CompareValidator to validate Start Date End Date using C# and VB.Net in ASP.Net

Hi Varsha7899,

Refer below sample.

HTML

<div>
    <div class="form-group">
        <b>
            <asp:Label ID="lbl_sdate" runat="server" CssClass="control-label col-sm-2 text-right">Start Date:</asp:Label></b>
        <div class="col-sm-4">
            <asp:TextBox ValidationGroup="diff_graph" ID="txt_StartDate" CssClass="form-control"
                runat="server"></asp:TextBox>
        </div>
        <div class="col-sm-3">
            <asp:RequiredFieldValidator ValidationGroup="diff_graph" Display="Dynamic" ID="rf_sdate"
                runat="server" ControlToValidate="txt_StartDate" Text="*" ForeColor="Red" ErrorMessage="RequiredFieldValidator"></asp:RequiredFieldValidator>
        </div>
    </div>
</div>
<div class="row">
    <div class="form-group">
        <asp:Label ID="lbl_edate" runat="server" CssClass="control-label col-sm-2 text-right"><b>End Date:</b></asp:Label></br>
        <div class="col-sm-4">
            <asp:TextBox ID="txt_EndDate" ValidationGroup="diff_graph" CssClass="form-control"
                runat="server" AutoPostBack="true" OnTextChanged="change"></asp:TextBox>
        </div>
        <div class="col-sm-3">
            <asp:RequiredFieldValidator ValidationGroup="diff_graph" Display="Dynamic" ID="rf_edate"
                runat="server" ControlToValidate="txt_EndDate" Text="*" ForeColor="Red" ErrorMessage="RequiredFieldValidator"></asp:RequiredFieldValidator>
            <asp:CustomValidator ID="CustomValidator1" OnServerValidate="CustomValidator1_ServerValidate"
                ErrorMessage="" runat="server" Display="None"></asp:CustomValidator>
            <asp:CompareValidator ID="cmpv" runat="server" ControlToCompare="txt_StartDate" ControlToValidate="txt_EndDate"
                Display="Dynamic" ErrorMessage=""></asp:CompareValidator>
        </div>
    </div>
</div>

Code

C#

protected void change(object sender, EventArgs e)
{
    Page.Validate();
    if (Page.IsValid)
    {
        //processing done after a successful submit here!
    }
}

protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
{
    DateTime dt1 = Convert.ToDateTime(txt_StartDate.Text);
    DateTime dt2 = Convert.ToDateTime(txt_EndDate.Text);
    if (dt2 > dt1.AddYears(1))
    {
        cmpv.ErrorMessage = "";
    }
    else
    {
        cmpv.ErrorMessage = "End date should be greater than 1 year !";
    }
}

VB.Net

    Protected Sub change(ByVal sender As Object, ByVal e As EventArgs)
        Page.Validate()

        If Page.IsValid Then
        End If
    End Sub

    Protected Sub CustomValidator1_ServerValidate(ByVal source As Object, ByVal args As ServerValidateEventArgs)
        Dim dt1 As DateTime = Convert.ToDateTime(txt_StartDate.Text)
        Dim dt2 As DateTime = Convert.ToDateTime(txt_EndDate.Text)
        If dt2 > dt1.AddYears(1) Then
            cmpv.ErrorMessage = ""
        Else
            cmpv.ErrorMessage = "End date should be greater than 1 year !"
        End If
    End Sub

Screenshot