What are init only setters in C# 9.0?

Init only setters in C# 9.0 provide a method to initialize object members.

C# 9.0 allows the user to create init accessors instead of set accessors. init accessors allow the members of an instance of the object to be set only at the time of the construction. Once the construction of the instance is complete, the members become immutable.

However, the set accessor allows the members to be changed even after the initialization is complete.

using System;
namespace c9 {
    // create an object
    public class Student {
    // replace set with init
    public int Id {get; init;} 
    public string firstName { get;init;}
    public string lastName { get;set;}
    }   
    class Program {
        static void Main(string[] args) {
            Student studentOne = new Student{Id = 1, firstName = "Sam"};
             
            // set the last name after initialization 
            
            studentOne.lastName = "Carlos";
            //display properties
            Console.WriteLine("ID : " + studentOne.Id);
            Console.WriteLine("Name : " + studentOne.firstName);
            Console.WriteLine("Last Name : " + studentOne.lastName);
            
        }
    }
}

What are init only setters in C# 9.0?

Example

Output