Assigning a value to an inherited readonly field?

Question

So I have a base class that has many children. This base class defines some readonly properties and variables that have default values. These can be different, depending on the child.

Readonly properties/fields allow you to change the value of the variable inside the constructor and also the definition, but nowhere else. I get a 'readonly variable can only be assigned to in a constructor' error if I try to change the value of an inherited readonly variable in the child class' constructor. Why is this and how can I work around this, without Reflection?

My intention: To allow user extensibility through scripts where they can only change certain fields once.

Answer 1

The reason is that you can only assign to readonly fields in the constructor of that class.
According to the definition of readonly in the C# Reference (emphasis mine):

When a field declaration includes a readonly modifier, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class.

To work around this, you could make a protected constructor in the base that takes a parameter for the readonly property.

An example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Base b = new Child();
            Console.WriteLine(b.i);
            Console.Read();
        }
    }

    class Base
    {
        public readonly int i;

        public Base()
        {
            i = 42;
        }

        protected Base(int newI)
        {
            i = newI;
        }
    }

    class Child : Base
    {
        public Child()
            : base(43)
        {}
    }
}

Answer 2

You could get the exact behavior you are looking for by using virtual get only properties.

public class BSE
{
    virtual public int Prop 
    {
        get
        {
            return 6;
         }
     }
}
public class Derived : BSE
{
    public override int Prop
    {
         get
         {
             return 10;
         }
    }
 }

Fields are out side the inheritance and overloading model and should not be used to provide polymorphic features.

Answer 3

Adam has the right answer. If you're worried about the space it will take up (number of parameters in the constructor?) then you should address that as a different problem with a different solution: create a BaseConfig class, that contains all those properties and that is all that needs to be passed in. Base can then either assign all it's readonly fields from BaseConfig's properties, or you can instead have Base hold just one readonly field of type BaseConfig and refer to that for the values.

As to why this is, see C# constructor execution order regarding when each class's readonly fields would be initialized/initializable.

Answer 4

You can use property with public get accessor and protected set accessor. Derived classes can set value of this property.

An example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Base b = new Child();
            Console.WriteLine(b.I);
            Console.Read();
        }
    }

    class Base
    {
        public int I { get; protected set; }

        public Base()
        {
            I = 42;
        }
    }

    class Child : Base
    {
        public Child()
        {
            I = 43;
        }
    }
}

Answer 5

this is impossible by design. try passing the values to a protected base class constructor

Answer 6

This is essentially what @rerun came up with, but with abstract instead of virtual:

protected abstract class BaseClass
{
    protected abstract int PrivateI { get; }
}

public class DerivedClass : BaseClass
{
    private readonly int _i;
    protected override int PrivateI => _i;

    public DerivedClass(int i)
    {
        _i = i;
    }
}