C# 9.0 Init-Only Setters: Immutable Object Initialization

Before C# 9, to make a property immutable, you had to use constructor injection. This broke object initializers (`new Obj { Prop = val }`). The `init` accessor solves this by allowing setting a property only during object initialization.

The ‘init’ Keyword

public class User
{
    public string Id { get; init; } // Can only be set during construction
    public string Username { get; set; } // Mutable
}

// Valid
var user = new User 
{ 
    Id = "123", 
    Username = "nitrix" 
};

// Invalid - Compiler Error
user.Id = "456"; 

Why this matters for DTOs

It allows for consistent, valid state without massive constructors.

Key Takeaways

• Replace `private set` with `init` for properties that shouldn’t change after creation.
• Works perfect with serialized data (JSON deserializers can set init properties).