuadro = '" + DropDownAcordosQuadros_ListarPropostas.SelectedValue + "') " +
            //                                                      " GROUP BY dbo.TblLoteCoContratanteAdjudicado.IDLote, CAST(dbo.TblLote.NrLote AS varchar) + '  ' + dbo.TblLote.DescricaoLote, dbo.TblLote.IDAcordoQuadro", c.con);
            string _query = "SELECT '%' as IDLote, 'Todos' as DescricaoLote UNION ALL SELECT CAST(dbo.TblLoteCoContratanteAdjudicado.IDLote AS VARCHAR), 'Nº' + CAST(dbo.TblLote.NrLote AS varchar) + ' - ' + dbo.TblLote.DescricaoLote AS DescricaoLote FROM dbo.TblLoteCoContratanteAdjudicado INNER JOIN dbo.TblLote ON dbo.TblLoteCoContratanteAdjudicado.IDLote = dbo.TblLote.IDLote AND dbo.TblLoteCoContratanteAdjudicado.IDLote = dbo.TblLote.IDLote WHERE(dbo.TblLoteCoContratanteAdjudicado.NIFCoContratante = '" + id_Vat + "') AND(dbo.TblLote.IDAcordoQuadro = '" + DropDownAcordosQuadros_ListarPropostas.SelectedValue + "')";

            SqlDataAdapter AtualizaDropLotes = new SqlDataAdapter(_query, c.con);

            AtualizaDropLotes.Fill(dt);

            if (dt != null)
            {
                DropDownLotes_ListaPropostas.DataSource = dt;
                DropDownLotes_ListaPropostas.DataBind();


            }

            c.CloseConnection();
            coverScreen.Visible = false;
        }

---

根据Alex Kudryashev和Gus给我的信息，我能够找到解决方案。我所做的是解决问题：

我在asp:UpdatePanel之间插入了加载屏幕，上面是asp:ScriptManager

<asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>

        <asp:UpdateProgress ID="UpdateProgress1" runat="server">
            <ProgressTemplate>
                <div id="coverScreen" class="LockOn" runat="server">
                </div>
            </ProgressTemplate>
        </asp:UpdateProgress>

我在主div中插入了以下代码

<div id="Main">
 <asp:UpdatePanel ID="UpdatePanel2" runat="server">
            <ContentTemplate>

//your code

    </ContentTemplate>
  </asp:UpdatePanel>
</div>

---

根据Alex Kudryashev和Gus给我的信息，我能够找到解决方案。我所做的是解决问题：

我在asp:UpdatePanel之间插入了加载屏幕，上面是asp:ScriptManager

<asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>

        <asp:UpdateProgress ID="UpdateProgress1" runat="server">
            <ProgressTemplate>
                <div id="coverScreen" class="LockOn" runat="server">
                </div>
            </ProgressTemplate>
        </asp:UpdateProgress>

我在主div中插入了以下代码

<div id="Main">
 <asp:UpdatePanel ID="UpdatePanel2" runat="server">
            <ContentTemplate>

//your code

    </ContentTemplate>
  </asp:UpdatePanel>
</div>

---

一般来说，我希望您的加载屏幕在客户端用javascript实现。这个问题可能很有用。asp:UpdatePanel和asp:UpdateProgress就是为此而创建的。一般来说，我希望您的加载屏幕能够在客户端用javascript实现。这个问题可能有用。asp:UpdatePanel和asp:UpdateProgress就是为此而创建的。