use-case-2.0（用例 2 0）

Use Case 2.0

A modern, lightweight approach to use case modeling that integrates with agile practices while preserving the benefits of structured requirements.

When to Use This Skill

Keywords: use case, use case 2.0, Ivar Jacobson, use case slice, narrative, basic flow, alternate flow, exception, actor, goal, scenario, precondition, postcondition, trigger, user story from use case

Use this skill when:

• Modeling system behavior from user perspective
• Deriving user stories from use cases
• Planning releases using use case slices
• Documenting complex interactions with multiple paths
• Understanding actor goals and system responses
• Creating testable requirements from narratives

What is Use Case 2.0?

Use Case 2.0 modernizes classic use cases for agile contexts while retaining their power for:

• Capturing functional requirements comprehensively
• Understanding system boundaries and actors
• Identifying all paths through a scenario (happy path + alternatives + exceptions)
• Creating traceable, testable specifications

Key Principles

Use Case 2.0 Lifecycle

┌──────────────┐    ┌──────────────┐    ┌──────────────┐
│   FIND       │ →  │   SLICE      │ →  │   PREPARE    │
│ Use Cases    │    │ Use Cases    │    │ Use Case     │
│              │    │              │    │ Slices       │
└──────────────┘    └──────────────┘    └──────────────┘
                                               │
                    ┌──────────────┐    ┌──────┴───────┐
                    │   INSPECT    │ ←  │   ANALYZE    │
                    │   & ADAPT    │    │   Slices     │
                    └──────────────┘    └──────────────┘

Phase Details

use_case_lifecycle:
  find:
    purpose: "Identify use cases from goals and actors"
    activities:
      - "Identify actors (primary, supporting, offstage)"
      - "Capture actor goals"
      - "Name use cases (verb-noun)"
      - "Create use case diagram"
    output: "Use case model (diagram + brief descriptions)"

  slice:
    purpose: "Break use cases into implementable slices"
    activities:
      - "Identify basic flow (happy path)"
      - "Identify alternative flows"
      - "Identify exception flows"
      - "Group flows into slices"
    output: "Sliced use cases with prioritization"

  prepare:
    purpose: "Detail slices for implementation"
    activities:
      - "Write narrative for selected slice"
      - "Define test cases"
      - "Identify special requirements"
    output: "Ready-for-development slice"

  analyze:
    purpose: "Verify slice is implementable"
    activities:
      - "Review with stakeholders"
      - "Validate test cases"
      - "Confirm acceptance criteria"
    output: "Validated slice"

  inspect_adapt:
    purpose: "Learn and improve"
    activities:
      - "Review implemented slices"
      - "Update use case model"
      - "Refine remaining slices"
    output: "Updated backlog"

Use Case Elements

Actor Types

actor_types:
  primary_actor:
    definition: "Actor whose goal the use case satisfies"
    examples:
      - "Customer placing an order"
      - "Administrator managing users"
    notation: "Stick figure connected to use case"

  supporting_actor:
    definition: "Actor that provides a service to the system"
    examples:
      - "Payment Gateway processing payment"
      - "Email Service sending notifications"
    notation: "Stick figure connected to use case (labeled)"

  offstage_actor:
    definition: "Actor with interest but no direct interaction"
    examples:
      - "Auditor requiring transaction logs"
      - "Regulator requiring compliance"
    notation: "Listed in stakeholders section"

Use Case Brief

Lightweight initial documentation:

use_case_brief:
  template:
    id: "UC-{domain}-{number}"
    name: "{Verb} {Noun}"
    primary_actor: "{Actor name}"
    goal: "{What the actor wants to achieve}"
    brief_description: "{1-2 sentences}"

  example:
    id: "UC-ORD-001"
    name: "Place Order"
    primary_actor: "Customer"
    goal: "Purchase products from the catalog"
    brief_description: "Customer selects products, provides shipping and payment information, and submits order for processing."

Use Case Narrative

Full documentation format:

## Use Case: {Name}

**ID:** UC-{XXX}-{NNN}
**Version:** {N.N}

### Overview

| Element | Description |
|---------|-------------|
| **Primary Actor** | {Actor name} |
| **Goal** | {What the actor wants to achieve} |
| **Scope** | {System boundary} |
| **Level** | {User goal / Subfunction / Summary} |
| **Trigger** | {What starts the use case} |

### Stakeholders and Interests

- **{Stakeholder 1}**: {Interest/concern}
- **{Stakeholder 2}**: {Interest/concern}

### Preconditions

- {Condition that must be true before use case can start}
- {Another precondition}

### Success Guarantee (Postconditions)

- {Condition guaranteed to be true after successful completion}
- {Another postcondition}

### Basic Flow (Main Success Scenario)

1. {Actor} {action}
2. System {response}
3. {Actor} {action}
4. System {response}
5. ...
6. System {final response indicating success}

### Alternative Flows

#### {Alternative Name} (at step {N})

**Condition:** {When this alternative applies}

{N}a. {Alternative action}
{N}b. System {alternative response}
{N}c. Return to step {M} / Use case ends

#### {Another Alternative} (at step {N})

...

