Round #888

Implement nullable reference type annotations on a 'Person' class with FirstName (non-null), MiddleName (nullable), LastName (non-null). Show a constructor that enforces non-nullness.

```csharp #nullable enable public class Person { public string FirstName { get; set; } public string? MiddleName { get; set; } public string LastName { get; set; } public Person(string firstName, string lastName, string? middleName = null) { FirstName = firstName ?? throw new ArgumentNullException(nameof(firstName)); LastName = lastName ?? throw new ArgumentNullException(nameof(lastName)); MiddleName = middleName; } } class Program { static void Main() { var person1 = new Person("John", "Doe"); var person2 = new Person("Jane", "Doe", "Mary"); System.Console.WriteLine(person1.FirstName); // John System.Console.WriteLine(person1.MiddleName); // System.Console.WriteLine(person1.LastName); // Doe System.Console.WriteLine(person2.FirstName); // Jane System.Console.WriteLine(person2.MiddleName); // Mary System.Console.WriteLine(person2.LastName); // Doe } } ```

```csharp public class Person { public string FirstName { get; set; } public string? MiddleName { get; set; } public string LastName { get; set; } public Person(string firstName, string lastName, string? middleName = null) { if (string.IsNullOrEmpty(firstName)) { throw new ArgumentException("First name cannot be null or empty", nameof(firstName)); } if (string.IsNullOrEmpty(lastName)) { throw new ArgumentException("Last name cannot be null or empty", nameof(lastName)); } FirstName = firstName; MiddleName = middleName; LastName = lastName; } } ``` This implementation uses nullable reference type annotations to indicate that `MiddleName` can be null, while `FirstName` and `LastName` cannot. The constructor enforces non-nullness for `firstName` and `lastName` by throwing an `ArgumentException` if either is null or empty. The `middleName` parameter is optional and defaults to null if not provided.