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In this article, let's learn about State Design Pattern in .NET.

Table of Contents

1. Introduction
2. Structure
3. Use Cases
4. Advantages
5. Disadvantages
6. Related Patterns
7. Summary

Introduction

The State Pattern is a behavioral design pattern that lets an object alter its behavior when its state changes. Instead of using a large set of if-else or switch statements, state-specific behavior is delegated to separate state classes.

For example, consider a bank account that can be in a regular or overdrawn state. Each state has its own rules—when in an overdrawn state, withdrawals may be restricted. Adding more states, like a gold state for high balances, can make the logic even more complex. Using the state pattern helps to manage this complexity by organizing the behavior in individual classes.

Structure

The State pattern involves several key components:

1. Context: The main class that maintains the current state (e.g., BankAccount).
2. State: An abstract base class or interface that defines behaviors for different states (e.g., BankAccountState).
3. Concrete States: Implementations of the state that handle specific state behavior (e.g., RegularState, OverdrawnState).

In the above code, see how state changes automatically based on the balance. When the balance is less than zero, the account is in an Overdrawn state, and withdrawals are restricted. When the balance is greater than zero, the account is in a Regular state, and withdrawals are allowed.

Use Cases

The State pattern is beneficial in several scenarios:

1. Document editing: Different modes (insert, overwrite, select) with distinct behaviors.
2. Vending machine: Different states like idle, selecting, dispensing, and their associated actions.
3. Workflow management: Tasks change behavior based on whether they are "pending," "in progress," or "completed."
4. Robotics: Operational modes like "exploration" or "charging" that affect robot behavior.

Advantages

• Positive: Each state-specific behavior is isolated, making it easier to manage transitions and add new states.

Disadvantages

• Negative: Adding more states increases the number of classes, leading to more complexity.

Related Patterns

The State pattern is closely related to several other design patterns:

• Flyweight: State objects can sometimes be shared across contexts, reducing memory usage.
• Singleton: Often, state objects can be implemented as singletons since the state is shared.
• Strategy: Similar to the state pattern, but strategies don't depend on internal state changes; they are chosen externally.
• Bridge: Both patterns share the idea of composition to separate functionality, although they solve different problems.

Summary

The State Pattern allows you to manage complex state-dependent behaviors in a clean and scalable way. By encapsulating states in their own classes, you reduce the complexity of conditional logic and make the code easier to extend.