日期/没有 - 为什么这不起作用？ [英] Dates / Nothing -- Why doesn't this work?
问题描述
您好。我在这里有两个非常相似的代码片段。一个工作，
但另一个不行。有人可以解释原因吗？
片段1：本地日期变量设置为空。编译好，设置
日期到1/1/0001 12:00:00 AM。
Private Sub Button1_Click（ByVal sender As System.Object，ByVal e as
System.EventArgs）处理Button1.Click
Dim x As Date
x = Nothing
Debug.WriteLine（x）
结束子
片段2：可选日期参数包含默认值没有价值。 .NET
2003拒绝编译。给出从'System.Object''转换为
'的错误'''''不能出现在常量表达式中。
私有Sub ThisDoesntWork（可选ByVal x As Date = Nothing）
Debug.WriteLine（x）
End Sub
为什么我收到此错误？这对我来说没有意义。如果x是一个不同类型的
（例如Integer），.NET根本不会抱怨。那么为什么
日期处理方式不同？
提前感谢您的任何反馈！
Scott
Scott，
我不知道其他人，但我再也不会使用可选项了。尝试
重载方法。
Private Sub ThisDoesntWork（）
ThisDoesntWork（Nothing）
End Sub
Private Sub ThisDoesntWork（ByVal x As Date）
Debug.WriteLine（x）
End Sub
希望这会有所帮助。
Chris。
" Scott Hembrough"写道：
你好。我在这里有两个非常相似的代码片段。一个工作，
但另一个不起作用。有人可以解释原因吗？
片段1：本地日期变量设置为空。编译好，将日期设置为1/1/0001 12:00:00 AM。
Private Sub Button1_Click（ByVal sender As System.Object，ByVal e As
System。 EventArgs）处理Button1.Click
Dim x As Date
x = Nothing
Debug.WriteLine（x）
End Sub
Snippet 2：可选日期参数包含默认值no。 .NET
2003拒绝编译。给出从'System.Object''转换为
''''的错误不能出现在常量表达式中。
Private Sub ThisDoesntWork（可选ByVal x As Date） =没什么）
Debug.WriteLine（x）
结束子
为什么我会收到此错误？这对我来说没有意义。如果x是一个不同类型（如Integer），.NET根本就不会抱怨。那么为什么
日期的处理方式不同？
提前感谢您的任何反馈！
Scott
Chris，
我认为这是一个假设的问题。据我所知有这种方法
没有任何意义。
Private Sub ThisDoesntWork（可选ByVal x As Date = Nothing）
因为当没有设置日期时它是Nothing，当它被设置时它是
东西
为什么它给出了错误在这个页面上也有描述。
http://msdn.microsoft.com/library/de...tionalargs.asp
Cor
" Chris Podmore" < Ch ********** @ discussion.microsoft.com>
Scott，
我不知道其他人们，但我再也不会使用可选项了。尝试
重载方法。
Private Sub ThisDoesntWork（）
ThisDoesntWork（Nothing）
End Sub
Private Sub ThisDoesntWork（ByVal x As Date）
Debug.WriteLine（x）
End Sub
希望这会有所帮助。
Chris。
" Scott Hembrough"写道：
你好。我在这里有两个非常相似的代码片段。一个
有效，
但另一个没有。有人可以解释原因吗？
片段1：本地日期变量设置为空。编译好，将日期设置为1/1/0001 12:00:00 AM。
Private Sub Button1_Click（ByVal sender As System.Object，ByVal e As
System。 EventArgs）处理Button1.Click
Dim x As Date
x = Nothing
Debug.WriteLine（x）
End Sub
Snippet 2：可选的日期参数包含默认值。
.NET
2003拒绝编译。给出从'System.Object'转换为'
'''''''''''''''''''''''''''''''''''''''''''''''''' ByVal x As Date = Nothing）
Debug.WriteLine（x）
End Sub
为什么我收到此错误？这对我来说没有意义。如果x是一个不同类型（如Integer），.NET根本就不会抱怨。那么为什么
日期处理方式不同？
提前感谢您的任何反馈！
Scott
Cor，
我认为Scott发布了一些示例代码而不是他的实际代码
他遇到了问题。
克里斯。
Cor Ligthert写道：
克里斯，
我认为这是一个假设的问题。据我所知，这种方法没有任何意义。
Private Sub ThisDoesntWork（可选ByVal x As Date = Nothing）
因为当没有设置日期时它是Nothing，当它被设置时它是
为什么它给出错误在本页面上有描述。
http://msdn.microsoft.com/library/de...tionalargs.asp
Chris Podmore < Ch ********** @ discussion.microsoft.com>
Scott，
我不是''知道其他人但我从不再使用可选项。尝试
重载方法。
Private Sub ThisDoesntWork（）
ThisDoesntWork（Nothing）
End Sub
Private Sub ThisDoesntWork（ByVal x As Date）
Debug.WriteLine（x）
End Sub
希望这会有所帮助。
Chris。
" Scott Hembrough"写道：
你好。我在这里有两个非常相似的代码片段。一个
有效，
但另一个没有。有人可以解释原因吗？
片段1：本地日期变量设置为空。编译好，将日期设置为1/1/0001 12:00:00 AM。
Private Sub Button1_Click（ByVal sender As System.Object，ByVal e As
System。 EventArgs）处理Button1.Click
Dim x As Date
x = Nothing
Debug.WriteLine（x）
End Sub
Snippet 2：可选的日期参数包含默认值。
.NET
2003拒绝编译。给出从'System.Object'转换为'
'''''''''''''''''''''''''''''''''''''''''''''''''' ByVal x As Date = Nothing）
Debug.WriteLine（x）
End Sub
为什么我收到此错误？这对我来说没有意义。如果x是一个不同类型（如Integer），.NET根本就不会抱怨。那么为什么
日期处理方式不同？
提前感谢您的任何反馈！
Scott
Hello. I have two snippets of code here that are very similar. One works,
but the other doesn''t. Can someone explain why?
