XML Finite State Machine in C#

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Introduction

This article demonstrates using the .NET Framework to implement a finite state machine (FSM). This project was inspired by the MSDN article Design Patterns: Solidify Your C# Application Architecture with Design Patterns. Like the article's author, I believe that using State objects is a more powerful way of implementing this particular design pattern. However, for those that belive in XP's principle of "doing the simplest thing that could possibly work", table-driven FSMs are quite simple to implement.

To make the implementation of the FSM more interesting, I chose to define the table using XML. The format is very simple and defines state and transition elements. Using the example from the MSDN article, here is the state table in XML.

<?xml version="1.0" ?>

<fsm name="Vending Machine">
    <states>
        <state name="start">
            <transition input="nickel" next="five" />
            <transition input="dime" next="ten" />
            <transition input="quarter" next="start" action="dispense" />
        </state>
        <state name="five">
            <transition input="nickel" next="ten" />
            <transition input="dime" next="fifteen" />
            <transition input="quarter" next="start" action="dispense" />
        </state>
        <state name="ten">
            <transition input="nickel" next="fifteen" />
            <transition input="dime" next="twenty" />
            <transition input="quarter" next="start" action="dispense" />
        </state>
        <state name="fifteen">
            <transition input="nickel" next="twenty" />
            <transition input="dime" next="start" action="dispense" />
            <transition input="quarter" next="start" action="dispense" />
        </state>
        <state name="twenty">
            <transition input="nickel" next="start" action="dispense" />
            <transition input="dime" next="start" action="dispense" />
            <transition input="quarter" next="start" action="dispense" />
        </state>
    </states>
</fsm>

Pretty simple. The class XMLStateMachine has three properties (Action, CurrentState, and StateTable) and one method, Next. To use it, simply create an instance of XMLStateMachine, set the StateTable property to the XML file you wish to use and set the CurrentState property to the initial state.

XMLStateMachine fsm = new XMLStateMachine();
fsm.StateTable = "vending.xml";
fsm.CurrentState ="start";

Then, call the Next method with an appropriate input argument. Next returns the new state as well as modifies the CurrentState property.

NAnt is used to build the sample project. It is based on the Apache Ant project, which is used to build many Java-based projects. If you cannot afford Visual Studio .NET, or prefer to use your own editor and the command line compilers I highly recommend NAnt.

The code presented here could be expanded in many ways. The Action property could be replaced with an event. The System.Xml.XPath.XPathNavigator class could be used instead of System.Xml.XmlTextReader. But, that would defeat the purpose of implementing "the simplest thing that could possibly work"!