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Exercise 3: .NET Player Stats API Modernization with Custom Skill

Duration: 20 minutes
Difficulty: ⭐⭐ Easy
Prerequisites: VS Code, GitHub Copilot extension with app modernization tools enabled, .NET 8 SDK installed

The Challenge

The Player Stats API tracks player performance, rankings, match history, and achievements across all Game Arena Legends tournaments. Built with .NET Framework 4.8, it's struggling to keep up:

  • ASP.NET Web API 2 controllers with blocking I/O
  • Entity Framework 6 with slow LINQ queries
  • No support for modern minimal APIs
  • Cannot deploy to containers or cloud-native platforms
  • Security vulnerabilities in old .NET Framework

Your mission: Create a custom .NET Modernization skill, then use it to automatically migrate to .NET 8 with a single prompt.

Prerequisites Check

Install Github Copilot app modernization for dotnet. App Modernization Extension Check these tools are available (✓ indicates enabled) from configure tools:

  • appmod-dotnet-run-assessment
  • appmod-dotnet-build-project
  • appmod-dotnet-run-test
  • appmod-dotnet-cve-check

App Modernization Tools

Four-Step Modernization

Step 1: Create .NET Modernization Skill (5 minutes)

  1. Create skill directory from copilot chat settings: Skills folder structure

  2. Create .github/skills/dotnet-modernizer/SKILL.md and paste this content:

    ---
name: dotnet-modernizer
description: Analyzes .NET Framework applications and migrates them to .NET 8 using app modernization tools for assessment, building, testing, and CVE scanning.
---

# .NET Modernization Expert

You are a .NET modernization specialist that migrates legacy .NET Framework applications to modern .NET 8.

## Your Capabilities

You have access to app modernization tools:
- **appmod-dotnet-run-assessment**: Analyze .NET Framework code and assess migration readiness
- **appmod-dotnet-build-project**: Build .NET projects and verify compilation
- **appmod-dotnet-run-test**: Execute tests to validate functionality
- **appmod-dotnet-cve-check**: Scan for security vulnerabilities in NuGet packages

## Workflow

When asked to modernize a .NET application:

1. **Run assessment first** using `appmod-dotnet-run-assessment` to identify:
   - Current .NET Framework version and compatibility issues
   - Components that can be auto-migrated vs. need manual intervention
   - Migration complexity and effort estimates

2. **Execute migration** with modern .NET 8 patterns:
   - File-scoped namespaces and nullable reference types
   - Convert EF6 to EF Core 8 with async/await patterns
   - Transform ASP.NET Web API 2/MVC to minimal APIs
   - Replace Web.config with appsettings.json
   - Update all NuGet packages to .NET 8

3. **Validate the migration**:
   - Use `appmod-dotnet-build-project` for successful compilation (zero errors)
   - Use `appmod-dotnet-run-test` to verify all tests pass
   - Use `appmod-dotnet-cve-check` for security vulnerabilities
   - Confirm no .NET Framework dependencies remain

## Success Criteria

- Assessment completed with migration plan
- All code migrated to .NET 8 with modern patterns
- Build succeeds with zero errors
- All tests pass
- No critical security vulnerabilities

  3. Reload VS Code window (Ctrl+Shift+P → "Developer: Reload Window")

Step 2: Open Legacy Project (2 minutes)

Open the legacy codebase in VS Code:

cd legacy-code/dotnet-stats-api
code .

Step 3: Use the Skill with Prompt (1 minute)

Open Copilot Chat (Ctrl+Shift+I / Cmd+Shift+I) and use this prompt:

Using Skill #file:dotnet-modernizer  Modernize this .NET Framework 4.8 Player Stats API to .NET 8 with modern minimal APIs, Entity Framework Core 8, file-scoped namespaces, nullable reference types, and async/await patterns. Include build validation, test execution, and CVE checking.

Step 4: Expected Output (12 minutes)

The skill will automatically execute assessment, migration, and validation. Monitor the progress in Copilot Chat and verify:

  • Assessment report shows migration plan
  • Code changes applied (models, DbContext, Program.cs, appsettings.json)
  • Build output: zero compilation errors
  • Test output: all tests passing
  • CVE scan: no critical vulnerabilities

What You Learned

  • Creating custom Copilot skills with YAML frontmatter
  • Using app modernization tools via skills
  • Single-prompt workflow for complex migrations
  • Automated assessment, migration, and validation

Next Steps