Return error with Must instead of a boolean

[AndreuCodina]

C# doesn't have sum types, so currently we have to specify the error before a Must, and return a boolean:

var email = "alex@spam.com";
Validate.That(email)
    .WithError(new EmailMarkedAsSpamError())
    .Must(CheckEmailNotMarkedAsSpam);

private bool CheckEmailNotMarkedAsSpam(string email)
{
    var isMarkedAsSpam = email.EndsWith("spam.com");
    return !isMarkedAsSpam;
}

Instead of a boolean, we can add method overloads (for Must and MustAsync) to return the error as nullable:

var email = "alex@spam.com";
Validate.That(email)
    .Must(CheckEmailNotMarkedAsSpam);

private EmailMarkedAsSpamError? CheckEmailNotMarkedAsSpam(string email)
{
    var isMarkedAsSpam = email.EndsWith("spam.com");
    return isMarkedAsSpam ? new EmailMarkedAsSpamError() : null;
}

or

var email = "alex@spam.com";
Validate.That(email)
    .Must(CheckEmailNotMarkedAsSpam);

private EmailMarkedAsSpamError? CheckEmailNotMarkedAsSpam(string email)
{
    var isMarkedAsSpam = email.EndsWith("spam.com");
    
    if (isMarkedAsSpam)
    {
        return new EmailMarkedAsSpamError();
    }

    return null;
}

Another option can be return void/Task and catch a possible exception:

var email = "alex@spam.com";
Validate.That(email)
    .Must(CheckEmailNotMarkedAsSpam);

private void CheckEmailNotMarkedAsSpam(string email)
{
    var isMarkedAsSpam = email.EndsWith("spam.com");

    if (isMarkedAsSpam)
    {
        throw new EmailMarkedAsSpamError();
    }
}

Benefits:

• Remove boilerplate.
• The code is much more readable (the validation and the private method).
• Increase the performance because we'll only instantiate the error if there's an error.

Considerations:

• It has to work with ValidationError and EnsureError.
• Benchmark the different options.