How do init and required work together in C#? Explain init-only setters (C# 9), required members (C# 11), what the compiler enforces, SetsRequiredMembers, how they interact with constructors, object initialisers, with expressions and System.Text.Json, and when you would choose a constructor instead.

1. How init is encoded

An init accessor is a setter with modreq(System.Runtime.CompilerServices.IsExternalInit) on its return type. Compilers that don't understand IsExternalInit can't call it (the modreq makes the signature unrecognisable), preventing older languages from mutating init-only properties. Reflection can still set them (PropertyInfo.SetValue) — init is a compile-time guarantee.

2. How required is encoded

• Members get [RequiredMember]; the type gets [RequiredMember].
• Constructors without [SetsRequiredMembers] get [CompilerFeatureRequired("RequiredMembers")] and [Obsolete] with a special message so older compilers refuse to use them.
• The compiler checks object creation expressions against the required member set.

3. Records with required members

public sealed record OrderPlaced
{
    public required Guid OrderId { get; init; }
    public required DateTimeOffset At { get; init; }
}

Nominal records with required give named, immutable message types without long constructors. Positional records already require all parameters via the constructor.

4. Validation patterns with init

public required string Email
{
    get;
    init => field = MailAddress.TryCreate(value, out _) ? value : throw new ArgumentException("Invalid email", nameof(value));
}

C# 14's field keyword makes per-property validation in init concise. Cross-property validation still needs a constructor, factory, or an IValidatableObject-style step.

5. Reflection and serialisers

• System.Text.Json sets init-only properties via reflection or source generation; with required, missing members raise errors (JsonRequiredAttribute equivalent).
• Newtonsoft.Json sets init-only properties but doesn't enforce required (use [JsonProperty(Required = Required.Always)]).
• Configuration binding (IOptions<T>) can bind init-only properties in recent .NET versions; validate options with ValidateDataAnnotations().ValidateOnStart().

Try it yourself

1. Create a class with required members and try creating it without them.
2. Add a [SetsRequiredMembers] constructor and observe the compiler accepting it.
3. Deserialise JSON missing a required property with System.Text.Json.
4. Use with on a record with required members.

using System.Diagnostics.CodeAnalysis;
using System.Text.Json;

public sealed class Customer
{
    public required string Name { get; init; }
    public required string Email
    {
        get;
        init => field = value.Contains('@') ? value : throw new ArgumentException("Invalid email", nameof(Email)); // C# 14 field keyword
    }
    public string? Phone { get; init; }
    public List<string> Tags { get; init; } = [];   // init does not make the list immutable

    public Customer() { }

    [SetsRequiredMembers]
    public Customer(string name, string email)
    {
        Name = name;
        Email = email;
    }
}

public sealed record Booking
{
    public required DateOnly Start { get; init; }
    public required DateOnly End { get; init; }

    // Cross-property rule: object initialisers run after constructors, so use a factory
    public static Booking Create(DateOnly start, DateOnly end) =>
        end >= start ? new Booking { Start = start, End = end }
                     : throw new ArgumentException("End must not be before Start");
}

public static class Program
{
    public static void Main()
    {
        var mia = new Customer { Name = "Mia", Email = "[email protected]" };
        var leo = new Customer("Leo", "[email protected]");   // ctor satisfies required members
        // mia.Name = "Changed";                                  // CS8852: init-only
        mia.Tags.Add("vip");                                     // allowed: the list itself is mutable

        var booking = Booking.Create(new DateOnly(2026, 12, 1), new DateOnly(2026, 12, 5));
        var extended = booking with { End = new DateOnly(2026, 12, 9) }; // init setters on the copy
        Console.WriteLine($"{leo.Name} {extended}");

        try
        {
            JsonSerializer.Deserialize<Customer>("""{"Email":"[email protected]"}""");
        }
        catch (JsonException ex)
        {
            Console.WriteLine(ex.Message);   // required property 'Name' missing
        }
    }
}