C# Academy · Lesson

Custom and Conditional Rules

Build complex, conditional validation logic.

Lesson 3 of 413 steps

Custom and Conditional Rules is a free C# Academy lesson on CoddyKit. This is lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

Beyond Built-In Validators

Real-world validation often needs custom logic: a value passing a domain check, or a rule that only applies under certain conditions. FluentValidation supports both with Must, When and custom validator types.

The Must Predicate

Must takes a predicate that returns true when the value is valid. It is the simplest way to express arbitrary logic inline.

RuleFor(c => c.Username)
    .Must(name => !name.Contains(" "))
    .WithMessage("Username cannot contain spaces.");

Must With Access To The Whole Object

An overload of Must gives you the parent object plus the property value, enabling cross-property checks.

RuleFor(o => o.DiscountedPrice)
    .Must((order, discounted) => discounted <= order.OriginalPrice)
    .WithMessage("Discounted price cannot exceed original price.");

Conditional Rules With When

When applies a rule only if a condition holds. Here, a shipping address is required only for physical orders.

RuleFor(o => o.ShippingAddress)
    .NotEmpty()
    .When(o => o.RequiresShipping);

The Unless Modifier

Unless is the inverse of When: the rule runs unless the condition is true.

RuleFor(o => o.CouponCode)
    .NotEmpty()
    .Unless(o => o.IsGift);

Grouping Conditions

Wrap several rules in a single When(...) block to share a condition cleanly across all of them.

When(o => o.RequiresShipping, () =>
{
    RuleFor(o => o.ShippingAddress).NotEmpty();
    RuleFor(o => o.PostalCode).NotEmpty();
});

Async Custom Logic With MustAsync

When your check needs the database or an API, use MustAsync. Validate the model with ValidateAsync.

RuleFor(u => u.Email)
    .MustAsync(async (email, cancellation) =>
        !await _repository.EmailExistsAsync(email))
    .WithMessage("Email is already registered.");

Writing A Reusable Custom Validator

For logic you reuse across projects, write an extension method on IRuleBuilder. This turns custom rules into first-class, chainable validators.

public static class CustomValidators
{
    public static IRuleBuilderOptions<T, string> ValidSlug<T>(
        this IRuleBuilder<T, string> rule)
    {
        return rule.Matches("^[a-z0-9-]+$")
            .WithMessage("Must be a lowercase slug.");
    }
}

Using The Custom Validator

Once defined, the extension chains like any built-in validator, keeping your validators expressive.

RuleFor(p => p.Slug).NotEmpty().ValidSlug();

The Custom Method For Full Control

For complex results, Custom lets you add validation failures imperatively using a context.

RuleFor(o => o.Items).Custom((items, context) =>
{
    if (items.Count == 0)
        context.AddFailure("An order must contain at least one item.");
});

Cascade Modes

By default all validators in a chain run even after one fails. Set CascadeMode.Stop to halt at the first failure, avoiding redundant or unsafe checks.

RuleFor(c => c.Email)
    .Cascade(CascadeMode.Stop)
    .NotEmpty()
    .EmailAddress();

Quick Check

Test conditional and custom rules.

Answer it in the interactive lesson

Recap

Custom and conditional rules make FluentValidation flexible: Must for inline predicates (with an overload exposing the whole object), When/Unless for conditions, grouped When blocks, MustAsync for I/O checks, reusable IRuleBuilder extensions, the imperative Custom method, and Cascade modes to control evaluation flow.

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Frequently Asked Questions

Is the “Custom and Conditional Rules” lesson free?

Yes — the full text of “Custom and Conditional Rules” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO. The C# Academy course includes 4 lessons in total.

What will I learn in “Custom and Conditional Rules”?

Build complex, conditional validation logic. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C# Academy?

No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

How long does the “Custom and Conditional Rules” lesson take?

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