Custom Dictionary TryGetValue doesn't find keys

Question

I'm trying to create a base class that works like a state machine and that can accept any type of enum:

    public class BaseFSM <T> where T : struct, IConvertible
{

    //Basic class that denote the transition between one state and another
    public class StateTransition
    {
        public  T currentState { get; set; }
        public  T nextState { get; set; }           

        //StateTransition Constructor
        public StateTransition(T currentState, T nextState)
        {
            this.currentState = currentState;
            this.nextState = nextState;
        }

        public override int GetHashCode()
        {
            return 17 + 31 * this.currentState.GetHashCode() + 31 * this.nextState.GetHashCode();;
        }

        public override bool Equals(object obj)
        {
            StateTransition other = obj as StateTransition;
            return other != null && this.currentState as Enum == other.currentState as Enum && this.nextState as Enum == other.nextState as Enum;
        }
    }

    protected Dictionary<StateTransition, T> transitions; //All the transitions inside the FSM
    public T currentState;
    public T previusState;

    protected BaseFSM() {
        // Throw Exception on static initialization if the given type isn't an enum.
        if(!typeof (T).IsEnum) 
            throw new Exception(typeof(T).FullName + " is not an enum type.");
    }

    private T GetNext(T next)
    {
        StateTransition transition = new StateTransition(currentState, next);
        T nextState;
        if (!transitions.TryGetValue(transition, out nextState))
            throw new Exception("Invalid transition: " + currentState + " -> " + next);
        return nextState;


    }
}

As you can see I defined both GetHashCode() and Equals(object obj). This is my implementation of my child class:

public class FSMPlayer : BaseFSM<PlayerState>
{   
    public FSMPlayer() : base()
    {           
        this.currentState = PlayerState.Idle;
        this.transitions = new Dictionary<StateTransition, PlayerState>
        {
            { new StateTransition(PlayerState.Idle, PlayerState.Run), PlayerState.Run }, //0
            { new StateTransition(PlayerState.Run, PlayerState.Jump), PlayerState.Jump }, //1
        };  
    }
}

As you can see in my child class I'm using my PlayerState Enum to define the state transitions. The problem it's when I try to use the getNext function because the TryGetValue always return false. The GetHashCode functions seams to work very well so I can't understand where the problem is. Thanks.

Jeroen Mostert · Accepted Answer · 2015-02-22T14:32:09.680

3

The problem is here:

this.currentState as Enum == other.currentState as Enum

Enum is a reference type, so your enum gets boxed into a (new, unique) object. As a result it no longer compares equal to any other boxed instance.

enum types do the Right Thing for overriding Equals, though (as @hvd so correctly points out), so you can just do

this.currentState.Equals(other.currentState)

edited Feb 22 '15 at 14:32

answered Feb 22 '15 at 14:10

Jeroen Mostert

27,176
2
52
85

2

Wouldn't a simple `this.currentState.Equals(other.currentState)` work? – Feb 22 '15 at 14:28
@hvd: yes, absolutely, I'm being silly. (Also, enums don't implement `IComparable`, but they do implement `IComparable`.) If you want to post this as an answer yourself, I'll delete mine. – Jeroen Mostert Feb 22 '15 at 14:32
You did identify the real problem, and you did post a valid way of solving it. I'm happy to just keep your answer. :) – Feb 22 '15 at 14:34

score 0 · Answer 2 · edited May 23 '17 at 10:24

0

You may also want to read the answers to this question to see why calling getHashCode on an enum is not recommended.

Using GetHashCode for getting Enum int value

edited May 23 '17 at 10:24

Community

1
1

answered Feb 22 '15 at 14:10

Wayne Tanner

1,346
11
18

This is perfectly fine. The answer in that question discusses why you should not rely on `GetHashCode()` to return the numerical value of the enum. If you are actually using it to hash it (and so not relying on that property at all), there's no problem. – Jeroen Mostert Feb 22 '15 at 14:14

Custom Dictionary TryGetValue doesn't find keys

2 Answers2