Aim

To accept a date from the user, decompose it into day, month and year components, and map the identical logic onto a WinForms MonthCalendar control feeding three TextBox fields.

Theory

DateTime is a struct (value type) that packs an entire date-time into a single 64-bit field: 62 bits of ticks (1 tick = 100 nanoseconds elapsed since 0001-01-01) plus 2 bits of DateTimeKind. Properties such as .Day, .Month and .Year are not stored fields — they are computed on demand from the tick count by calendar arithmetic, which is why a DateTime occupies only 8 bytes. Parsing is culture-sensitive: DateTime.TryParse honours the current culture and reports failure through a bool return plus an out parameter instead of throwing — the TryParse pattern is .NET's idiomatic non-throwing validation and is orders of magnitude cheaper than catching FormatException in a loop. When a wire format is fixed by contract, DateTime.TryParseExact with "yyyy-MM-dd" and CultureInfo.InvariantCulture is the deterministic choice. The WinForms half is event-driven programming: MonthCalendar raises DateChanged, whose DateRangeEventArgs e exposes e.Start; the console program prints the exact subscription lambda you would place in a Forms project. A plain console app cannot host controls because it runs no Windows message loop (Application.Run) — which is why this practical ships as a console equivalent plus the wiring recipe.

Requirements

• .NET SDK 8.0+ for the console version (dotnet new console).
• For the GUI variant: Windows with dotnet new winforms -n DatePicker, or Visual Studio 2022 with the ".NET desktop development" workload.

Procedure

1. Create the project: dotnet new console -n DateParts, cd DateParts, paste the code, run dotnet run.
2. Enter 2026-07-03 (ISO yyyy-MM-dd) and record the four decomposed fields.
3. Enter nonsense ("hello") and observe the graceful fallback to today's date instead of a crash.
4. GUI extension: scaffold dotnet new winforms, drop a MonthCalendar and three TextBox controls onto the form, and paste the printed event-handler lambda into the form's constructor.

Explanation of the Code

• Console.ReadLine() ?? DateTime.Today.ToString("yyyy-MM-dd") — the null-coalescing operator substitutes today's ISO string when the input stream is closed (returns null).
• if (!DateTime.TryParse(input, out DateTime date)) — C# 7 inline out variable declaration; on failure the program logs a message and assigns DateTime.Today: validation without exceptions.
• date.Day, date.Month, date.Year — computed properties decoding the tick count; date.ToString("MMMM") applies a format specifier for the full month name (culture-sensitive); date.DayOfWeek returns a value of the DayOfWeek enum.
• The final Console.WriteLine block prints the WinForms wiring: monthCalendar.DateChanged += (_, e) => { ... } — a lambda subscribed to the event, with the discard _ ignoring the unused sender argument, copying e.Start.Day/Month/Year into the three text boxes.

Expected Output

For input 2026-07-03:

=== Date Selection Demo ===
Enter a date (yyyy-MM-dd): 2026-07-03

Day   : 3
Month : 7  (July)
Year  : 2026
Day of Week : Friday

WinForms equivalent:
  monthCalendar.DateChanged += (_, e) => {
      dayBox.Text   = e.Start.Day.ToString();
      monthBox.Text = e.Start.Month.ToString();
      yearBox.Text  = e.Start.Year.ToString();
  };

🎯 Viva Questions

1. Why TryParse instead of Parse? — It signals failure via a bool instead of a costly FormatException; ideal for user input loops.
2. Is DateTime a class? — No, a struct (value type) storing 64 bits: tick count plus DateTimeKind.
3. What is one tick? — 100 nanoseconds, counted from 0001-01-01 00:00.
4. What does the ?? operator do? — Null-coalescing: yields the left operand unless it is null, otherwise the right.
5. Which event does MonthCalendar raise on selection? — DateChanged, carrying DateRangeEventArgs with e.Start and e.End.
6. Why can a console app not show WinForms controls? — It runs no Windows message loop (Application.Run) and no STA UI thread is configured.