SurrogateAttribute.Fody 0.6.1

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

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

In a nutshell

Instead of having:

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 create attributes that implement ISurrogateAttribute and have reusable abstractions suitable to your requirements, e.g.

[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 }
    ];
}

so Cat can be simplified:

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

Attributes implementing ISurrogateAttribute are replaced with their TargetAttributes and then removed from the assembly at build time using IL weaving.

Supported features

TODO