State Example .NET 5

StateExample.cs

using System;

try
{
    Client server = new (new Idle());
    server.Click();
    server.Success();
    server.Click();
    server.Failure();
    server.Retry();
    server.Retry();
}
catch (Exception e)
{
    Console.WriteLine(e.Message);
}


class Client
{
    public IApiState State { get; set; }

    public Client(IApiState _state)
    {
        State = _state;
        OnStateChange();
    }

    private void OnStateChange() => Console.WriteLine($"New state is {State.GetType()}");

    public void Click()
    {
        State.Click(this);
        OnStateChange();
    }
    public void Failure()
    {
        State.Failure(this);
        OnStateChange();
    }
    public void Retry()
    {
        State.Retry(this);
        OnStateChange();
    }
    public void Success()
    {
        State.Success(this);
        OnStateChange();
    }
}

class Utils
{
    public static Exception IncorrectMoveException(IApiState _state) => new($"Incorrect move for state {_state.GetType()}");
}

interface IApiState
{
    void Click(Client _client);
    void Failure(Client _client);
    void Retry(Client _client);
    void Success(Client _client);
}

class Idle : IApiState
{
    public void Click(Client _client) => _client.State = new Fetching();
    public void Failure(Client _client) => throw Utils.IncorrectMoveException(this);
    public void Retry(Client _client) => throw Utils.IncorrectMoveException(this);
    public void Success(Client _client) => throw Utils.IncorrectMoveException(this);
}

class Fetching : IApiState
{
    public void Click(Client _client) => throw Utils.IncorrectMoveException(this);
    public void Failure(Client _client) => _client.State = new Error();
    public void Retry(Client _client) => throw Utils.IncorrectMoveException(this);
    public void Success(Client _client) => _client.State = new Idle();
}

class Error : IApiState
{
    public void Click(Client _client) => throw Utils.IncorrectMoveException(this);
    public void Failure(Client _client) => throw Utils.IncorrectMoveException(this);
    public void Retry(Client _client) => _client.State = new Fetching();
    public void Success(Client _client) => throw Utils.IncorrectMoveException(this);

}