Question

I’m trying to create an abstract class that defines a property with a getter. I want to leave it up to derived classes to decide if they want to implement a setter for the property or not. Is this possible?

What I have so far:

public abstract class AbstractClass {
    public abstract string Value { get; }
    public void DoSomething() {
        Console.WriteLine(Value);
    }
}

public class ConcreteClass1 : AbstractClass {
    public override string Value { get; set; }
}

public class ConcreteClass2 : AbstractClass {
    private string _value;
    public override string Value {
        get { return _value; }
    }
    public string Value {
        set { _value = value; }
    }
}

public class ConcreteClass3 : AbstractClass {
    private string _value;
    public override string Value {
        get { return _value; }
    }
    public void set_Value(string value) {
        _value = value;
    }
}

In ConcreteClass1, I get an error on the set. It can’t override set_Value because no overridable set accessor exists in AbstractClass.

In ConcreteClass2, I get an error on both Value‘s because a member with the same name is already declared.

ConcreteClass3 doesn’t give an error, but even though Value’s set accessor would be compiled into set_Value, it doesn’t work the other way around. Defining a set_Value does not mean that Value gets a set accessor. So I can’t assign a value to a ConcreteClass3.Value property. I can use ConcreteClass3.set_Value(“value”), but that’s not what I’m trying to achieve here.

Is it possible to have the abstract class demand a public getter, while allowing an optional setter to be defined in a derived class?

In case you’r wondering, this is just a theoretical question. I don’t have a real situation where something like this is needed. But I can imagine an abstract class that doesn’t care how a property gets set, but that does need to be able to get the property.

Answer 1

Unfortunately, you can’t do exactly what you want. You can do this with interfaces though:

public interface IInterface {
    string MyProperty { get; }
}

public class Class : IInterface {
    public string MyProperty { get; set; }
}

The way I would do it is to have a separate SetProperty method in the concrete classes:

public abstract class AbstractClass {
    public abstract string Value { get; }
}

public class ConcreteClass : AbstractClass {

    private string m_Value;
    public override string Value {
        get { return m_Value; }
    }

    public void SetValue(string value) {
        m_Value = value;
    }
}