Validate ASP.Net DropDownList selected month with current month using C#

Hi comunidadmexi...,

Refer below code.

HTML

<asp:DropDownList runat="server" ID="ddlmonth" AutoPostBack="true" OnSelectedIndexChanged="ddlmonth_SelectedIndexChanged">
</asp:DropDownList>

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        mtmonthbind();
    }
}

private void mtmonthbind()
{
    System.Globalization.DateTimeFormatInfo info = System.Globalization.DateTimeFormatInfo.GetInstance(null);
    int currentMonth = DateTime.Now.Month;

    for (int i = 1; i < 13; i++)
    {
        bool isMonthInPast = ((i + 1) < currentMonth) || (i + 1 == currentMonth && DateTime.Now.Day > 20);

        if (!isMonthInPast)
            ddlmonth.Items.Add(new ListItem(info.GetMonthName(i), i.ToString()));
    }
}

protected void ddlmonth_SelectedIndexChanged(object sender, EventArgs e)
{
    DateTime currentDate = DateTime.Now.AddMonths(2);
    string month = currentDate.ToString("MMMM");
    DateTime dt = new DateTime(1, Int32.Parse(ddlmonth.SelectedValue), 1);
    if (currentDate.Month <= Convert.ToInt32(ddlmonth.SelectedValue))
    {
        Page.ClientScript.RegisterStartupScript(this.GetType(), "Msg", "alert('Stop! The selected month " + dt.ToString("MMMM").ToUpper()
            + " is different from the month of " + month.ToString().ToUpper() + "');", true);
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not IsPostBack Then
        mtmonthbind()
    End If
End Sub

Private Sub mtmonthbind()
    Dim info As System.Globalization.DateTimeFormatInfo = System.Globalization.DateTimeFormatInfo.GetInstance(Nothing)
    Dim currentMonth As Integer = DateTime.Now.Month

    For i As Integer = 1 To 13 - 1
        Dim isMonthInPast As Boolean = ((i + 1) < currentMonth) OrElse (i + 1 = currentMonth AndAlso DateTime.Now.Day > 20)
        If Not isMonthInPast Then ddlmonth.Items.Add(New ListItem(info.GetMonthName(i), i.ToString()))
    Next
End Sub

Protected Sub ddlmonth_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim currentDate As DateTime = DateTime.Now.AddMonths(2)
    Dim month As String = currentDate.ToString("MMMM")
    Dim dt As DateTime = New DateTime(1, Int32.Parse(ddlmonth.SelectedValue), 1)

    If currentDate.Month <= Convert.ToInt32(ddlmonth.SelectedValue) Then
        Page.ClientScript.RegisterStartupScript(Me.GetType(), "Msg", "alert('Stop! The selected month " & dt.ToString("MMMM").ToUpper() & " is different from the month of " & month.ToString().ToUpper() & "');", True)
    End If
End Sub