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GitHub Copilot Train-the-Trainer
Overview

Session M2-07: Migrate a legacy Java service to modern .NET

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Module: Module 2: Copilot in Practice

Difficulty: Advanced

Prerequisites: Sessions M1-01 to M1-03, M2-01 to M2-04

Duration: 3 hours (1 hour trainer content + 2 hours lab)

Overview

Migrate one complete order-pricing slice from a Java 8-compatible HTTP service to a .NET 10 ASP.NET Core Minimal API. Run the legacy service, trace its request path, record the behavior that must survive, then implement the same contract in .NET and prove parity with shared JSON cases.

The exercise uses synthetic orders and integer-cent arithmetic. It needs no database, cloud service, or production data.

Important

Check GitHub Copilot access in the selected coding environment before the session. If access is unavailable, stop and do not start the lab. Resolve access first.

Learning outcomes

  • map a legacy request from HTTP entry point to domain calculation and response;
  • separate observed behavior from incidental Java structure;
  • capture pricing rules in shared contract fixtures;
  • migrate one vertical slice to a .NET 10 Minimal API;
  • preserve rounding, validation, shipping, discount, and tax behavior;
  • limit Copilot prompts to the pricing slice and review every generated change;
  • compare Java and .NET results with repeatable tests;
  • create a migration backlog without turning the exercise into a full rewrite.

Scenario

The legacy service prices synthetic orders through POST /orders/price.

The rules are small enough to inspect. Pay attention to the details that can break a migration:

  • discounts run in a fixed order;
  • all amounts use integer cents;
  • percentage calculations round half up;
  • free shipping uses the discounted merchandise amount;
  • tax applies to discounted merchandise, not shipping;
  • invalid enum-like values and line items return clear errors.

You will migrate this slice. Leave the health and sample-order endpoints in Java and record them as backlog items.

Session structure

Block Duration Work
Trainer content 1 hour Migration boundaries, behavior capture, contract tests, .NET mapping, and prepared demo
Lab 2 hours Inspect the Java service, implement the .NET slice, prove parity, and write the next backlog

Materials

Resource Location
Trainer guide trainer-content/README.md
Marp slides slides.md
Lab guide lab/README.md
Java baseline, fixtures, and .NET scaffold lab/starter/
.NET reference implementation and contract tests lab/solution/

Environment

Use the repository Dev Container or Codespaces. It supplies Java 21, which compiles the legacy source for Java 8, and the .NET 10 SDK. The Maven Wrapper pins Maven 3.9.9 for the baseline.

The lab also includes a short native setup route for learners who cannot use the container. A global Maven installation is not required.

Scope

In scope

  • local HTTP services;
  • synthetic JSON;
  • behavior extraction;
  • one pricing endpoint with validation, calculation, and response handling;
  • package-free .NET contract tests;
  • a reviewed migration backlog.

Out of scope

  • databases and persistence migration;
  • messaging and distributed transactions;
  • authentication and authorization;
  • cloud hosting;
  • a whole-service rewrite;
  • production performance claims.