Creating Custom Validators

You can easily add custom validation by creating an attribute with an IsValid method that accepts the value to be validated as the first parameter. There is no need to implement any interface — just decorate your custom attribute with the [Validator] attribute, allowing the Source Generator to discover it easily (performance optimization).

Here's an example:

[Validator]
[ValidatorDescription("must be one of the defined enum members")]
[AttributeUsage(AttributeTargets.Property)]
public class InEnumAttribute : Attribute
{
    private static readonly ValidationMessage ValidationMessage =
        new("Must be one of the defined enum members.", "Valigator.Validations.InEnum");

    public ValidationMessage? IsValid<TEnum>(TEnum value)
        where TEnum : struct, Enum
    {
        return !Enum.IsDefined(typeof(TEnum), value) ? ValidationMessage : null;
    }
}

The ValidatorDescription attribute is optional but recommended. It serves as a description for your custom validation, making it easier to identify or document, especially if you want to list all validations on an API request object for documentation or user reference.

Method Parameters

You can use method parameters for dependency injection, allowing you to retrieve services directly from the service provider.

Return Value

The validation method supports a variety of return types, offering maximum flexibility to fit your workflow:

• Single Message or Validation result:

  • ValidationMessage?

  • Validation.Error(), Validation.Success()

• Collection of Messages:

  • IEnumerable<ValidationMessage>

• Asynchronous Variants:

  • IAsyncEnumerable<ValidationMessage>

  • Task<ValidationMessage?>

  • Task<Validation>

  • ValueTask<ValidationMessage?>

  • ValueTask<Validation>

This versatility means you can write validation logic synchronously or asynchronously, handle single errors or multiple.