Validate selected Time with current Time using C# and VB.Net in ASP.Net

Hi nauna,

Refer below code.

HTML

<asp:DropDownList runat="server" ID="ddlTimes" AppendDataBoundItems="true">
    <asp:ListItem Text="Select" Value="0" />
</asp:DropDownList>
<asp:Button Text="Appoint" runat="server" OnClick="OnAppoint" />

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        List<ListItem> times = GetTime();
        ddlTimes.DataSource = times;
        ddlTimes.DataTextField = "Text";
        ddlTimes.DataValueField = "Value";
        ddlTimes.DataBind();
    }
}

private List<ListItem> GetTime()
{
    List<ListItem> times = new List<ListItem>();

    TimeSpan timeSpan = new TimeSpan(08, 00, 00);
    string time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt");
    times.Add(new ListItem { Text = "8:00AM To 10:00AM", Value = time });

    timeSpan = new TimeSpan(10, 00, 00);
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt");
    times.Add(new ListItem { Text = "10:00AM To 12:00PM", Value = time });

    timeSpan = new TimeSpan(12, 00, 00);
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt");
    times.Add(new ListItem { Text = "12:00PM To 14:00PM", Value = time });

    timeSpan = new TimeSpan(14, 00, 00);
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt");
    times.Add(new ListItem { Text = "14:00PM To 17:00PM", Value = time });

    timeSpan = new TimeSpan(17, 00, 00);
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt");
    times.Add(new ListItem { Text = "17:00PM To 19:00PM", Value = time });

    timeSpan = new TimeSpan(19, 00, 00);
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt");
    times.Add(new ListItem { Text = "19:00PM To 22:00PM", Value = time });

    return times;
}

protected void OnAppoint(object sender, EventArgs e)
{
    if (Convert.ToDateTime(ddlTimes.SelectedValue).Subtract(DateTime.Now).TotalHours <= 1)
    {
        ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('Slot Not Available.')", true);
    }
    else
    {
        ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('Slot Available.')", true);
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not IsPostBack Then
        Dim times As List(Of ListItem) = GetTime()
        ddlTimes.DataSource = times
        ddlTimes.DataTextField = "Text"
        ddlTimes.DataValueField = "Value"
        ddlTimes.DataBind()
    End If
End Sub

Private Function GetTime() As List(Of ListItem)
    Dim times As List(Of ListItem) = New List(Of ListItem)()
    Dim timeSpan As TimeSpan = New TimeSpan(8, 0, 0)
    Dim time As String = DateTime.Today.Add(timeSpan).ToString("hh:mm tt")
    times.Add(New ListItem With {.Text = "8:00AM To 10:00AM", .Value = time})
    timeSpan = New TimeSpan(10, 0, 0)
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt")
    times.Add(New ListItem With {.Text = "10:00AM To 12:00PM", .Value = time})
    timeSpan = New TimeSpan(12, 0, 0)
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt")
    times.Add(New ListItem With {.Text = "12:00PM To 14:00PM", .Value = time})
    timeSpan = New TimeSpan(14, 0, 0)
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt")
    times.Add(New ListItem With {.Text = "14:00PM To 17:00PM", .Value = time})
    timeSpan = New TimeSpan(17, 0, 0)
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt")
    times.Add(New ListItem With {.Text = "17:00PM To 19:00PM", .Value = time})
    timeSpan = New TimeSpan(19, 0, 0)
    time = DateTime.Today.Add(timeSpan).ToString("hh:mm tt")
    times.Add(New ListItem With {.Text = "19:00PM To 22:00PM", .Value = time})
    Return times
End Function

Protected Sub OnAppoint(ByVal sender As Object, ByVal e As EventArgs)
    If Convert.ToDateTime(ddlTimes.SelectedValue).Subtract(DateTime.Now).TotalHours <= 1 Then
        ClientScript.RegisterClientScriptBlock(Me.GetType(), "", "alert('Slot Not Available.')", True)
    Else
        ClientScript.RegisterClientScriptBlock(Me.GetType(), "", "alert('Slot Available.')", True)
    End If
End Sub

Screenshot