Blog / Design Patterns
The State Design Pattern in C# and .Net
  • Jan 04, 2021
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The next design pattern in the Behavioral Category is the State Design Pattern. Let's take a look at the definition of this design pattern and how to implement it in C# and .Net.

Note: Code can be downloaded at my Github.


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1. What is the State Design Pattern?

State is a behavioral design pattern that lets an object alter its behavior when its internal state changes. It appears as if the object changed its class.

The state pattern is a behavioral software design pattern that allows an object to alter its behavior when its internal state changes. This pattern is close to the concept of finite-state machines.


2. When to implement the State Design Pattern?

The State Design Pattern should be implemented when:

  • you have an object that behaves differently depending on its current state, the number of states is enormous, and the state-specific code changes frequently.
  • you have a class polluted with massive conditionals that alter how the class behaves according to the current values of the class’s fields.
  • you have a lot of duplicate code across similar states and transitions of a condition-based state machine.


3. How to implement the State Design Pattern?

Below is how to implement the State Design Pattern in C# and .Net:

Step 1: Create a Context class

The Context defines the interface of interest to clients. It also maintains a reference to an instance of a State subclass, which represents the current state of the Context.

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class Context
{
    // A reference to the current state of the Context.
    private State _state = null;
 
    public Context(State state)
    {
        this.TransitionTo(state);
    }
 
    // The Context allows changing the State object at runtime.
    public void TransitionTo(State state)
    {
        Console.WriteLine($"Context: Transition to {state.GetType().Name}.");
        this._state = state;
        this._state.SetContext(this);
    }
 
    // The Context delegates part of its behavior to the current State
    // object.
    public void Request1()
    {
        this._state.Handle1();
    }
 
    public void Request2()
    {
        this._state.Handle2();
    }
}


Step 2: Create State classes

The base State class declares methods that all Concrete State should implement and also provides a backreference to the Context object, associated with the State. This backreference can be used by States to transition the Context to another State.

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abstract class State
{
    protected Context _context;
 
    public void SetContext(Context context)
    {
        this._context = context;
    }
 
    public abstract void Handle1();
 
    public abstract void Handle2();
}

Concrete States implement various behaviors, associated with a state of the Context.

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class ConcreteStateA : State
{
    public override void Handle1()
    {
        Console.WriteLine("ConcreteStateA handles request1.");
        Console.WriteLine("ConcreteStateA wants to change the state of the context.");
        this._context.TransitionTo(new ConcreteStateB());
    }
 
    public override void Handle2()
    {
        Console.WriteLine("ConcreteStateA handles request2.");
    }
}
 
class ConcreteStateB : State
{
    public override void Handle1()
    {
        Console.Write("ConcreteStateB handles request1.");
    }
 
    public override void Handle2()
    {
        Console.WriteLine("ConcreteStateB handles request2.");
        Console.WriteLine("ConcreteStateB wants to change the state of the context.");
        this._context.TransitionTo(new ConcreteStateA());
    }
}

Done, you can now implement this in client:

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class Program
{
    static void Main(string[] args)
    {
        // The client code.
        var context = new Context(new ConcreteStateA());
        context.Request1();
        context.Request2();
    }
}

Output:



4. Conclusion

I hope this article is helpful for your projects. Please let me know your thoughts by commenting in the section below.


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