[Solved] Prevent Bootstrap Modal Close if Validation (fails) error exist in ASP.Net

Hi Mehram,

Please refer below sample.

You have to set ValidationGroup="btnSave" on Save button.

HTML

<center>
    <asp:Button ID="btnShowPopup" runat="server" Text="Show Popup" OnClick="ShowPopup"
        CssClass="btn btn-info btn-lg" />
</center>
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true" data-backdrop="static" data-keyboard="false">
    <div class="modal-dialog modal-dialog-centered">
        <div class="modal-dialog modal-dialog-centered" role="document">
            <!-- Modal content-->
            <div class="modal-content">
                <div class="modal-header">
                    <button type="button" class="close" data-dismiss="modal">&times;</button>
                    <h4 class="modal-title">Tax Voucher</h4>
                </div>
                <div class="modal-body">
                    <div class="box-body table-responsive">
                        <div class="row" id="divSearch" runat="server" visible="true">
                            <div class="col-xs-12">
                                <asp:Label ID="lblTaxMessage" runat="server" Style="display: none;"></asp:Label>
                                <table class="table table-bordered" style="width: 100%">
                                    <tbody>
                                        <tr>
                                            <td style="vertical-align: middle; width: 200px;">ID</td>
                                            <td><asp:Label ID="lblTaxID" runat="server" Text="" Enabled="false"></asp:Label></td>
                                        </tr>
                                        <tr>
                                            <td style="vertical-align: middle">Tax Apply
                                                    <asp:RequiredFieldValidator ID="RequiredFieldValidator19" runat="server"
                                                        ControlToValidate="cboTaxApply" Display="Dynamic" InitialValue="0"
                                                        ErrorMessage="*" ValidationGroup="btnSave">
                                                    </asp:RequiredFieldValidator>
                                            </td>
                                            <td><asp:DropDownList ID="cboTaxApply" Width="150px" runat="server" CssClass="chzn-select img-rounded " Style="border: 1px solid #000; color: black;" AutoPostBack="false">
                                                    <asp:ListItem Text="Select" Selected="True" Value="0"></asp:ListItem>
                                                    <asp:ListItem Text="1%" Value="1"></asp:ListItem>
                                                    <asp:ListItem Text="2%" Value="2"></asp:ListItem>
                                                    <asp:ListItem Text="3%" Value="3"></asp:ListItem>
                                                    <asp:ListItem Text="4.5%" Value="4.5"></asp:ListItem>
                                                    <asp:ListItem Text="17%" Value="17"></asp:ListItem>
                                                </asp:DropDownList>
                                            </td>
                                        </tr>
                                    </tbody>
                                </table>
                            </div>
                        </div>
                    </div>
                </div>
                <div class="modal-footer">
                    <asp:Button ID="btnSave" Width="100px" runat="server" Text="Save" CssClass="btn btn-primary" ValidationGroup="btnSave" />
                    <button type="button" class="btn btn-primary" style="width: 100px;" data-dismiss="modal">Close</button>
                </div>
            </div>
        </div>
    </div>
</div>

jQuery

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script type="text/javascript" src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/js/bootstrap.min.js"></script>
<script type="text/javascript">
    function ShowPopup() {
        $("#myModal").modal("show");
    }
</script>

Code

C#

protected void ShowPopup(object sender, EventArgs e)
{
    ClientScript.RegisterStartupScript(this.GetType(), "Popup", "ShowPopup();", true);
}

VB.Net

Protected Sub ShowPopup(ByVal sender As Object, ByVal e As EventArgs)
    ClientScript.RegisterStartupScript(Me.GetType(), "Popup", "ShowPopup();", True)
End Sub

Screenshot