How to reuse data in multiple rules using FluentValidation

In web projects, I often use FluentValidation to validate models in .NET Core backend. Not only is it a good way to validate models, it also kind of forces the developer to separate validation from other logic.

However, when using FluentValidation I always find myself in a situation where I need to use the same external value in multiple validation rules. By external I mean not in the current context. This can be some value from the database or from a rest API, and it would be expensive to fetch this value for every evaluated rule.

In order to resolve this, we can make use of a lifecycle event - PreValidate. This method executes before all rules are evaluated, which enables us to pre-cache any expensive lookup.

Let's look into some code. This is a validator that validates a order number provided as a string. It will validate that the order exists and that its status is at least delivered.

public class ReturnOrderValidator : AbstractValidator<string>
{
    private const string OrderContextKey = nameof(OrderContextKey);

    private readonly IPurchaseOrderRepository _purchaseOrderRepository;

    public ReturnOrderValidator(IPurchaseOrderRepository purchaseOrderRepository)
    {
        _purchaseOrderRepository = purchaseOrderRepository;

        RuleFor(x => x)
            .Must((model, _, context) => context.GetContext<IPurchaseOrder>(OrderContextKey) != null)
                .WithMessage("Order was not found.")
            .Must((model, _, context) => context.GetContext<IPurchaseOrder>(OrderContextKey).OrderStatus == OrderStatus.Completed)
                .WithMessage("Order has not yet been delivered.");
    }

    protected override bool PreValidate(ValidationContext<string> context, ValidationResult result)
    {
        var order = _purchaseOrderRepository.Load(context.InstanceToValidate);

        context.SetContext(OrderContextKey, order);

        return base.PreValidate(context, result);
    }
}

In order to easily access the validation context, I use these extension methods.

public static class FluentValidationExtensions
{
    public static T GetContext<T>(this IValidationContext validationContext, string key) where T : class
    {
        return validationContext.RootContextData[key] as T;
    }

    public static void SetContext<T>(this IValidationContext validationContext, string key, T value) where T : class
    {
        validationContext.RootContextData[key] = value;
    }
}

That's it!