.net Multiple inheritance from abstract classes

Question

it is possible to implement this functionality? I've two abstract classes, and i want to force a third class to implement all the abstract methods in the two classes

public abstract class A
{
    private void MethodA()
    {
        var fool = Prop*2;
    }
    public abstract int Prop { get; }
}
public abstract class B
{
    private void MethodB()
    {
        //.... 
        SomeMethod();
        //....
    }
    public abstract void SomeMethod();
}

public class C: A, B
{
    public int Prop { .... }
    public void SomeMethod { .... }
}

The main problem here is that the implemented final class methods are then used in base abstract classes, and this cannot be achieved with interfaces. So there is no a workaround?

I've read many similar q/a but nothing responding to my problem. Thanks

Answer 1

As of C# 8.0 on .NET Core 3.0, C# supports default implementations for Interface members.

public interface ILight
{
    void SwitchOn();
    void SwitchOff();
    bool IsOn();
}

// Basic Implementation
public class OverheadLight : ILight
{
    private bool isOn;
    public bool IsOn() => isOn;
    public void SwitchOff() => isOn = false;
    public void SwitchOn() => isOn = true;

    public override string ToString() => $"The light is {(isOn ? "on" : "off")}";
}

// Interface with default implementation
public interface ITimerLight : ILight
{
    // defines default implementation
    public async Task TurnOnFor(int duration)
    {
        Console.WriteLine("Using the default interface method for the ITimerLight.TurnOnFor.");
        SwitchOn();
        await Task.Delay(duration);
        SwitchOff();
        Console.WriteLine("Completed ITimerLight.TurnOnFor sequence.");
    }
}

// Implementation with interface
public class OverheadLight : ITimerLight
{
    private bool isOn;
    public bool IsOn() => isOn;
    public void SwitchOff() => isOn = false;
    public void SwitchOn() => isOn = true;

    public override string ToString() => $"The light is {(isOn ? "on" : "off")}";

    // Automatically gets the interface implementation
    //
    // To provide your own implementation: (no use of override)
    // public async Task TurnOnFor(int duration)
    // {
    // }
}