Prevent Bootstrap Collapse toggle after postback

Question

I am using bootstrap collapse in my project,in that collapse I have some buttons and dropdowns but when I click on any button or change dropdown index, postback occurs and collapse got uncollapsed,How can I stop this ? Here is my Code

 <h4><a href="#" data-toggle="collapse" data-target="#div_search" style="font-weight: bold;">Search Training Profile</a><br />
                </h4>
                <div id="div_search" class="collapse" style="overflow-x: auto;">
                    <table class="table table-bordered table-striped">
                        <tr>
                            <td>Employee Name</td>
                            <td>
                                <asp:TextBox runat="server" ID="txt_name"></asp:TextBox>
                                <span class="err">optional</span>

                            </td>
                            <td>e.g First Name, Middle Name, Last Name</td>
                        </tr>
                        <tr>
                            <td>Designation</td>
                            <td><span class="err"></span>
                                <asp:DropDownList runat="server" ID="DDL_Desig" OnSelectedIndexChanged="DDL_Desig_SelectedIndexChanged" AutoPostBack="true">
                                </asp:DropDownList>
                                <span class="err">optional</span>
                            </td>
                            <td>
                                 <asp:TextBox runat="server" ID="txt_desig" ReadOnly="true"></asp:TextBox>
                                <asp:Button Text="Clear" runat="server" />
                            </td>
                        </tr>
                        <tr>
                            <td>Location</td>
                            <td>
                                 <asp:DropDownList runat="server" ID="DDL_Loc" OnSelectedIndexChanged="DDL_Loc_SelectedIndexChanged" AutoPostBack="true">
                                    <asp:ListItem>Select</asp:ListItem>
                                </asp:DropDownList>
                                <span class="err">optional</span>
                                </td>

                            <td>
                                 <asp:TextBox runat="server" ID="txt_loc" ReadOnly="true"></asp:TextBox>
                                <asp:Button Text="Clear" runat="server" />
                            </td>
                        </tr>
                        <tr>
                            <td>Division</td>
                            <td>
                                 <asp:DropDownList runat="server" ID="DDL_Divis" OnSelectedIndexChanged="DDL_Divis_SelectedIndexChanged" AutoPostBack="true">
                                </asp:DropDownList>
                                <span class="err"> optional </span>

                            </td>
                            <td>
                                  <asp:TextBox runat="server" ID="txt_divis" ReadOnly="true"></asp:TextBox>
                                <asp:Button Text="Clear" runat="server" />
                                </td>
                        </tr>
                        <tr>
                            <td>Department</td>
                            <td>
                                <asp:TextBox runat="server" ID="tx"></asp:TextBox>
                                <span class="err">optional</span></td>
                            <td>e.g ISD, MKT, HR etc</td>
                        </tr>
                        <tr>
                            <td>Filter By</td>
                            <td>
                                <asp:DropDownList runat="server" ID="DDL_Assc">
                                </asp:DropDownList>
                            </td>
                            <td>e.g OLP, SOLC, MAF, etc</td>
                        </tr>
                        <tr>

                            <td colspan="3">&nbsp;</td>
                        </tr>
                        <tr>
                            <td colspan="3" style="text-align:center;">
                                <asp:Button Text="Search" runat="server" CssClass="btn" />
                                <asp:Button Text="Reset" runat="server"  CssClass="btn" />
                            </td>
                        </tr>
                    </table>

                </div>

Answer 1

Create public variable

string state = "collapse";

during the postback or dropdown changed set the value as

state = "expand";

and aspx page use this as below:-

<div id="div_search" class='<%= state %>' style="overflow-x: auto;">

Answer 2

Aditya's answer helped me initially and is a nice solution. As my code evolved wrapping the div inside an update panel and having the collapse div contain a couple calendar controls that cause postbacks to refresh data in a grid, I came up with another solution I thought worth sharing as it is handled server side only.

<asp:Panel ID="pnlDateRange" ClientIDMode="Static" runat="server" CssClass="collapse col-xs-12 col-sm-12 col-md-12 col-lg-12">
    <div id="divFrom" class="col-xs-12 col-sm-12 col-md-2 col-md-offset-4 col-lg-2 col-lg-offset-4">
     <asp:Calendar ID="cStartDate" runat="server" CssClass="calendar" Font-Size="X-Small"
                                DayNameFormat="FirstTwoLetters" DayHeaderStyle-Font-Bold="false"
                                TitleStyle-Font-Bold="true" TitleStyle-Font-Size="Small" Caption="From"
                                OnSelectionChanged="cStartDate_SelectionChanged">
                                <SelectedDayStyle Font-Bold="true" />
                                <OtherMonthDayStyle ForeColor="SlateGray" />
                            </asp:Calendar>
                        </div>
      <div id="divTo" class="col-xs-12 col-sm-12 col-md-2 col-lg-2">
          <asp:Calendar ID="cEndDate" runat="server" CssClass="calendar" Font-Size="X-Small"
                                DayNameFormat="FirstTwoLetters" DayHeaderStyle-Font-Bold="false"
                                TitleStyle-Font-Bold="true" TitleStyle-Font-Size="Small" Caption="To"
                                OnSelectionChanged="cEndDate_SelectionChanged">
                                <SelectedDayStyle Font-Bold="true" />
                                <OtherMonthDayStyle ForeColor="SlateGray" />
               </asp:Calendar>
            </div>
       </asp:Panel>

The control was originally an ordinary with the same classes applied, it is the collapse target. In the c# on server side I have defined a private boolean variable set to true in all cases when the code loads to run. In the OnPreRender event I do this:

if (_collapseRange)
{
    pnlDateRange.CssClass = pnlDateRange.CssClass.Replace("collapse in", "collapse");
}
else
{
    if (!pnlDateRange.CssClass.Contains("collapse in"))
    {
        pnlDateRange.CssClass = pnlDateRange.CssClass.Replace("collapse", "collapse in");
    }
}