C# - Readonly properties and fields cannot be set in derived class constructors [duplicate]

Question

I have a class structure like the following

abstract class AMyAbstractClass {
    public readonly int MyReadonlyField;
    public int MyReadonlyProperty { get; }//read-only auto-property (syntactic sugar)
}
class MyConcreteClass : AMyAbstractClass {
    MyConcreteClass() {
        this.MyReadonlyField = 1;
        this.MyReadonlyProperty = 1;
    }
}

Which throws compilation errors

A readonly field cannot be assigned to (except in a constructor or a variable initializer)

And

Property or indexer 'AMyAbstractClass.MyReadonlyProperty cannot be assigned to -- it is read only

Respectively.

In the first case, the error message is erroneous, since it is being set in a constructor!

---

I can come up with alternative ways to have immutable object properties, but why is this situation disallowed? What are good practices for this type of encapsulation?

Answer 1

You need to propagate them via base class' constructor:

abstract class AMyAbstractClass {
    public readonly int MyReadonlyField;
    public int MyReadonlyProperty { get; }//syntactic sugar
    protected AMyAbstractClass (int fieldValue, int propertyValue) {
        this.MyReadonlyField = fieldValue;
        this.MyReadonlyProperty = propertyValue;
    }
}
class MyConcreteClass : AMyAbstractClass {
    public MyConcreteClass() 
        : base(fieldValue: 1, propertyValue:1) {
    }
}

Regarding readonly fields. Reference from ECMA-334 C# Language Specification, chapter 15.5.3:

When a field - declaration includes a readonly modifier, the fields introduced by the declaration are readonly fields . Direct assignments to readonly fields ca n only occur as part of that declaration or in an instance constructor or static constructor in the same class.

And the description for read-only auto-properties from c# 6:

... properties can be set only in the body of a constructor: