Disable (Prevent) current month selection in ASP.Net DropDownList using C# and VB.Net

Hi comunidadmexi,

Please refer below Sample.

HTML

Month Names:
<asp:DropDownList ID="ddlMonthNames" runat="server" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged">
    <asp:ListItem Value="" Selected="True"></asp:ListItem>
</asp:DropDownList>

Namespace

C#

using System.Globalization;

VB.Net

Imports System.Globalization

Code

C#

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

private void mtmonthbind()
{
    DateTimeFormatInfo info = 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)
            ddlMonthNames.Items.Add(new ListItem(info.GetMonthName(i), i.ToString()));
    }
}

protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
    int currentMonth = DateTime.Now.Month;
    if (currentMonth == int.Parse(ddlMonthNames.SelectedValue))
    {
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You can not select current month.');", true);
    }

}

VB.Net

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

Private Sub mtmonthbind()
    Dim info As DateTimeFormatInfo = 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 ddlMonthNames.Items.Add(New ListItem(info.GetMonthName(i), i.ToString()))
    Next
End Sub

Protected Sub OnSelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim currentMonth As Integer = DateTime.Now.Month

    If currentMonth = Integer.Parse(ddlMonthNames.SelectedValue) Then
        ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('You can not select current month.');", True)
    End If
End Sub

Screenshot