DRY Principle

Overview

DRY means Do Not Repeat Yourself. Knowledge should have a single, authoritative representation in a codebase.

Why DRY Matters

Duplicate logic increases maintenance cost and bug risk. A change applied to one copy but missed in another introduces inconsistent behavior.

flowchart TD
    A[Duplicate Logic] --> B[Inconsistent Fixes]
    B --> C[Production Defects]
    D[Shared Abstraction] --> E[Single Update Point]
    E --> F[Consistent Behavior]

Practical Example

Validation duplicated across services:

// service A
if (!email.Contains("@")) throw new ArgumentException("Invalid email");

// service B
if (!email.Contains("@")) throw new ArgumentException("Invalid email");

Refactor into shared utility:

public static class EmailValidator
{
    public static void ValidateEmail(string email)
    {
        if (!email.Contains("@"))
        {
            throw new ArgumentException("Invalid email");
        }
    }
}

Production Guidance

• Share rules through libraries or domain modules
• Keep shared code versioned and documented
• Avoid coupling unrelated modules through oversized utility packages

Common Pitfalls

• Premature abstraction for patterns that are not stable
• Merging different concerns into one generic helper
• Ignoring domain boundaries while deduplicating

Key Takeaways

• DRY reduces inconsistency and maintenance overhead.
• Stable duplication signals a refactoring opportunity.
• Abstractions should reflect real domain commonality.