Make property in abstract class only settable in constructor of child class?

Question

I have a get-only property in a concrete class. By definition, that can only be set in a constructor of that class. I later decide that I want this property to belong to a constructor-less abstract class that the concrete class inherits from. How can I make the property that is now in the abstract class only capable of being set within the constructors of the child classes?

Solutions that won't work:

• If I give the property a protected set modifier, then allows the child class more power than I'm asking for.
• init isn't an option because I only want it set in the constructor.
• readonly won't always work, because I'm on C# 7.3.

Example original class:

public class FooOriginal
{
   public int IntHere { get; }
}

Example abstract class:

public abstract class FooAbstract
{
   public int IntHere { get; }
}

Example child class

public class FooChild : FooAbstract
{
   public FooChild()
   {
       // Won't compile because IntHere is get-only.
       intHere = 3;
   }
}

Answer 1

To achieve this, you can give the base class a constructor, you can make it protected if you like:

public abstract class FooAbstract
{
    public int IntHere { get; }

    protected FooAbstract(int intHere)
    {
        IntHere = intHere;
    }
}

And your old can call the base class constructor, for example:

public class FooOriginal : FooAbstract
{
    public FooOriginal(int intHere)
        : base(intHere) // <-- This is the magic
    {
        
    }
}

Or if you want to pass in a constant value as per your example:

public FooOriginal() : base(3)
{
}