SurrogateAttribute.Fody 0.6.7

A Fody add-in that allows creating C# attributes composed of other attributes, thus making C#'s attributes a bit more useful.

In a nutshell

Instead of having a mess like this:

class Cat
{
    [Required(ErrorMessageResourceType = typeof(Translations), ErrorMessageResourceName = nameof(Translations.NameRequired))]
    [MinLength(3, ErrorMessageResourceType = typeof(Translations), ErrorMessageResourceName = nameof(Translations.NameBadLength))]
    [MaxLength(10, ErrorMessageResourceType = typeof(Translations), ErrorMessageResourceName = nameof(Translations.NameBadLength))]
    [DisplayAttribute(ResourceType = typeof(Translations), Name = nameof(Translations.CatName))]
    public string Name { get; set; }
}

You can have:

class Cat
{
    [RequiredWithLength(MinLength = 3)]
    [Translation(nameof(Translations.CatName))]
    public string Name { get; set; }
}

By creating custom attributes that implement ISurrogateAttribute:

[AttributeUsage(AttributeTargets.Property)]
class RequiredWithLengthAttribute : Attribute, ISurrogateAttribute
{
    [PropertyDefaultValue(1)]
    public int MinLength { get; set; }

    [PropertyDefaultValue(10)]
    public int MaxLength { get; set; }

    [PropertyDefaultValue(typeof(Translations))]
    Type TranslationResourceType { get; }

    Attribute[] ISurrogateAttribute.TargetAttributes => [
        new RequiredAttribute { ErrorMessageResourceType = TranslationResourceType, ErrorMessageResourceName = nameof(Translations.NameRequired) },
        new MinLengthAttribute(MinLength) { ErrorMessageResourceType = TranslationResourceType, ErrorMessageResourceName = nameof(Translations.NameBadLength) },
        new MaxLengthAttribute(MaxLength) { ErrorMessageResourceType = TranslationResourceType, ErrorMessageResourceName = nameof(Translations.NameBadLength) },
    ];
}

[AttributeUsage(AttributeTargets.Property)]
class TranslationAttribute(string translationKey) : Attribute, ISurrogateAttribute
{
    Attribute[] ISurrogateAttribute.TargetAttributes => [
        new DisplayAttribute { ResourceType = typeof(Translations), Name = translationKey }
    ];
}

Usages of those custom attributes are replaced by their corresponding TargetAttributes at build time using IL weaving.

Installation

Install Fody and SurrogateAttribute.Fody to each project in which you want to use surrogate attributes:

Install-Package Fody
Install-Package SurrogateAttribute.Fody

And make sure both dependencies have PrivateAssets="All" like so, because they are only needed during build:

<PackageReference Include="Fody" Version="6.8.1" PrivateAssets="All" />
<PackageReference Include="SurrogateAttribute.Fody" Version="0.6.7" PrivateAssets="All" />

A FodyWeavers.xml file will be added automatically to the project on rebuild. If not, create the file with the following content:

<Weavers>
  <SurrogateAttribute />
</Weavers>

This includes SurrogateAttribute Fody add-in to the IL weaving process.

In case your project (most likely a library) has no surrogate attribute usages but has surrogate attribute implementations, you may want to install SurrogateAttribute.Core instead, which has the required types for implementing surrogate attributes but does not include Fody as a dependency:

Install-Package SurrogateAttribute.Core

See Samples for a working solution.

Features

TODO

Supported targets for surrogate attributes

The following table displays what's currently supported. The support will be extended to cover all targets in the future: