类型“T”必须是可转换的，以便在泛型类型或方法中将其用作参数“T”

Question

I have these two main classes. First the FSMSystem class:

public class FSMSystem<T> : MonoBehaviour where T : FSMSystem<T>
{
    private T m_Owner = default(T);

    protected FSMState<T> currentState;

    private Dictionary<int, FSMState<T>> m_states;

    public FSMSystem(T owner)
    {
        m_Owner = GameObject.FindObjectOfType(typeof(T)) as T; //owner;
        m_states = new Dictionary<int, FSMState<T>>();
    }

    protected void AddState( FSMState<T> state )
    {
        m_states.Add( state.GetStateID(), state );
    }
}

And the second class, FSMState:

public abstract class FSMState<T>
{   
    public abstract int GetStateID();

    public abstract void OnEnter (FSMSystem<T> fsm, FSMState<T> prevState);
    public abstract void OnUpdate (FSMSystem<T> fsm);
    public abstract void OnExit (FSMSystem<T> fsm, FSMState<T> nextState);
}

It leads to the following error:

error CS0309: The type ' T ' must be convertible to ' FSMSystem<T> ' in order to use it as parameter ' T ' in the generic type or method ' FSMSystem<T> '

Can someone tell me how to resolve this? I see many other posts similar to this one but I'm not seeing the relationship.

Answer 1

The T of FSMState must also be constrained, otherwise it cannot be used as the T of FSMSystem - which has constraints placed on it ( T : FSMSystem<T> ).

If you would have provided the line number of the compiler error, I suspect it would point to the methods OnEnter , etc.

Answer 2

It's big help to me. thank you all. I resolved the problem. I misunderstood about 'where' syntax.

Here is the revised version that works.

public class FSMSystem<T> : MonoBehaviour where T : FSMSystem<T>
{
    private T m_Owner = default(T);

    protected FSMState<T> currentState;

    private Dictionary<int, FSMState<T>> m_states;

    public FSMSystem()
    {
        m_Owner = this as T;
        m_states = new Dictionary<int, FSMState<T>>();
    }

    protected void AddState( FSMState<T> state )
    {
        m_states.Add( state.GetStateID(), state );
    }
}


public abstract class FSMState<T> where T : FSMSystem<T>
{
    public abstract int GetStateID();

    public abstract void OnEnter (T fsm, FSMState<T> prevState);
    public abstract void OnUpdate (T fsm);
    public abstract void OnExit (T fsm, FSMState<T> nextState);
}