Is letting a class pass itself as a parameter to a generic base class evil?

Question

I first saw a colleague do this when he implemented object pools. He passed the class that was going to be pooled as a parameter to a generic base class. This base class layed out the pooling code.

The odd thing is that the base class will know of its children. This is considered bad practice in every normal case. But in this case the parent is just a technical solution to avoid writing repetetive code. The base class is never referenced by any other code.

One drawback with this construction is that it "burns the base class". You cannot introduce the generic base class in the middle of a hierarchy. This problem might be outside the topic.

Below is a thinkable example:

public abstract class Singleton<T> where T : class
{
    public static T Instance { get; private set; }

    public Singleton()
    {
        if (Instance != null)
            throw new Exception("Singleton instance already created.");
        Instance = (T) (object) this;
    }
}

public class MyClass : Singleton<MyClass>
{
}

Improved code:

public abstract class Singleton<T> where T : Singleton<T>
{
    public static T Instance { get; private set; }

    public Singleton()
    {
        if (Instance != null)
            throw new Exception("Singleton instance already created.");
        Instance = (T) this;
    }
}

public class MyClass : Singleton<MyClass>
{
}

Answer 1

No; this is a well-known pattern called the CRTP.
It is especially useful in C++ as an alternative to virtual methods.

You can see it inside the .Net framework in IComparable<T> and IEquatable<T>.

For added robustness, you should add where T : Singleton<T>