HC validation example

AddTemporaryFileInputValidator.cs

public sealed class AddTemporaryFileInputValidator : AbstractValidator<AddTemporaryFileInput>
{
    public AddTemporaryFileInputValidator()
    {
        this.RuleFor(f => f.TemporaryFileName)
            .NotEmptyWithLength(FileName.MinimumLength, FileName.MaximumLength) // using VO metadata
            .Must(FileName.ValidateCharacters) // using VO validation logic
                .WithErrorCode(nameof(FileName.Errors.FileNameDisallowedCharacters)); // using VO validation message/code
    }
}

FileConfiguration.cs

internal sealed class FileConfiguration : IEntityTypeConfiguration<File>
{
    public void Configure(EntityTypeBuilder<File> builder)
    {
        // Properties.

        builder
            .Property(f => f.Name)
            .HasMinLength(FileName.MinimumLength)
            .HasMaxLength(FileName.MaximumLength); // using VO metadata
            
        // ...
    }
}

FileName.cs

public sealed class StringValueAttribute(Conversions conversions = Conversions.None)
    : ValueObjectAttribute<string>(conversions);

[StringValue]
public sealed partial class FileName
{
    public const int MinimumLength = 1;
    public const int MaximumLength = 200;

    public string Extension => this.extension ??= Path.GetExtension(this._value);

    private string? extension;

    // Based on System.IO.Path.GetInvalidFileNameChars (in a Windows environment).
    // On Unix, there are only 2 invalid characters ('\0' and '/'), which is too permissive.
    private static readonly SearchValues<char> DisallowedFileNameChars = SearchValues.Create(
    [
#pragma warning disable IDE0055 // TODO: https://github.com/dotnet/roslyn/issues/70951.
        '\"', '<', '>', '|', '\0',
        (char)1, (char)2, (char)3, (char)4, (char)5, (char)6, (char)7, (char)8, (char)9, (char)10,
        (char)11, (char)12, (char)13, (char)14, (char)15, (char)16, (char)17, (char)18, (char)19,
        (char)20, (char)21, (char)22, (char)23, (char)24, (char)25, (char)26, (char)27, (char)28,
        (char)29, (char)30, (char)31, ':', '*', '?', '\\', '/'
#pragma warning restore IDE0055
    ]);

    /// <summary>
    /// Returns <c>true</c> if the file's extension is contained within the given list.
    /// </summary>
    public bool HasExtension(IList<string> extensions)
    {
        return extensions.Contains(this.Extension);
    }

    public static bool ValidateCharacters(string value)
    {
        return value.AsSpan().IndexOfAny(DisallowedFileNameChars) == -1;
    }

    private static Validation Validate(string value)
    {
        var result = StringValidator.Validate(value, MinimumLength, MaximumLength);

        if (result != Validation.Ok)
        {
            return result;
        }

        // Validate characters.
        if (!ValidateCharacters(value))
        {
            return Validation.Invalid(Errors.FileNameDisallowedCharacters);
        }

        return Validation.Ok;
    }

    public static class Errors
    {
        public const string FileNameDisallowedCharacters
            = "File name contains disallowed characters.";

        public static IReadOnlyDictionary<string, string> ToDictionary()
        {
            return new Dictionary<string, string>()
            {
                { nameof(FileNameDisallowedCharacters), FileNameDisallowedCharacters }
            }.AsReadOnly();
        }
    }
}