如何将具有约束参数类型的 class 的实例引用传递给该约束类型的实例？

Question

Let's say I have these two classes:

public abstract class ASpawnedObject {}

public abstract class ASpawner<T> where T : ASpawnedObject {}

The ASpawner class creates some ASpawnedObject instances. I want it to pass itself to the ASpawnedObject constructor and keep a reference of it as a member variable:

public abstract class ASpawnedObject {}
{
    ASpawner<ASpawnedObject> spawner;

    public void Init(ASpawner<ASpawnedObject> spawner)
    {
        this.spawner = spawner;
    }
}

public abstract class ASpawner<T> where T : ASpawnedObject
{
    public void Spawn()
    {
        T spawnedObject = System.Activator.CreateInstance<T>();
        spawnedObject.Init(this);
    }
}

BUT: spawnedObject.Init(this) is causing a conversion error in ASpawner . It can't convert ASpawner<T> to ASpawner<ASpawnedObject> even with the type constraint. I don't get it. Doesn't it follow the polymorphism principles?

Answer 1

This test class compiles and runs without any exceptions

[TestClass]
public class Test
{
    [TestMethod]
    public void X()
    {
        var ghostSpawner = new GhostSpawner();
        ghostSpawner.Spawn();
    }
}

What you have to do is constrain your spawner and your spawned object in terms of your spawned object. Yes, that means for the latter, in terms of itself:

public abstract class ASpawnedObject<T> where T : ASpawnedObject<T>
{
    ASpawner<T> spawner;
    public void Init(ASpawner<T> spawner) => this.spawner = spawner;
}

public abstract class ASpawner<T> where T : ASpawnedObject<T>
{
    public void Spawn()
    {
        T spawnedObject = Activator.CreateInstance<T>();
        spawnedObject.Init(this);
    }
}

As you can see, that means the ASpawnedObject base class must be made generic. You can then create non-generic concrete classes that make the test pass.

public class GhostSpawner : ASpawner<Ghost>
{}

public class Ghost : ASpawnedObject<Ghost>
{}