### Exception Flows

#### {Exception Name} (at step {N})

**Condition:** {When this exception occurs}

{N}a. System {error detection}
{N}b. System {error handling}
{N}c. Use case ends in failure / Return to step {M}

### Special Requirements

- {Non-functional requirement affecting this use case}
- {Performance, security, usability requirement}

### Technology and Data Variations

- {Step N}: {Variation in technology or data format}

### Related Information

- **Frequency:** {How often this use case occurs}
- **Related Use Cases:** {Links to included/extended use cases}
- **Business Rules:** {BR-xxx, BR-yyy}

Use Case Slicing

What is a Slice?

use_case_slice:
  definition: "A subset of a use case that delivers value and is independently testable"

  characteristics:
    - "Implements part of use case flows"
    - "Can be developed in one iteration"
    - "Has clear acceptance criteria"
    - "Delivers incremental value"

  slice_types:
    basic_flow_slice:
      description: "Happy path only (simplest implementation)"
      example: "Place Order - basic checkout with existing customer"

    flow_variation_slice:
      description: "Basic flow + one alternative"
      example: "Place Order - with new customer registration"

    exception_slice:
      description: "Basic flow + exception handling"
      example: "Place Order - payment declined handling"

    complete_slice:
      description: "Full use case with all paths"
      example: "Place Order - complete implementation"

Slicing Strategies

slicing_strategies:
  by_actor:
    description: "Different slices for different actors"
    example: "Customer checkout vs. Admin override checkout"

  by_data_variation:
    description: "Different data types or volumes"
    example: "Single item order vs. bulk order"

  by_business_rule:
    description: "Different rules applied"
    example: "Standard pricing vs. promotional pricing"

  by_interface:
    description: "Different UI or integration points"
    example: "Web checkout vs. API checkout"

  by_quality:
    description: "Different quality attributes"
    example: "Basic validation vs. full validation"

Slice Template

use_case_slice:
  id: "UC-{XXX}-{NNN}-S{NN}"
  use_case: "UC-{XXX}-{NNN}"
  name: "{Use Case Name} - {Slice Description}"

  scope:
    flows_included:
      - "Basic flow steps 1-6"
      - "Alternative 3a (guest checkout)"
    flows_excluded:
      - "Alternative 2a (saved addresses)"
      - "Exception 5a (payment failure)"

  value_statement: "{What this slice enables for the user}"

  acceptance_criteria:
    - given: "{Precondition}"
      when: "{Action}"
      then: "{Expected outcome}"

  test_scenarios:
    - "Happy path with valid data"
    - "Guest checkout without account"

  estimate: "{Story points or ideal days}"
  priority: "{Value/Risk ranking}"

User Story Derivation

Convert use case slices to user stories:

slice_to_story:
  pattern: "Each slice becomes one or more user stories"

  mapping:
    use_case_name: "Epic name"
    slice: "User story"
    acceptance_criteria: "Acceptance criteria"
    test_scenarios: "Test cases"

  example:
    use_case: "Place Order"
    slices:
      - slice: "Basic checkout"
        story: |
          As a Customer
          I want to place an order with items in my cart
          So that I can receive my purchased products

      - slice: "Guest checkout"
        story: |
          As a Guest
          I want to checkout without creating an account
          So that I can complete my purchase quickly

      - slice: "Payment retry"
        story: |
          As a Customer
          I want to retry payment if it fails
          So that I can complete my order without starting over

Story Mapping from Use Cases

Use Case Model              →    Story Map
─────────────────                ─────────────────
UC: Place Order                  Epic: Order Placement
├── Basic Flow               →   ├── Basic Checkout
├── Alt: Guest Checkout      →   ├── Guest Checkout
├── Alt: Saved Address       →   ├── Address Management
├── Alt: Gift Wrapping       →   ├── Gift Options
├── Exc: Payment Failed      →   ├── Payment Error Handling
└── Exc: Out of Stock        →   └── Inventory Handling

