How to write [DefaultValue (null)] in VB.NET? <DefaultValue (Nothing)> does not compile

Question

This is not as trivial as it seems. This is a continuation of this issue .

Say I have a Windows Forms user control with the property:

// using System.ComponentModel; [DefaultValue(null)] public object DataSource { … } 

If I transfer this to VB.NET, I would try this:

 'Imports System.ComponentModel <DefaultValue(Nothing)> Public Property DataSource As Object … End Property 

This will not work, because the compiler has problems with choosing the correct overload of the DefaultValueAttribute constructor :

An overload error is not possible because the available New not the most specific for these arguments:

• Public Sub New(value As Boolean) : Not the most specific.
• Public Sub New(value As Byte) : Not the most specific.
• Public Sub New(value As Char) : not the most specific.

I am pretty sure that this is because Nothing in VB.NET not only means "null reference" ( null in C #), but also the "default value for" of the parameter type ( default(T) in C #). Because of this ambiguity, every constructor overload is a potential coincidence.

<DefaultValue(CObj(Nothing))> also not work because it is not a constant value.

How exactly do I write this in VB.NET?

PS: <DefaultValue(GetType(Object), Nothing)> is an option, but it bypasses the problem. I'm really wondering if there is a way to use the same constructor for DefaultValueAttribute as the C # version.

Answer 1

"[...] will not work, because the compiler has problems choosing the right overload of the DefaultValueAttribute constructor [.]"

The compiler can be provided with the Nothing type-type for the desired type:

 <DefaultValue(DirectCast(Nothing, Object))> 

" <DefaultValue(CObj(Nothing))> also not work because it is not a constant value.

Fortunately, unlike CObj(Nothing) , the compiler considers DirectCast(Nothing, Object) - and, suprisingly, CType(Nothing, Object) , is also a constant value, so it is accepted.