Question

How do I make an abstract class that shall force each derived classes to be Singleton ? I use C#.

Answer 1

When you want to enfore compile time checking, this is not possible. With runtime checking you can do this. It’s not pretty, but it’s possible. Here’s an example:

public abstract class Singleton
{
    private static readonly object locker = new object();
    private static HashSet<object> registeredTypes = new HashSet<object>();

    protected Singleton()
    {
        lock (locker)
        {
            if (registeredTypes.Contains(this.GetType()))
            {
                throw new InvalidOperationException(
                    "Only one instance can ever  be registered.");
            }
            registeredTypes.Add(this.GetType());
        }
    }
}

public class Repository : Singleton
{
    public static readonly Repository Instance = new Repository();

    private Repository()
    {
    }
}