Use Case Diagram

UML Notation

┌─────────────────────────────────────────────────────────────────┐
│                    Order Management System                       │
│  ┌────────────────────────────────────────────────────────────┐ │
│  │                                                            │ │
│  │  ╭──────────────╮       ╭─────────────────╮               │ │
│  │  │ Place Order  │───────│ Process Payment │               │ │
│  │  ╰──────────────╯       ╰─────────────────╯               │ │
│  │         │                        │                         │ │
│  │         │ «include»              │ «include»               │ │
│  │         ▼                        ▼                         │ │
│  │  ╭──────────────╮       ╭─────────────────╮               │ │
│  │  │ Select Items │       │ Validate Card   │               │ │
│  │  ╰──────────────╯       ╰─────────────────╯               │ │
│  │                                                            │ │
│  │  ╭──────────────╮                                         │ │
│  │  │ Cancel Order │                                         │ │
│  │  ╰──────────────╯                                         │ │
│  │         ▲                                                  │ │
│  │         │ «extend»                                         │ │
│  │         │ [within cancellation window]                     │ │
│  │  ╭──────────────╮                                         │ │
│  │  │ View Order   │                                         │ │
│  │  ╰──────────────╯                                         │ │
│  │                                                            │ │
│  └────────────────────────────────────────────────────────────┘ │
└─────────────────────────────────────────────────────────────────┘

     ○                                               ○
    /|\  ────────── Place Order                     /|\  ────────── Process Payment
    / \                                             / \
  Customer                                    Payment Gateway

Relationships

use_case_relationships:
  include:
    notation: "«include»"
    meaning: "Base use case ALWAYS includes the behavior"
    example: "Place Order «include» Process Payment"
    arrow: "Dashed arrow from base to included"

  extend:
    notation: "«extend»"
    meaning: "Extension use case MAY add behavior under conditions"
    example: "View Order «extend» Cancel Order [within window]"
    arrow: "Dashed arrow from extension to base"

  generalization:
    notation: "Triangle arrow"
    meaning: "Child use case inherits from parent"
    example: "Pay by Credit Card inherits from Make Payment"

.NET/C# Implementation

Use Case Model

// Domain model for Use Case 2.0
namespace Requirements.UseCases;

public record UseCase
{
    public required string Id { get; init; }
    public required string Name { get; init; }
    public required Actor PrimaryActor { get; init; }
    public required string Goal { get; init; }
    public required UseCaseLevel Level { get; init; }
    public string? Trigger { get; init; }
    public string? Scope { get; init; }

    public List<Stakeholder> Stakeholders { get; init; } = [];
    public List<string> Preconditions { get; init; } = [];
    public List<string> Postconditions { get; init; } = [];
    public List<FlowStep> BasicFlow { get; init; } = [];
    public List<AlternativeFlow> AlternativeFlows { get; init; } = [];
    public List<ExceptionFlow> ExceptionFlows { get; init; } = [];
    public List<string> SpecialRequirements { get; init; } = [];
    public List<UseCaseSlice> Slices { get; init; } = [];
}

public enum UseCaseLevel
{
    Summary,      // High-level business process
    UserGoal,     // Primary level - user achieves goal
    Subfunction   // Supporting functionality
}

public record Actor(
    string Name,
    ActorType Type,
    string? Description = null
);

public enum ActorType
{
    Primary,
    Supporting,
    Offstage
}

public record FlowStep(
    int StepNumber,
    string ActorOrSystem,
    string Action
);

public record AlternativeFlow(
    string Name,
    int BranchAtStep,
    string Condition,
    List<FlowStep> Steps,
    FlowReturn Return
);

public record ExceptionFlow(
    string Name,
    int OccursAtStep,
    string Condition,
    List<FlowStep> Steps,
    bool EndsInFailure
);

public record FlowReturn(
    FlowReturnType Type,
    int? ReturnToStep = null
);

public enum FlowReturnType
{
    ReturnToBasicFlow,
    EndUseCase,
    EndInFailure
}

Use Case Slice Model

public record UseCaseSlice
{
    public required string Id { get; init; }
    public required string UseCaseId { get; init; }
    public required string Name { get; init; }
    public required string ValueStatement { get; init; }

    public List<string> IncludedFlows { get; init; } = [];
    public List<string> ExcludedFlows { get; init; } = [];
    public List<AcceptanceCriterion> AcceptanceCriteria { get; init; } = [];
    public List<string> TestScenarios { get; init; } = [];

    public int? StoryPoints { get; init; }
    public SlicePriority Priority { get; init; }
    public SliceStatus Status { get; init; }
}

public record AcceptanceCriterion(
    string Given,
    string When,
    string Then
);

public enum SlicePriority
{
    MustHave,
    ShouldHave,
    CouldHave,
    WontHave
}

public enum SliceStatus
{
    Identified,
    Prepared,
    Analyzed,
    Implementing,
    Done
}

