Workaround for C# generic attribute limitation

Question

As discussed here, C# doesn't support generic attribute declaration. So, I'm not allowed to do something like:

[Audit<User> (UserAction.Update)]
public ActionResult SomeMethod(int id){ ...

that would fit like a charm in my attribute impl class, cause I need to call a method from a generic repository:

User fuuObj = (User) repository.LoadById<T>(_id);

I tried to use this solution without success. I can pass something like typeOf(User), but how can I call LoadById just with type or magic string?

*Both, T and User, extend a base class called Entity.

Do you need a specific class or is Entity enough to work with? — Bas, Mar 20 '12 at 14:26
@Daniel, yes... but I actually prefer center all auditory process and avoid maximum as possible future code in this context. — Custodio, Mar 20 '12 at 14:28
I really hope by .NET5 they address these huge glaring pain points for dealing with generics, it's absolutely silly i can't do `var t = typeof(Cat); new Repository().DoMagic();` — Chris Marisic, Mar 20 '12 at 14:56
Finally on the roadmap for C# 10.0 ... [github discussion](https://github.com/dotnet/csharplang/issues/124) — McGuireV10, Sep 30 '20 at 11:22

score 17 · Accepted Answer · answered Mar 20 '12 at 14:33

You could use reflection to load by id:

public class AuditAttribute : Attribute
{
    public AuditAttribute(Type t)
    {
        this.Type = t;
    }

    public  Type Type { get; set; }

    public void DoSomething()
    {
        //type is not Entity
        if (!typeof(Entity).IsAssignableFrom(Type))
            throw new Exception();

        int _id;

        IRepository myRepository = new Repository();
        MethodInfo loadByIdMethod =  myRepository.GetType().GetMethod("LoadById");
        MethodInfo methodWithTypeArgument = loadByIdMethod.MakeGenericMethod(this.Type);
        Entity myEntity = (Entity)methodWithTypeArgument.Invoke(myRepository, new object[] { _id });
    }
}

score 4 · Answer 2 · answered Mar 20 '12 at 14:24

4

You have at least these three possibilities:

You could use reflection to call LoadById
You could create an expression tree that calls LoadById
You could provide a LoadById method in your repository that is not generic.

answered Mar 20 '12 at 14:24

Daniel Hilgarth

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score 1 · Answer 3 · edited Dec 24 '22 at 21:07

1

Since C# 11 there is no need for workarounds because generic attributes are supported:

public class AuditAttribute<T> : Attribute where T : Entity
{
   // ... 
}

edited Dec 24 '22 at 21:07

halfer

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answered Dec 23 '22 at 17:25

Guru Stron

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score 1 · Answer 4 · answered Mar 20 '12 at 14:30

1

You could use reflection to invoke the LoadById method. The following msdn article should point you in the right direction:

http://msdn.microsoft.com/en-us/library/b8ytshk6(v=vs.100).aspx

answered Mar 20 '12 at 14:30

Kev

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Workaround for C# generic attribute limitation

4 Answers4

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