Attribute instantiation

20 Nov 2006 9:39 AM

Theo

Hi,
i created a custom attribute that accepts an argument in the constructor to fill a property. The constructor is supposed to be called when then attribute is set but that doesn't seem to happen.
Sample code:

    <System.AttributeUsage(System.AttributeTargets.All, AllowMultiple:=True)> _
    Public Class CustomAtt
        Inherits System.Attribute

        Public Sub New(ByVal Arg As String)
            Me.Arg = Arg
            if Arg = string.empty then throw new exception
        End Sub

        Private _Arg As String

        Public Property Arg() As String
            Get
                Return _Arg
            End Get
            Set(ByVal Value As String)
                _Arg = Value
            End Set
        End Property
    End Class

    <CustomAtt("something")> _
    Public Class SomeClass

        Public Sub New()
            'Do something
        End Sub
    End Class

Running this in debug mode i noticed that the debuger does not go through the constructor of the attribute and there is not exception even if pass an empty string parameter.

How can i make this work?

Regards,
Theodore

20 Nov 2006 2:01 PM

Chris Dunaway

Theo wrote:

Show quoteHide quote

> i created a custom attribute that accepts an argument in the constructor to fill a property. The constructor is supposed to be called when then attribute is set but that doesn't seem to happen.
> Sample code:
>
>     <System.AttributeUsage(System.AttributeTargets.All, AllowMultiple:=True)> _
>     Public Class CustomAtt
>         Inherits System.Attribute
>
>         Public Sub New(ByVal Arg As String)
>             Me.Arg = Arg
>             if Arg = string.empty then throw new exception
>         End Sub
>
>         Private _Arg As String
>
>         Public Property Arg() As String
>             Get
>                 Return _Arg
>             End Get
>             Set(ByVal Value As String)
>                 _Arg = Value
>             End Set
>         End Property
>     End Class
>
>     <CustomAtt("something")> _
>     Public Class SomeClass
>
>         Public Sub New()
>             'Do something
>         End Sub
>     End Class
>
>
> Running this in debug mode i noticed that the debuger does not go through the constructor of the attribute and there is not exception even if pass an empty string parameter.

Did you actually create an instance of the SomeClass somewhere? I
think your attribute gets instantiated when the class is, but I'm not
100% sure.

By the way, it is standard convention that attribute classes acutally
have the word 'Attribute' as part of the class name: Public Class
CustomAttribute.

I think then, that you can apply the attribute using only 'Custom' as
in:

<Custom("something")> _
Public Class SomeClass
End Class

Chris

20 Nov 2006 2:24 PM

Theo

Hi Chris,
I did create an instance of the SomeClass and it didn't work. I tried an
example I found in msdn library but didn't work. However, I noticed that the
attribute was instantiating when I was attempting to list the assembly's
attributes. Something triggered the instantiation that didn't occur before.
Upon instantiation the attribute executes a method that succeeds under
specific conditions and defines whether or not to continue loading the
class. Therefore is crucial to me that the attribute instantiates properly.

Thanks,
Theodore

Show quoteHide quote

"Chris Dunaway" <dunaw***@gmail.com> wrote in message
news:1164031269.626710.208920@j44g2000cwa.googlegroups.com...
> Theo wrote:
>
>> i created a custom attribute that accepts an argument in the constructor
>> to fill a property. The constructor is supposed to be called when then
>> attribute is set but that doesn't seem to happen.
>> Sample code:
>>
>>     <System.AttributeUsage(System.AttributeTargets.All,
>> AllowMultiple:=True)> _
>>     Public Class CustomAtt
>>         Inherits System.Attribute
>>
>>         Public Sub New(ByVal Arg As String)
>>             Me.Arg = Arg
>>             if Arg = string.empty then throw new exception
>>         End Sub
>>
>>         Private _Arg As String
>>
>>         Public Property Arg() As String
>>             Get
>>                 Return _Arg
>>             End Get
>>             Set(ByVal Value As String)
>>                 _Arg = Value
>>             End Set
>>         End Property
>>     End Class
>>
>>     <CustomAtt("something")> _
>>     Public Class SomeClass
>>
>>         Public Sub New()
>>             'Do something
>>         End Sub
>>     End Class
>>
>>
>> Running this in debug mode i noticed that the debuger does not go through
>> the constructor of the attribute and there is not exception even if pass
>> an empty string parameter.
>
> Did you actually create an instance of the SomeClass somewhere? I
> think your attribute gets instantiated when the class is, but I'm not
> 100% sure.
>
> By the way, it is standard convention that attribute classes acutally
> have the word 'Attribute' as part of the class name: Public Class
> CustomAttribute.
>
> I think then, that you can apply the attribute using only 'Custom' as
> in:
>
> <Custom("something")> _
> Public Class SomeClass
> End Class
>
> Chris
>

20 Nov 2006 4:17 PM

Chris Dunaway

Theo wrote:

> I did create an instance of the SomeClass and it didn't work. I tried an
> example I found in msdn library but didn't work. However, I noticed that the

>From what I have read, the Attribute is instantiated by the compiler

and then serialized to the target's meta data entry. That may be why
the breakpoint is not hit at runtime because the object has already
been instantiated before the debugger is attached to the process.

I think that in code that uses the class (SomeClass), if it calls
GetCustomAttributes, looking for your custom attribute, then the
attribute is instantiated from the parameters that were serialized to
the meta data. The constructor would be called at that time.

BTW: I found most of this information in Jeffrey Richter's book,
"Applied Microsoft .Net Framework Programming".

Regards,

Chris

21 Nov 2006 6:44 AM

Theo

Hi Chris,

thanks for the reply, what you actually say is accurate and I have confirmed
it. What I don't understand is how do other attributes instantiate without
calling GetCustomAttributes. I don't know if it is worth the trouble digging
so much when you have to meet some serious deadlines. Microsoft claims that
the attribute's constructor is called when the attribute is set or so it
says in the example you can find in the following link:
http://msdn2.microsoft.com/en-us/library/system.attribute.aspx

Thanks again for looking into it.

Regards,
Theodore

Show quoteHide quote

"Chris Dunaway" <dunaw***@gmail.com> wrote in message
news:1164039459.525840.323630@b28g2000cwb.googlegroups.com...

>
> I think that in code that uses the class (SomeClass), if it calls
> GetCustomAttributes, looking for your custom attribute, then the
> attribute is instantiated from the parameters that were serialized to
> the meta data. The constructor would be called at that time.
>
> BTW: I found most of this information in Jeffrey Richter's book,
> "Applied Microsoft .Net Framework Programming".
>
> Regards,
>
> Chris
>