Designing Interface Hierarchies for Specialized Domains

In this lesson, you will learn how to structure interface hierarchies to mirror the complex, multi-layered requirements of specialized business domains.

Building Logical Taxonomies

When working in a specialized domain—like a banking system or a medical records database—you often encounter entities that share core traits but require highly specific behaviors. Instead of creating one "God interface" with dozens of methods, we use interface inheritance to build a taxonomy. This allows you to group related rules logically.

Think of it as moving from general responsibilities to specific capabilities. A ITransaction might define the basics, but a IBankTransfer or ICryptoTransaction would inherit from it to enforce domain-specific rules.

Structuring the Hierarchy

Consider a financial domain. Every entity needs an ID, but specialized transactions need validation rules.

public interface IEntity
{
    Guid Id { get; }
}

public interface ITransaction : IEntity
{
    decimal Amount { get; }
    DateTime Timestamp { get; }
}

public interface IInternationalTransaction : ITransaction
{
    string CurrencyCode { get; }
    string SwiftCode { get; }
}

By structuring the hierarchy this way, you ensure that any component expecting an ITransaction can handle a standard transfer, while services that need to process IInternationalTransaction have access to the SwiftCode without forcing every other transaction type to implement fields they don't need.

Why This Matters for Specialized Domains

Specialized domains are rarely static. Business rules evolve, and regulations change. By keeping your interfaces modular, you can update one branch of your hierarchy without breaking the entire codebase. When you design these trees, ask yourself: "Does this child interface add a necessary domain-specific constraint, or am I just grouping things by coincidence?"

If you find yourself casting interfaces frequently to check for types, it is a sign that your hierarchy isn't granular enough. By pushing specific behaviors into child interfaces, you keep your code clean, type-safe, and reflective of the actual business logic used by domain experts.

Key Takeaway: Design your interface hierarchies by moving from generic capabilities to specialized rules, ensuring each layer adds clear, meaningful context to your domain model.