这是不是琐碎这似乎是。这是一个后续这个问题。
让我们说我有一个Windows窗体用户控件的属性为:
使用System.ComponentModel //; [默认值(NULL)公共DataSource对象{...}如果我翻译这VB.NET,我会尝试这样的:
'进口System.ComponentModel <默认值(没有)GT; 公共财产的DataSource作为对象:端物业这不会工作,因为编译器选择正确的 DefaultValueAttribute 的构造过载:
重载决策失败,因为没有访问新是最具体的这些参数:
- 的Public Sub New(价值布尔):不是最具体
- 的Public Sub New(价值字节):不是最具体
- 的Public Sub New(价值为CHAR):没有最具体
我敢肯定,这是因为没有在VB.NET不仅意味着(在C#空)的空引用,也有参数类型为默认值(默认(T)在C#)。因为这种模糊的,每一个构造函数重载是一个潜在的匹配
<默认值(CObj(没有))> 也不会工作,因为它不是一个恒定值。
究竟是如何的不的我写这篇文章的VB.NET?
PS: <默认值(的GetType(对象),为Nothing)> 是一个选择,但它绕过问题。我如果有任何的方式来使用 DefaultValueAttribute 相同的构造函数的C#版本一样。
解决方案[...]将无法工作,因为编译器有选择 DefaultValueAttribute 构造函数[。]
编译器可以用没事的类型转换辅助所需类型:
<默认值(DirectCast(没什么,对象) )>
<默认值(CObj(没有))方式> 也不会工作,因为它不是一个恒定的值
幸运的是,不像 CObj(没有),编译器会认为 DirectCast(没什么,对象)&MDASH;令人惊讶和 CTYPE(没什么,对象),太&MDASH;一个恒定值,所以它已被接受
This isn't as trivial as it appears to be. It's a follow-up to this question.
Let's say I have a Windows Forms user control with a property:
// using System.ComponentModel; [DefaultValue(null)] public object DataSource { … }If I translate this to VB.NET, I would try this:
'Imports System.ComponentModel <DefaultValue(Nothing)> Public Property DataSource As Object … End PropertyThis won't work because the compiler has problems choosing the correct overload of DefaultValueAttribute's constructor:
Overload resolution failed because no accessible New is most specific for these arguments:
- Public Sub New(value As Boolean): Not most specific.
- Public Sub New(value As Byte): Not most specific.
- Public Sub New(value As Char): Not most specific.
I am quite certain this is because Nothing in VB.NET not only means "the null reference" (null in C#), but also "the default value for" the parameter type (default(T) in C#). Because of this ambiguity, every constructor overload is a potential match.
<DefaultValue(CObj(Nothing))> also won't work because it is not a constant value.
How exactly do I write this in VB.NET?
P.S.: <DefaultValue(GetType(Object), Nothing)> is an option, but it circumvents the problem. I'm really interested if there's any way to use the same constructor for DefaultValueAttribute as the C# version does.
解决方案"[…] won't work because the compiler has problems choosing the correct overload of DefaultValueAttribute's constructor[.]"
The compiler can be assisted with a type-cast of Nothing to the desired type:
<DefaultValue(DirectCast(Nothing, Object))>
"<DefaultValue(CObj(Nothing))> also won't work because it is not a constant value."
Fortunately, unlike CObj(Nothing), the compiler considers DirectCast(Nothing, Object) — and suprisingly, CType(Nothing, Object), too — a constant value, so it is accepted.
更多推荐
我怎样写[默认值(NULL)在VB.NET? <默认值(没有)GT;不编译
发布评论