Values of Unknown Custom Attributes

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pagates - 29 Mar 2007 15:56 GMT

Hello All,

I am sure that I am just overlooking something, but here's something I can't
quite get right...

I want to be able to get the value of a parameter of an unknown custom
attribute at runtime. All of the examples I have seen use casting of known
custom attributes to get these values.

Thanks,
pagates

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Nicholas Paldino [.NET/C# MVP] - 29 Mar 2007 16:10 GMT

pagates,

Custom attributes are classes, they are not methods with parameters.
Getting any information from them using reflection is like using reflection
to get information from any other class. You have to have some expectation
of what you are looking for.

Can you provide an example of what you are trying to do?

Signature

- Nicholas Paldino [.NET/C# MVP]
- mvp@spam.guard.caspershouse.com

> Hello All,
>
[quoted text clipped - 9 lines]
> Thanks,
> pagates

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pagates - 29 Mar 2007 16:26 GMT

Hi Nicholas,

Sure - it's a general question, but I wasn't sure how to word it...

Say I have something like this:
public class Class1
{
public Class1()
{
}

string _someProperty;
[ACustomAttribute("Some Custom Attribute Value")]
public string SomeProperty
{
get { return _someProperty; }
set { _someProperty = value; }
}
}

I simply want to use reflection to get "Some Custom Attribute Value" from
the ACustomAttribute attribute in a program that loads the assembly that
contains Class1. I can use GetCustomAttributes to get the attribute itself,
but I can't quite get how to get the value of the parameter contained therein.

Does that make sense? I can see what I want in my head, but getting it out
of there might be a problem.... : )

Thanks,
pagates

> pagates,
>
[quoted text clipped - 18 lines]
> > Thanks,
> > pagates

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Nicholas Paldino [.NET/C# MVP] - 29 Mar 2007 17:01 GMT

pagates,

Ok, I understand now.

So, when you have the attribute like this:

[ACustomAttribute("Some Custom Attribute Value")]
public string SomeProperty

The runtime constructs an instance of ACustomAttribute passing the value
of "Some Custom Attribute Value". What the constructor does with this is up
to the attribute class. It's like any other class in this sense.
Typically, however, it will be exposed as a property. So, on the
ACustomAttribute class, there should be some sort of property that exposes
the value. When you call GetCustomAttributes, you can cast the return value
to an instance of ACustomAttribute and then access the property to get the
value (assuming it is exposed as such).

Signature

- Nicholas Paldino [.NET/C# MVP]
- mvp@spam.guard.caspershouse.com

> Hi Nicholas,
>
[quoted text clipped - 54 lines]
>> > Thanks,
>> > pagates

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bonk - 14 May 2007 09:51 GMT

This approach is only possible if you can actually cast to the
CustomAttribute. If I reflect on an Assembly that also declares the
CustomAttribute Type I will not be able to cast to this Attribute type
because I do not reference that assembly. How could this issue be
solved ?

On Mar 29, 6:01 pm, "Nicholas Paldino [.NET/C# MVP]"
<m...@spam.guard.caspershouse.com> wrote:

> pagates,
>
[quoted text clipped - 82 lines]
>
> - Show quoted text -

Reply to this Message

Marc Gravell - 14 May 2007 10:04 GMT

Fist - you can't access *parameters*, but you can access *properties*;
and there-in lies the key.

If you don't want to reference the assembly, then perhaps use the
System.ComponentModel to access the values at runtime - i.e. (notepad
code; not compile-tested)

foreach(Attribute untypedAttribute in ...) { // presumably
GetAttributes() etc
Debug.WriteLine(untypedAttribute.GetType().Name);
foreach(PropertyDescriptor property in
TypeDescriptor.GetProperties(untypedAttribute)) {
Debug.WriteLine(string.Format("{0}={1}", property.Name,
property.GetValue(untypedAttribute)));
}
}

If you want to work with a *specific* attribute, then my first choice
would be to reference it! Otherwise, perhaps filter on FQN on
FullName.

Marc

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Values of Unknown Custom Attributes

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