Snippet 1: Local "date" variable is set to nothing. Compiles fine, sets
date to 1/1/0001 12:00:00 AM.
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
Dim x As Date
x = Nothing
Debug.WriteLine(x)
End Sub
Snippet 2: Optional date parameter contains default value of nothing. .NET
2003 refuses to compile. Gives error of "Conversion from ''System.Object'' to
''Date'' cannot occur in a constant expression."
Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
Debug.WriteLine(x)
End Sub
Why am I getting this error? It doesn''t make sense to me. If x is a
different type (such as Integer), .NET doesn''t complain at all. So why is
Date treated differently?
Thanks in advance for any feedback!
Scott
Scott,
I don''t know about other people but I never use optional anymore. Try
overloading the method.
Private Sub ThisDoesntWork()
ThisDoesntWork(Nothing)
End Sub
Private Sub ThisDoesntWork(ByVal x As Date)
Debug.WriteLine(x)
End Sub
Hope this helps.
Chris.
"Scott Hembrough" wrote:
Hello. I have two snippets of code here that are very similar. One works,
but the other doesn''t. Can someone explain why?
Snippet 1: Local "date" variable is set to nothing. Compiles fine, sets
date to 1/1/0001 12:00:00 AM.
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
Dim x As Date
x = Nothing
Debug.WriteLine(x)
End Sub
Snippet 2: Optional date parameter contains default value of nothing. .NET
2003 refuses to compile. Gives error of "Conversion from ''System.Object'' to
''Date'' cannot occur in a constant expression."
Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
Debug.WriteLine(x)
End Sub
Why am I getting this error? It doesn''t make sense to me. If x is a
different type (such as Integer), .NET doesn''t complain at all. So why is
Date treated differently?
Thanks in advance for any feedback!
Scott
Chris,
I assume it is a hypothetical question. As far as I can see has this method
not any sence.
Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
Because when the Date is not set it is Nothing and when it is set it is
Something
Why it gives the error is as well described on this page.
http://msdn.microsoft.com/library/de...tionalargs.asp
Cor
"Chris Podmore" <Ch**********@discussions.microsoft.com>
Scott,
I don''t know about other people but I never use optional anymore. Try
overloading the method.
Private Sub ThisDoesntWork()
ThisDoesntWork(Nothing)
End Sub
Private Sub ThisDoesntWork(ByVal x As Date)
Debug.WriteLine(x)
End Sub
Hope this helps.
Chris.
"Scott Hembrough" wrote:Hello. I have two snippets of code here that are very similar. One
works,
but the other doesn''t. Can someone explain why?
Snippet 1: Local "date" variable is set to nothing. Compiles fine, sets
date to 1/1/0001 12:00:00 AM.
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
Dim x As Date
x = Nothing
Debug.WriteLine(x)
End Sub
Snippet 2: Optional date parameter contains default value of nothing.
.NET
2003 refuses to compile. Gives error of "Conversion from ''System.Object''
to
''Date'' cannot occur in a constant expression."
Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
Debug.WriteLine(x)
End Sub
Why am I getting this error? It doesn''t make sense to me. If x is a
different type (such as Integer), .NET doesn''t complain at all. So why
is
Date treated differently?
Thanks in advance for any feedback!
Scott
Cor,
I assumed that Scott had posted some sample code rather than his actual code
which he was having problems with.
Chris.
"Cor Ligthert" wrote:
Chris,
I assume it is a hypothetical question. As far as I can see has this method
not any sence.Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
Because when the Date is not set it is Nothing and when it is set it is
Something
Why it gives the error is as well described on this page.
http://msdn.microsoft.com/library/de...tionalargs.asp
Cor
"Chris Podmore" <Ch**********@discussions.microsoft.com>Scott,
I don''t know about other people but I never use optional anymore. Try
overloading the method.
Private Sub ThisDoesntWork()
ThisDoesntWork(Nothing)
End Sub
Private Sub ThisDoesntWork(ByVal x As Date)
Debug.WriteLine(x)
End Sub
Hope this helps.
Chris.
"Scott Hembrough" wrote:Hello. I have two snippets of code here that are very similar. One
works,
but the other doesn''t. Can someone explain why?
Snippet 1: Local "date" variable is set to nothing. Compiles fine, sets
date to 1/1/0001 12:00:00 AM.
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
Dim x As Date
x = Nothing
Debug.WriteLine(x)
End Sub
Snippet 2: Optional date parameter contains default value of nothing.
.NET
2003 refuses to compile. Gives error of "Conversion from ''System.Object''
to
''Date'' cannot occur in a constant expression."
Private Sub ThisDoesntWork(Optional ByVal x As Date = Nothing)
Debug.WriteLine(x)
End Sub
Why am I getting this error? It doesn''t make sense to me. If x is a
different type (such as Integer), .NET doesn''t complain at all. So why
is
Date treated differently?
Thanks in advance for any feedback!
Scott
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