Authoring rules

Fluent definitions

Inherit ValidatorDefinition<T> and override Configure:

[ValiantValidator]
public sealed partial class ProductValidator : ValidatorDefinition<Product>
{
    public override void Configure(ValidationContext<Product> context)
    {
        context.Property(product => product.Name)
            .Required()
            .LengthBetween(3, 80);

        context.Property(product => product.Price)
            .GreaterThan(0m);
    }
}

The instance method also works naturally with constructor-injected dependencies and reusable validators.

Attribute definitions

Derive from ValidatorDefinition<T> to read Valiant validation attributes from the model:

[ValiantValidator]
public sealed partial class ProductValidator : ValidatorDefinition<Product>;

public sealed class Product
{
    [Required("NameRequired", "Enter a product name.")]
    [LengthBetween(3, 80)]
    public string? Name { get; init; }

    [GreaterThan(0)]
    public decimal Price { get; init; }
}

These are attributes from Valiant.Validation, not Data Annotations. Fluent and attribute rules use the same generated checks and metadata.

Custom predicates

Use Satisfies for a rule that is clearer as code:

context.Property(order => order.Reference)
    .Satisfies(
        value => value is null || value.StartsWith("ORD-", StringComparison.Ordinal),
        code: "ReferenceFormat",
        message: "Reference must start with ORD-.");

Async predicate overloads are available for work that accepts a cancellation token.

Conditions

Apply a condition to a property chain:

context.Property(request => request.InviteCode)
    .Required()
    .When(request => request.RequiresInvite);

For larger branches, use normal C# control flow. See Composition and control flow.

Codes, messages, and severity

Every built-in check accepts optional code, message, and severity arguments:

context.Property(user => user.DisplayName)
    .MaxLength(
        40,
        code: "DisplayNameLong",
        message: "Display name is longer than 40 characters.",
        severity: ValiantSeverity.Warning);

ValidationResult.Errors determine IsValid. Warnings and Infos are preserved separately for callers that want non-blocking feedback.