Slice to User Story Converter

public class SliceToStoryConverter
{
    public UserStory Convert(UseCaseSlice slice, UseCase useCase)
    {
        return new UserStory
        {
            Id = $"US-{slice.Id}",
            Title = slice.Name,
            Narrative = GenerateNarrative(slice, useCase),
            AcceptanceCriteria = slice.AcceptanceCriteria
                .Select(ac => $"Given {ac.Given}, When {ac.When}, Then {ac.Then}")
                .ToList(),
            Epic = useCase.Name,
            StoryPoints = slice.StoryPoints,
            Priority = MapPriority(slice.Priority)
        };
    }

    private string GenerateNarrative(UseCaseSlice slice, UseCase useCase)
    {
        return $"""
            As a {useCase.PrimaryActor.Name}
            I want to {ExtractAction(slice, useCase)}
            So that {useCase.Goal}
            """;
    }

    private string ExtractAction(UseCaseSlice slice, UseCase useCase)
    {
        // Extract the main action from included flows
        var mainFlow = slice.IncludedFlows.FirstOrDefault() ?? useCase.Name.ToLowerInvariant();
        return mainFlow.Replace("Basic flow", useCase.Name.ToLowerInvariant());
    }

    private UserStoryPriority MapPriority(SlicePriority priority) =>
        priority switch
        {
            SlicePriority.MustHave => UserStoryPriority.Critical,
            SlicePriority.ShouldHave => UserStoryPriority.High,
            SlicePriority.CouldHave => UserStoryPriority.Medium,
            SlicePriority.WontHave => UserStoryPriority.Low,
            _ => UserStoryPriority.Medium
        };
}

public record UserStory
{
    public required string Id { get; init; }
    public required string Title { get; init; }
    public required string Narrative { get; init; }
    public List<string> AcceptanceCriteria { get; init; } = [];
    public string? Epic { get; init; }
    public int? StoryPoints { get; init; }
    public UserStoryPriority Priority { get; init; }
}

public enum UserStoryPriority
{
    Critical,
    High,
    Medium,
    Low
}

Best Practices

Writing Good Use Cases

naming:
  pattern: "Verb + Noun"
  good_examples:
    - "Place Order"
    - "Register Customer"
    - "Generate Report"
  bad_examples:
    - "Order" # No verb
    - "Customer Registration System" # System, not goal
    - "Handle Click" # Too low-level

flow_writing:
  guidelines:
    - "Write in active voice"
    - "Actor or System starts each step"
    - "One observable action per step"
    - "Number steps sequentially"
    - "Keep steps at same abstraction level"

  good_steps:
    - "1. Customer enters shipping address"
    - "2. System validates address format"
    - "3. System calculates shipping options"
    - "4. Customer selects shipping method"

  bad_steps:
    - "1. Click submit button" # Too detailed
    - "2. Stuff happens" # Too vague
    - "3. The system processes" # No outcome

level_selection:
  user_goal: "Primary level - use most often"
  summary: "For business process overview"
  subfunction: "For shared functionality (include targets)"

Slicing Guidelines

slice_sizing:
  ideal: "1-5 story points per slice"
  too_big: "Cannot complete in one iteration"
  too_small: "No incremental value delivered"

prioritization:
  value_first: "Basic flow slice before alternatives"
  risk_reduction: "Risky slices early for learning"
  dependency_aware: "Prerequisites before dependents"

coverage:
  minimal_viable: "Basic flow slice = MVP"
  enhanced: "Add high-value alternatives"
  complete: "All flows including exceptions"

Integration with Other Skills

Upstream

• stakeholder-simulation - Identify actors and goals
• interview-conducting - Elicit use case details
• gap-analysis - Find missing use cases

Downstream

• user-story-mapping - Derive stories from slices
• prioritization-methods - Prioritize slices
• business-rules-analysis - Extract rules from flows

Related Skills

• ears-authoring (spec-driven-development) - Convert to EARS format
• gherkin-authoring (spec-driven-development) - Convert to Gherkin scenarios

Output Format

Use Case Catalog

use_case_catalog:
  project: "{Project Name}"
  version: "1.0"
  last_updated: "{ISO-8601}"

  actors:
    - name: "Customer"
      type: "primary"
      description: "End user purchasing products"

    - name: "Payment Gateway"
      type: "supporting"
      description: "External payment processor"

  use_cases:
    - id: "UC-ORD-001"
      name: "Place Order"
      actor: "Customer"
      goal: "Purchase products from catalog"
      level: "UserGoal"
      slices:
        - id: "UC-ORD-001-S01"
          name: "Basic Checkout"
          status: "Done"
          priority: "MustHave"

        - id: "UC-ORD-001-S02"
          name: "Guest Checkout"
          status: "Implementing"
          priority: "ShouldHave"

  coverage:
    total_use_cases: 12
    total_slices: 34
    slices_done: 18
    slices_remaining: 16

References

For additional guidance:

• Flow Writing Examples
• Slicing Patterns

Version History

• v1.0.0 (2025-12-26): Initial release - Use Case 2.0 skill

Last Updated: 2025-12-26