Passing an array of objects to Attribute constructor

Question

I have custom attribute that takes array of objects. I am trying to pass an array of objects to it, but receive compiler error:

An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type.

Here is the code:

[AttributeUsage(AttributeTargets.Class)]
public class MyAttribute : Attribute
{
    public MyAttribute(Person[] people)
    {
         People = people;
    }

    public Person[] People{get; private set;}
}

public class Person
{
    public string Name{get;set;}
    public string LastName{get;set;}
}

I saw some articles online, but haven't seen any closer to mine. Am I doing something wrong or it is bug? If it is not bug can someone explain why I can't pass array of Person? I tried array of string and it works fine.

Mike Perrenoud · Accepted Answer · 2013-09-09 20:05:17Z

5

It's because an array of string is a compile-time constant whereas an array of Person is not. This isn't a bug, it's a limitation. The best way around it, that I've found, is by using reflection to set the attributes properties on creation of the object - but it's not pretty.

answered Sep 9, 2013 at 20:05

Mike Perrenoud

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9 Comments

Dilshod Over a year ago

Thanks for the explaining. Do you know why it is limited? What is wrong passing array of objects?

Mike Perrenoud Over a year ago

@Dilshod, attributes are applied to the Type, not the object. This means that there's no real way for the CLR to apply them to individual objects.

Mike Perrenoud Over a year ago

@Dilshod, in your case the best approach would be to pass an array of strings to the attribute and then load those Person objects at run-time.

Mike Perrenoud Over a year ago

@Dilshod, here is some more reading by Eric Lippert.

Dilshod Over a year ago

Yes, it looks like there is no more options. Thanks for the help!

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Jeppe Stig Nielsen · Accepted Answer · 2013-09-09 20:09:29Z

1

I think the error quoted comes when you try to apply your attribute to some code element. As in:

[My(new[] { new Person(), })]

You cannot use an array like that. While the expression in () is an array creation expression, it is not an array creation expression of an attribute parameter type since, by definition, the class Person is not an attribute parameter type.

answered Sep 9, 2013 at 20:09

Jeppe Stig Nielsen

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