Skip to content


Repository files navigation

Stateless Build status NuGet Pre Release Join the chat at Stack Overflow

Create state machines and lightweight state machine-based workflows directly in .NET code:

var phoneCall = new StateMachine<State, Trigger>(State.OffHook);

    .Permit(Trigger.CallDialled, State.Ringing);

    .OnEntry(t => StartCallTimer())
    .OnExit(t => StopCallTimer())
    .InternalTransition(Trigger.MuteMicrophone, t => OnMute())
    .InternalTransition(Trigger.UnmuteMicrophone, t => OnUnmute())
    .InternalTransition<int>(_setVolumeTrigger, (volume, t) => OnSetVolume(volume))
    .Permit(Trigger.LeftMessage, State.OffHook)
    .Permit(Trigger.PlacedOnHold, State.OnHold);

// ...

Assert.AreEqual(State.Ringing, phoneCall.State);

This project, as well as the example above, was inspired by Simple State Machine (Archived).


Most standard state machine constructs are supported:

  • Generic support for states and triggers of any .NET type (numbers, strings, enums, etc.)
  • Hierarchical states
  • Entry/exit actions for states
  • Guard clauses to support conditional transitions
  • Introspection

Some useful extensions are also provided:

  • Ability to store state externally (for example, in a property tracked by an ORM)
  • Parameterised triggers
  • Reentrant states
  • Export to DOT graph

Hierarchical States

In the example below, the OnHold state is a substate of the Connected state. This means that an OnHold call is still connected.

    .Permit(Trigger.TakenOffHold, State.Connected)
    .Permit(Trigger.PhoneHurledAgainstWall, State.PhoneDestroyed);

In addition to the StateMachine.State property, which will report the precise current state, an IsInState(State) method is provided. IsInState(State) will take substates into account, so that if the example above was in the OnHold state, IsInState(State.Connected) would also evaluate to true.

Entry/Exit actions

In the example, the StartCallTimer() method will be executed when a call is connected. The StopCallTimer() will be executed when call completes (by either hanging up or hurling the phone against the wall.)

The call can move between the Connected and OnHold states without the StartCallTimer() and StopCallTimer() methods being called repeatedly because the OnHold state is a substate of the Connected state.

Entry/Exit action handlers can be supplied with a parameter of type Transition that describes the trigger, source and destination states.

Internal transitions

Sometimes a trigger needs to be handled, but the state shouldn't change. This is an internal transition. Use InternalTransition for this.

Initial state transitions

A substate can be marked as initial state. When the state machine enters the super state it will also automatically enter the substate. This can be configured like this:



Due to Stateless' internal structure, it does not know when it is "started". This makes it impossible to handle an initial transition in the traditional way. It is possible to work around this limitation by adding a dummy initial state, and then use Activate() to "start" the state machine.

        .OnActivate(() => sm.Fire(LetsGo))
        .Permit(LetsGo, StateA)

External State Storage

Stateless is designed to be embedded in various application models. For example, some ORMs place requirements upon where mapped data may be stored, and UI frameworks often require state to be stored in special "bindable" properties. To this end, the StateMachine constructor can accept function arguments that will be used to read and write the state values:

var stateMachine = new StateMachine<State, Trigger>(
    () => myState.Value,
    s => myState.Value = s);

In this example the state machine will use the myState object for state storage.

Another example can be found in the JsonExample solution, located in the example folder.

Activation / Deactivation

It might be necessary to perform some code before storing the object state, and likewise when restoring the object state. Use Deactivate and Activate for this. Activation should only be called once before normal operation starts, and once before state storage.


The state machine can provide a list of the triggers that can be successfully fired within the current state via the StateMachine.PermittedTriggers property. Use StateMachine.GetInfo() to retrieve information about the state configuration.

