The type 'T' must be convertible in order to use it as parameter 'T' in the generic type or method

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.

I agree, why don't you remove the constraint all together? The code seems to compile fine without it. — NeddySpaghetti, May 22 '13 at 07:40
@Onots While I'm usually disinclined to give answers that are (albeit cryptically) explained by the compiler errors, I'll give it a crack. — J. Steen, May 22 '13 at 07:42
@J.Steen - in C++, it's called the Curiously Recurring Template Pattern. Eric Lippert has a blog post about it called [Curiouser and curiouser](http://blogs.msdn.com/b/ericlippert/archive/2011/02/03/curiouser-and-curiouser.aspx) where he points out that it's a flawed approach in C# (but it's perfectly compilable) — Damien_The_Unbeliever, May 22 '13 at 07:44
@Damien_The_Unbeliever Thank you for a name to put to the construct. I agree it's flawed, I agree the code - as it stands - can be compiled, but can it actually **be used** in C#? Can you instantiate a class of type `FSMSystem`? — J. Steen, May 22 '13 at 07:46
Consider `class MyFSM : FSMSystem{}` You can't directly instantiate a `FSMSystem`, but you can instantiate a class that *derives* from it. — Damien_The_Unbeliever, May 22 '13 at 07:48
@Damien_The_Unbeliever D'oh. I've even USED that construct. It was so long ago that I completely forgot. — J. Steen, May 22 '13 at 08:02

score 10 · Accepted Answer · answered May 22 '13 at 07:28

10

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.

answered May 22 '13 at 07:28

flq

22,247
8
55
77

the errors occurred these lines. public abstract void OnEnter (FSMSystem fsm, FSMState prevState); public abstract void OnUpdate (FSMSystem fsm); public abstract void OnExit (FSMSystem fsm, FSMState nextState); – Claire Lee May 23 '13 at 00:43
This was helpful for me, I put the constraint right after the class name instead of the interfaces – MichaelChan May 13 '19 at 04:58

score 0 · Answer 2 · edited May 23 '13 at 00:56

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);
}

The type 'T' must be convertible in order to use it as parameter 'T' in the generic type or method

2 Answers2

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