Composition and control flow

Write normal C# branches

Validator configuration supports native if/else, switch, loops, and foreach:

public void Configure(ValidationContext<CreateOrder> context)
{
    if (context.Instance.RequiresApproval)
    {
        context.Property(order => order.ApproverId).Required();
    }

    switch (context.Instance.Channel)
    {
        case SalesChannel.Store:
            context.Property(order => order.StoreId).Required();
            break;
        case SalesChannel.Web:
            context.Property(order => order.SessionId).Required();
            break;
    }
}

In Debuggable mode this method executes at runtime, so ordinary breakpoints and step-through debugging work.

Nested objects

Use Children to validate a nested context and retain the complete property path:

context.Property(order => order.Customer)
    .Required()
    .Children(customer =>
    {
        customer.Property(value => value.Name).Required();
        customer.Property(value => value.Email).MaxLength(200);
    });

Collections

Use ForEach for element rules. context.Item is the current item inside the nested validation context:

context.Property(order => order.Lines)
    .MinCount(1)
    .ForEach(line =>
    {
        line.Property(item => item.Sku).Required();
        line.Property(item => item.Quantity).GreaterThan(0);
    });

Failures include indexed paths such as Lines[0].Sku.

Reuse a root validator

public sealed partial class CheckoutValidator(AddressValidator addressValidator)
    : ValidatorDefinition<Checkout>
{
    public override void Configure(ValidationContext<Checkout> context)
    {
        context.UseValidator(addressValidator);
        context.Property(value => value.AcceptedTerms).Equal(true);
    }
}

Reuse a property validator

Create a reusable PropertyValidatorDefinition<TProperty> and attach it to a property:

context.Property(order => order.ShippingAddress)
    .Required()
    .UseValidator(addressPropertyValidator);

Property validators skip null values. Add Required() or NotNull() when absence itself is invalid. Attribute-authored models can use [UseValidator<TValidator>].

Failure behavior

Configure collection globally, then override it at validator or property-chain scope:

services.AddValiantValidators(options =>
    options.FailureBehavior = ValiantFailureBehavior.ValidateAll);

Use FailFast to stop within the configured scope after the first failure; use ValidateAll to collect all applicable failures. Narrower configuration overrides the global option.