Why dotnet program allow readonly property to assign value from Constructor?

Question

see a sample example which i am running on version 4.5.2. in this program i can assign ready only property with a value from constructor....why it is allowing ?

    public class Person
    {
        //line 1
        public string FirstName { get; }

        //line 2
        //public string LastName { get; } = null!;

        //assign null is possible
        //public string? MiddleName { get; } = null;

        public Person(string firstName, string lastName, string middleName)
        {
            FirstName = firstName;
            //LastName = lastName;
            //MiddleName = middleName;
        }

        public Person(string firstName, string lastName)
        {
            FirstName = firstName;
            //LastName = lastName;
            //MiddleName = null;
        }
    }

why program allow assign value to readonly property from CTOR ? please guide me. thanks

Answer 1

This is because there is a private setter which only works inside the class constructor. See the docs.

Example when implicitly defined

public string FirstName { get; private set; }  

 

Answer 2

Read-only property only work where you create and call an object of a class.
Read-only doesn't work inside your "Person" class.

Therefore, the constructor code below will work normally.
However, changing the value of a read-only attribute is not allowed.

        Person oP1 = new Person("AAA", "BBB");
        oP1.FirstName = "CCC"; // error CS8370:  Feature 'readonly members' is not available ......