Modifying Class Attribute on Runtime

Question

I am not sure if it is possible I have seen:
Change Attribute's parameter at runtime.
My case is very similar but I am trying to change the attribute of a class in Runtime:

[Category("Change me")]
public class Classic
{
    public string Name { get; set; }
}

One of the answers was:

Dim prop As PropertyDescriptor = TypeDescriptor
    .GetProperties(GetType(UserInfo))("Age")
Dim att As CategoryAttribute = DirectCast(
     prop.Attributes(GetType(CategoryAttribute)),
     CategoryAttribute)
Dim cat As FieldInfo = att.GetType.GetField(
     "categoryValue",
      BindingFlags.NonPublic Or BindingFlags.Instance)
cat.SetValue(att, "A better description")

Changed to more readable format, thanks to Marc Gravell:

TypeDescriptor.AddAttributes(table, new Category{ Name = "Changed" });

All is good when using TypeDescriptor but when using:

var attrs = (Category[])typeof(Classic).GetCustomAttributes(
    typeof(Category),
    true);
attrs[0].Name

Name has the "Change me" text.
Is there a way to change this attribute on runtime?

Edit:
I need this for Linq2Sql in the designer the generated code has the DB schema. I want to use the user's default schema without using XML mapping or change the generated code (the table is still in development stage and changes frequently).

The designer code is:

[global::System.Data.Linq.Mapping.TableAttribute(Name="DbSchema.MyTable")]
public partial class MyTable

I want the attribute to be:

[TableAttribute(Name="MyTable")]

Now I have dug into the Framework code and I think linq2sql uses:

TableAttribute[] attrs = (TableAttribute[])typeof(MyTable)
   .GetCustomAttributes(typeof(TableAttribute), true);

When I use TypeDescriptor to change the attribute the value isn't changed in GetCustomAttributes.

Why do you want to do this? Attributes are meant to provide metadata, not much else. Why not take the approach of having a "rule list" that is initially populated by the attributes, and changed from there? — vcsjones, Jan 27 '11 at 12:52
What are you trying to achieve? Localizing text in Category? — Sam B, Jan 27 '11 at 13:03
@vcsjones believe it or not, there are times where you need to add, change or delete attributes at runtime. I've had to do it in order to add a converter to WPF bindings so that they were serialized instead of being evaluated. — , Jan 27 '11 at 13:07
I'm confused - why do you expect changing the `Name` of `Classic` to impact the category? — Marc Gravell, Jan 27 '11 at 13:09
Also - that `TypeDescriptor` code itself is horribly brittle; a decent answer there would be to use `TypeDescriptor.AddAttributes` and `TypeDescriptor.GetAttributes` - not to use reflection on attribute instances. — Marc Gravell, Jan 27 '11 at 13:16
I am trying to change linq2sql Designer attributes - specifically the DB schema to a config value. Designer = [global::System.Data.Linq.Mapping.TableAttribute(Name="DbSchema.MyTable")] — Roy Dallal, Jan 27 '11 at 13:42

score 2 · Answer 1 · answered Jan 27 '11 at 13:19

Avoiding reflection entirely, you can do this via TypeDescriptor:

using System;
using System.ComponentModel;
using System.Linq;
[Category("nice")]
class Foo {  }
static class Program
{
    static void Main()
    {
        var ca = TypeDescriptor.GetAttributes(typeof(Foo))
              .OfType<CategoryAttribute>().FirstOrDefault();
        Console.WriteLine(ca.Category); // <=== nice
        TypeDescriptor.AddAttributes(typeof(Foo),new CategoryAttribute("naughty"));
        ca = TypeDescriptor.GetAttributes(typeof(Foo))
              .OfType<CategoryAttribute>().FirstOrDefault();
        Console.WriteLine(ca.Category); // <=== naughty
    }
}

Well it doesn't really work whenever I use TypeDescriptor I get the changed value. But linq2sql (what I need this for) uses (TableAttribute[])table.GetCustomAttributes(typeof(TableAttribute), true) which gets the original value... — Roy Dallal, Jan 27 '11 at 13:33

score 0 · Answer 2 · answered Jan 27 '11 at 13:03

If you use reflection, then not quite like this - reflection attributes can't be substituted - only the component-model view is impacted by TypeDescriptor. However, you can subclass CategoryAttribute to your purposes. Especially useful for i18n.

using System.ComponentModel;
using System;
[MyCategory("Fred")]
class Foo {  }
static class Program
{
    static void Main()
    {
        var ca = (CategoryAttribute)Attribute.GetCustomAttribute(typeof(Foo), typeof(CategoryAttribute));
        Console.WriteLine(ca.Category);
              // ^^^ writes "I was Fred, but not I'm EVIL Fred"
    }
}
class MyCategoryAttribute : CategoryAttribute
{
    public MyCategoryAttribute(string category) : base(category) { }
    protected override string GetLocalizedString(string value)
    {
        return "I was " + value + ", but not I'm EVIL " + value;
    }
}

Unfortunately I can't change the original attribute nor create my own version. Linq2Sql expects TableAttribute. — Roy Dallal, Jan 27 '11 at 13:55

score 0 · Answer 3 · answered Jan 27 '11 at 13:03

You need to code your attribute so that it supports runtime values. For example, the validation attributes support Internationalization of messages through the setting of a resource type and resource string as opposed to the static Message string.

Another approach is to use a IOC container such as StructureMap or Unity to provide some object/service that provides the values.

If you don;t want to couple your attribute to a particular container, use the Common ServiceLocator wrapper provided by the Patterns and Practices group.

Modifying Class Attribute on Runtime

3 Answers3

Linked