Compose Rules

Combine and reuse multiple rule sets.

Goal

Learn how to compose rule sets using Include and When for reusability.

Prerequisites

• JD.Domain.Rules package installed
• Understanding of rule types

Steps

1. Include Base Rules

// Base rules
var baseRules = new RuleSetBuilder<Customer>("Base")
    .Invariant("Name.Required", c => !string.IsNullOrWhiteSpace(c.Name))
    .Build();

// Extended rules
var extendedRules = new RuleSetBuilder<Customer>("Extended")
    .Include(baseRules)
    .Invariant("Email.Required", c => !string.IsNullOrWhiteSpace(c.Email))
    .Build();

2. Conditional Rules with When

var rules = new RuleSetBuilder<Customer>("Default")
    .Invariant("Email.Required", c => !string.IsNullOrWhiteSpace(c.Email))
    .When(c => c.IsPremiumCustomer)
    .WithMessage("Premium customers must have an email")
    
    .Invariant("Phone.Required", c => !string.IsNullOrWhiteSpace(c.Phone))
    .When(c => c.RequiresPhoneVerification)
    .WithMessage("Phone number is required for verification")
    
    .Build();

3. Rule Set Families

public static class CustomerRules
{
    public static RuleSetManifest Minimal() => 
        new RuleSetBuilder<Customer>("Minimal")
            .Invariant("Name.Required", c => !string.IsNullOrWhiteSpace(c.Name))
            .Build();
    
    public static RuleSetManifest Standard() =>
        new RuleSetBuilder<Customer>("Standard")
            .Include(Minimal())
            .Invariant("Email.Required", c => !string.IsNullOrWhiteSpace(c.Email))
            .Build();
    
    public static RuleSetManifest Premium() =>
        new RuleSetBuilder<Customer>("Premium")
            .Include(Standard())
            .Invariant("Phone.Required", c => !string.IsNullOrWhiteSpace(c.Phone))
            .Build();
}

Result

Rule composition enables:

• DRY (Don't Repeat Yourself) principle
• Progressive validation levels
• Context-specific rule application

Next Steps

• Validate in ASP.NET - Use composed rules in APIs

See Also

• Business Rules Tutorial