Question

I need a field that can be assigned to from where ever I want, but it should be possible to assign it only once (so subsequent assignments should be ignored). How can I do this?

Answer 1

That would not be a readonly field then. Your only options for initializing real readonly fields are field initializer and constructor.

You could however implement a kind of readonly functionality using properties. Make your field as properties. Implement a “freeze instance” method that flipped a flag stating that no more updates to the readonly parts are allowed. Have your setters check this flag.

Keep in mind that you’re giving up a compile time check for a runtime check. The compiler will tell you if you try to assign a value to a readonly field from anywhere but the declaration/constructor. With the code below you’ll get an exception (or you could ignore the update – neither of which are optimal IMO).

EDIT: to avoid repeating the check you can encapsulate the readonly feature in a class.

Revised implementation could look something like this:

class ReadOnlyField<T> {
    public T Value {
        get { return _Value; }
        set { 
            if (Frozen) throw new InvalidOperationException();
            _Value = value;
        }
    }
    private T _Value;

    private bool Frozen;

    public void Freeze() {
        Frozen = true;
    }
}


class Foo {
    public readonly ReadOnlyField<int> FakeReadOnly = new ReadOnlyField<int>();

    // forward to allow freeze of multiple fields
    public void Freeze() {
        FakeReadOnly.Freeze();
    }
}

Then your code can do something like

        var f = new Foo();

        f.FakeReadOnly.Value = 42;

        f.Freeze();

        f.FakeReadOnly.Value = 1337;

The last statement will throw an exception.