Imports System
Imports System.Globalization
Module CompareDatesVB
'
' Compare two dates in VB.NET
' ---------------------------
' This program demonstrates how to compare two dates using VB.NET's built‑in
' DateTime structure. It uses a modular design, clear comments, and a full test suite.
'
' Concepts:
' - Parsing dates safely (YYYY‑MM‑DD)
' - Comparing DateTime objects
' - Handling invalid formats
' - Edge‑case testing
'
' Architecture notes:
' - A dedicated function handles parsing.
' - Another function performs comparison.
' - Main runs multiple predefined test cases.
' - No external dependencies; only standard library.
'
' Performance notes:
' - DateTime comparisons are O(1).
' - Parsing is fast and predictable.
' - Memory usage is minimal.
'
' Pitfalls:
' - Invalid date strings must be handled.
' - Comparing raw strings is unsafe; always convert to DateTime.
' - DateTime is timezone‑aware; here we treat dates as date‑only values.
'
' Safely parse a date string in the format YYYY-MM-DD.
' Returns Nothing if the date is invalid.
Function ParseDate(s As String) As DateTime?
Try
Return DateTime.ParseExact(
s,
"yyyy-MM-dd",
CultureInfo.InvariantCulture,
DateTimeStyles.None
)
Catch
Return Nothing
End Try
End Function
' Compare two DateTime objects.
' Returns: "earlier", "later", or "equal".
Function CompareDates(a As DateTime, b As DateTime) As String
If a < b Then Return "earlier"
If a > b Then Return "later"
Return "equal"
End Function
' Run a single test case:
' - Parse both dates
' - Handle invalid input
' - Compare if valid
Sub RunTestCase(d1 As String, d2 As String)
Dim a = ParseDate(d1)
Dim b = ParseDate(d2)
If a Is Nothing OrElse b Is Nothing Then
Console.WriteLine($"Compare '{d1}' vs '{d2}' → invalid date format")
Exit Sub
End If
Console.WriteLine($"Compare '{d1}' vs '{d2}' → {CompareDates(a.Value, b.Value)}")
End Sub
' Main test suite:
' - Multiple test cases
' - Includes edge cases
' - Prints results cleanly
Sub Main()
Console.WriteLine("Date comparison tests:")
Console.WriteLine()
Dim tests(,) As String = {
{"2024-01-01", "2024-01-02"}, ' earlier
{"2024-01-02", "2024-01-01"}, ' later
{"2024-01-01", "2024-01-01"}, ' equal
{"1999-12-31", "2000-01-01"}, ' millennium boundary
{"2024-02-29", "2024-03-01"}, ' leap year
{"2024-02-29", "2023-02-28"}, ' leap vs non-leap
{"2024-13-01", "2024-01-01"}, ' invalid month
{"2024-00-10", "2024-01-01"}, ' invalid month
{"2024-01-32", "2024-01-01"}, ' invalid day
{"abcd-ef-gh", "2024-01-01"}, ' invalid format
{"2024-01-01", "abcd-ef-gh"} ' invalid format
}
For i As Integer = 0 To tests.GetLength(0) - 1
RunTestCase(tests(i, 0), tests(i, 1))
Next
End Sub
End Module
'
' run:
'
' Date comparison tests:
'
' Compare '2024-01-01' vs '2024-01-02' → earlier
' Compare '2024-01-02' vs '2024-01-01' → later
' Compare '2024-01-01' vs '2024-01-01' → equal
' Compare '1999-12-31' vs '2000-01-01' → earlier
' Compare '2024-02-29' vs '2024-03-01' → earlier
' Compare '2024-02-29' vs '2023-02-28' → later
' Compare '2024-13-01' vs '2024-01-01' → invalid date format
' Compare '2024-00-10' vs '2024-01-01' → invalid date format
' Compare '2024-01-32' vs '2024-01-01' → invalid date format
' Compare 'abcd-ef-gh' vs '2024-01-01' → invalid date format
' Compare '2024-01-01' vs 'abcd-ef-gh' → invalid date format
'
'
