[RESOLVED] Change Custom Attribute Values At Run Time

I want to know how to change the values of my custom attributes.

The Attribute:

Code:

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<AttributeUsage(AttributeTargets.Property, AllowMultiple:=True, Inherited:=True)> _
Public Class IndexAttribute
    Inherits System.Attribute

    Private _MaxLength As Integer
    Public Property MaxLength() As Integer
        Get
            Return _MaxLength
        End Get
        Set(ByVal value As Integer)
            _MaxLength = value
        End Set
    End Property


    Private _NotNull As Boolean
    Public Property NotNull() As Boolean
        Get
            Return _NotNull
        End Get
        Set(ByVal value As Boolean)
            _NotNull = value
        End Set
    End Property

End Class

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Attribute Setup:

Code:

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Public Class Index
    Private _Value As String
    <IndexAttribute(MaxLength:=0, NotNull:=False)> _
    Public Property Value() As String
        Get
            Return _Value
        End Get
        Set(ByVal value As String)
            _Value = value
        End Set
    End Property
End Class

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Attempt to Change Attribute Value at RunTime:

Code:

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'--Get and Set the Custom Attributes for the Index object.
            Dim _IndexAttribute As IndexAttribute
            _IndexAttribute = CType(Attribute.GetCustomAttribute(_Index.GetType.GetProperty("Value"), _
            GetType(IndexAttribute)), IndexAttribute)
            _IndexAttribute.MaxLength = 102

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Even though you are allowed to change the MaxLength, it doesn't stick for the _Index instance.

Attributes are metadata and are not instance based. Notice that you never refer to an instance of index to get the attribute instead you get the type of the instance. The Type is the blueprint if you will for what the structure is of the object but is not an instance of the object itself. It is at this level that the attribute lives and thus it does not persist values when changed. They are actually compiled into the Type.

One of the strengths of Attributes is that they allow you to query the metadata of an object without creating an instance. This allows you to find out about the object without creating it. This is very useful when creating a plug-in type scenerio.

Quote:

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Originally Posted by Edneeis

Attributes are metadata and are not instance based. Notice that you never refer to an instance of index to get the attribute instead you get the type of the instance. The Type is the blueprint if you will for what the structure is of the object but is not an instance of the object itself. It is at this level that the attribute lives and thus it does not persist values when changed. They are actually compiled into the Type.

One of the strengths of Attributes is that they allow you to query the metadata of an object without creating an instance. This allows you to find out about the object without creating it. This is very useful when creating a plug-in type scenerio.

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Interesting, I thought I was referring to an instance of the _Index object on this line of code, but I guess not! :lol:

Code:

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CType(Attribute.GetCustomAttribute(_Index.GetType.....

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Well after you call GetCustomAttribute you are dealing with an instance of the attribute but its a copy and not the same instance that is compiled as meta data for the type. At that point it is just like any other object but changes are not persisted back to the metadata.