Guard Clauses

The state machine will choose between multiple transitions based on guard clauses, e.g.:

    .PermitIf(Trigger.CallDialled, State.Ringing, () => IsValidNumber)
    .PermitIf(Trigger.CallDialled, State.Beeping, () => !IsValidNumber);

Guard clauses within a state must be mutually exclusive (multiple guard clauses cannot be valid at the same time.) Substates can override transitions by respecifying them, however substates cannot disallow transitions that are allowed by the superstate.

The guard clauses will be evaluated whenever a trigger is fired. Guards should therefore be made side effect free.

Parameterised Triggers

Strongly-typed parameters can be assigned to triggers:

var assignTrigger = stateMachine.SetTriggerParameters<string>(Trigger.Assign);

    .OnEntryFrom(assignTrigger, email => OnAssigned(email));

stateMachine.Fire(assignTrigger, "");

Trigger parameters can be used to dynamically select the destination state using the PermitDynamic() configuration method.

Ignored Transitions and Reentrant States

Firing a trigger that does not have an allowed transition associated with it will cause an exception to be thrown.

To ignore triggers within certain states, use the Ignore(TTrigger) directive:


Alternatively, a state can be marked reentrant so its entry and exit actions will fire even when transitioning from/to itself:

    .OnEntry(() => SendEmailToAssignee());

By default, triggers must be ignored explicitly. To override Stateless's default behaviour of throwing an exception when an unhandled trigger is fired, configure the state machine using the OnUnhandledTrigger method:

stateMachine.OnUnhandledTrigger((state, trigger) => { });

State change notifications (events)

Stateless supports 2 types of state machine events:

  • State transition
  • State machine transition completed

State transition

stateMachine.OnTransitioned((transition) => { });

This event will be invoked every time the state machine changes state.

State machine transition completed

stateMachine.OnTransitionCompleted((transition) => { });

This event will be invoked at the very end of the trigger handling, after the last entry action has been executed.

Export to DOT graph

It can be useful to visualize state machines on runtime. With this approach the code is the authoritative source and state diagrams are by-products which are always up to date.

    .PermitIf(Trigger.CallDialled, State.Ringing, IsValidNumber);
string graph = UmlDotGraph.Format(phoneCall.GetInfo());

The UmlDotGraph.Format() method returns a string representation of the state machine in the DOT graph language, e.g.:

digraph {
  OffHook -> Ringing [label="CallDialled [IsValidNumber]"];

This can then be rendered by tools that support the DOT graph language, such as the dot command line tool from or viz.js. See for instant gratification. Command line example: dot -T pdf -o phoneCall.pdf to generate a PDF file.

Async triggers

On platforms that provide Task<T>, the StateMachine supports async entry/exit actions and so on:

    .OnEntryAsync(async () => await SendEmailToAssignee());

Asynchronous handlers must be registered using the *Async() methods in these cases.

To fire a trigger that invokes asynchronous actions, the FireAsync() method must be used:

await stateMachine.FireAsync(Trigger.Assigned);

Note: while StateMachine may be used asynchronously, it remains single-threaded and may not be used concurrently by multiple threads.

Advanced Features

Retaining the SynchronizationContext

In specific situations where all handler methods must be invoked with the consumer's SynchronizationContext, set the RetainSynchronizationContext property on creation:

var stateMachine = new StateMachine<State, Trigger>(initialState)
    RetainSynchronizationContext = true

Setting this is vital within a Microsoft Orleans Grain for example, which requires the SynchronizationContext in order to make calls to other Grains.


Stateless runs on .NET runtime version 4+ and practically all modern .NET platforms by targeting .NET Framework 4.6.2, .NET Standard 2.0 and .NET 8.0. Visual Studio 2017 or later is required to build the solution.


We welcome contributions to this project. Check for more info.

Project Goals

This page is an almost-complete description of Stateless, and its explicit aim is to remain minimal.

Please use the issue tracker or the Discussions page if you'd like to report problems or discuss features.

(Why the name? Stateless implements the set of rules regarding state transitions, but, at least when the delegate version of the constructor is used, doesn't maintain any internal state itself.)