Business Rules

Learn how to define, compose, and evaluate business rules including invariants, validators, policies, and derivations.

Time: 60 minutes | Level: Intermediate

What You'll Learn

Invariants (always-true rules)
Validators (context-dependent)
Policies (authorization rules)
Derivations (computed properties)
Rule composition and reuse
Conditional rules with When

Topics Covered

Invariants

Define always-true rules:

new RuleSetBuilder<Customer>("Default")
    .Invariant("Email.Required", c => !string.IsNullOrWhiteSpace(c.Email))
    .WithMessage("Email is required")

Validators

Context-dependent validation:

.Validator("Email.Unique", async (c, ctx) =>
    await IsEmailUniqueAsync(c.Email))
.WithMessage("Email already exists")

Policies

Authorization and business policies:

.Policy("CanPlaceOrder", (c, ctx) =>
    c.IsActive && c.CreditLimit > 0)
.WithMessage("Customer cannot place orders")

Derivations

Computed properties:

.Derivation("FullName", c => $"{c.FirstName} {c.LastName}")

Rule Composition

Reuse and combine rules:

.Include(BaseCustomerRules.Default())
.When(c => c.IsPremium)

Prerequisites

Completion of Domain Modeling
Understanding of validation concepts

Table of Contents