有趣的结果在VB.Net [英] Interesting Results In VB.Net
问题描述
对于i as Integer = 0到5
Dim sb As StringBuilder
如果i = 0
sb = New StringBuilder（i。 ToString（））
下一页我是
Console.WriteLine（sb.ToString（））
下一页我是
上面的代码给出了以下输出：
0
0
0
0
0
有点奇怪StringBuilder（或任何对象）在第一次迭代之后如何保存它状态环。 StringBuilder的文本保持不变并且
没有重新初始化。
评论？
Mythran
Mythran写道：For i As整数= 0到5
Dim sb As StringBuilder
如果i = 0
sb =新的StringBuilder（i.ToString（））
接下来我
Console.WriteLine（ sb.ToString（））
接下来我上面的代码给出了以下输出：
0
0
0 > 0
0
有点奇怪StringBuilder（或任何对象）在循环的第一次迭代后如何保存它状态。 StringBuilder的文本保持不变，并且不会重新初始化。
评论？
Mythran
是否有任何保证短范围变量确实是短期寿命变量？它们可能与它们所处的
子例程或函数具有相同的生命周期，具体取决于编译器希望编译代码的方式。
Mythran写道：For i As Integer = 0 To 5
Dim sb As StringBuilder
如果i = 0
sb =新的StringBuilder（i.ToString（））
接下来我
Console.WriteLine（sb.ToString（））
下一个我
上面的代码给出了以下输出：
0
0
0
0
0
有点奇怪的是StringBuilder（或任何对象）在循环的第一次迭代后如何保存它状态。 StringBuilder的文本保持不变并且
不会重新初始化。
评论？
不是真的令人惊讶...... VB.NET支持块范围。所以，你
在范围的开头声明一个引用类型（范围是
for / next）：
Dim sb As StringBuilder
预留存储 - 但不初始化值。你只需要
初始化一次，当i = 0时。如果你修改了代码到
看起来像：
对于我作为整数= 0到5
Dim sb作为新的StringBuilder
如果i = 0那么
sb.Append（i.ToString（ ））
结束如果
Console.WriteLine（sb）
下一页
然后你会得到了非常不同的结果。
-
汤姆谢尔顿[MVP]
< blockquote>
" Tom Shelton" < to*@mtogden.com>在消息中写道
news：11 ********************** @ j33g2000cwa.googlegr oups.com ...
Mythran写道：For i As Integer = 0 to 5
Dim sb As StringBuilder
如果i = 0
sb = New StringBuilder（i.ToString（））
下一页我是
Console.WriteLine（sb.ToString（））
接下来我上面的代码给出了以下输出：
0
0
0
0
有点奇怪StringBuilder（或任何对象）如何保存状态
循环的第一次迭代之后。 StringBuilder的文本保持不变，并且没有重新初始化。
评论？
不是那么令人惊讶...... VB.NET支持块范围。所以，你在范围的开头声明一个引用类型（范围是
for / next）：
是的，不是很惊讶，但仍然有趣。我一直在想，它会被重新调暗（不是ReDim而是重新调整尺寸），因为在每次迭代结束时它将超出范围。
。 ..但我错了。它没有重新定义
。它的尺寸只有一次，并且似乎在每次迭代之后都没有看到它（我没有检查IL所以
不太确定）。 br />
Dim sb As StringBuilder
保留存储 - 但不初始化值。当i = 0时，你只需要初始化一次值。如果你修改了代码看起来像
对于i as Integer = 0到5
Dim sb As New StringBuilder
如果i = 0那么
sb.Append（i.ToString（））
结束如果
Console.WriteLine（sb）
Next
然后你会得到非常不同的结果。
是的，我知道。在我的原始帖子之前，我也测试了它。那不是很有意思，但已经知道了嘿嘿：P
-
Tom Shelton [MVP]
感谢Tom的回复...
Mythran
For i As Integer = 0 To 5
Dim sb As StringBuilder
If i = 0
sb = New StringBuilder(i.ToString())
Next i
Console.WriteLine(sb.ToString())
Next i
The above code gives the following output:
0
0
0
0
0
Kinda wierd how the StringBuilder (or any object) saves it state after the
first iteration of the loop. The StringBuilder''s text stays the same and
doesn''t re-initialize.
Comments?
Mythran
Mythran wrote:For i As Integer = 0 To 5
Dim sb As StringBuilder
If i = 0
sb = New StringBuilder(i.ToString())
Next i
Console.WriteLine(sb.ToString())
Next i
The above code gives the following output:
0
0
0
0
0
Kinda wierd how the StringBuilder (or any object) saves it state after the
first iteration of the loop. The StringBuilder''s text stays the same and
doesn''t re-initialize.
Comments?
Mythran
Is there any guarantee that short-scoped variables are really
short-life variables? They may have the same life-time as the
subroutine or function they are in, depending upon how the compiler
wishes to compile the code.
Mythran wrote:For i As Integer = 0 To 5
Dim sb As StringBuilder
If i = 0
sb = New StringBuilder(i.ToString())
Next i
Console.WriteLine(sb.ToString())
Next i
The above code gives the following output:
0
0
0
0
0
Kinda wierd how the StringBuilder (or any object) saves it state after the
first iteration of the loop. The StringBuilder''s text stays the same and
doesn''t re-initialize.
Comments?
Not really that suprising... VB.NET supports block scope. So, you
declare a reference type at the begining of the scope (the scope being
the for/next):
Dim sb As StringBuilder
That reserves storage - but does not initialize the value. You only
initialize the value once, when i = 0. If you had modified the code to
look like:
For i As Integer = 0 To 5
Dim sb As New StringBuilder
If i = 0 Then
sb.Append(i.ToString())
End If
Console.WriteLine(sb)
Next
Then you would have gotten very different results.
--
Tom Shelton [MVP]
"Tom Shelton" <to*@mtogden.com> wrote in message
news:11**********************@j33g2000cwa.googlegr oups.com...
Mythran wrote:For i As Integer = 0 To 5
Dim sb As StringBuilder
If i = 0
sb = New StringBuilder(i.ToString())
Next i
Console.WriteLine(sb.ToString())
Next i
The above code gives the following output:
0
0
0
0
0
Kinda wierd how the StringBuilder (or any object) saves it state after
the
first iteration of the loop. The StringBuilder''s text stays the same and
doesn''t re-initialize.
Comments?
Not really that suprising... VB.NET supports block scope. So, you
declare a reference type at the begining of the scope (the scope being
the for/next):
Aye, not suprising, but still interesting. I have been thinking all along
that it gets re-dimmed (not ReDim but re-dimensioned) because it would be
out of scope at the end of each iteration...but I had been mistaken. It
does NOT get re-dimensioned. It gets dimensioned a single time and seem to
not even look at it after every iteration after (I haven''t check the IL so
not too sure).
Dim sb As StringBuilder
That reserves storage - but does not initialize the value. You only
initialize the value once, when i = 0. If you had modified the code to
look like:
For i As Integer = 0 To 5
Dim sb As New StringBuilder
If i = 0 Then
sb.Append(i.ToString())
End If
Console.WriteLine(sb)
Next
Then you would have gotten very different results.
Yeah, I know. Before my original post, I tested that as well. That wasn''t
interesting though, already knew it heh :P
--
Tom Shelton [MVP]
Thanks for the reply Tom...
Mythran
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