Template Method Pattern Real-World Example in C#: Complete Implementation

A complete C# implementation of the Template Method design pattern applied to a real-world document processing pipeline. The pipeline handles PDF invoices, CSV reports, and XML data feeds, all sharing a fixed validate-parse-transform-audit sequence while each document type implements the steps differently. Covers the abstract base class with a sealed template method and hook methods, three concrete processor subclasses, a factory with dependency injection wiring, a resilience layer with retry logic and exponential backoff, and xUnit tests including a spy subclass that verifies step execution order. Also compares the pattern to Strategy, Decorator, and Composite patterns.