将VB.NET代码迁移到C#时，为什么for循环的行为会有所不同? [英] Why does a for loop behave differently when migrating VB.NET code to C#?
问题描述
我正在将项目从Visual Basic迁移到C#，并且不得不更改声明使用的for循环的方式.
I'm in the process of migrating a project from Visual Basic to C# and I've had to change how a for loop being used is declared.
在VB.NET中，for循环在下面声明:
In VB.NET the for loop is declared below:
Dim stringValue As String = "42"
For i As Integer = 1 To 10 - stringValue.Length
stringValue = stringValue & " " & CStr(i)
Console.WriteLine(stringValue)
Next
哪个输出:
42 1
42 1 2
42 1 2 3
42 1 2 3 4
42 1 2 3 4 5
42 1 2 3 4 5 6
42 1 2 3 4 5 6 7
42 1 2 3 4 5 6 7 8
在C#中，以下声明了for循环:
In C# the for loop is declared below:
string stringValue = "42";
for (int i = 1; i <= 10 - stringValue.Length; i ++)
{
stringValue = stringValue + " " + i.ToString();
Console.WriteLine(stringValue);
}
输出:
42 1
42 1 2
42 1 2 3
这显然是不正确的，因此我不得不稍稍更改代码，并包含一个可以容纳字符串长度的整数变量.
This obviously isn't correct so I had to change the code ever so slightly and included an integer variable that would hold the length of the string.
请参见下面的代码:
string stringValue = "42";
int stringValueLength = stringValue.Length;
for (int i = 1; i <= 10 - stringValueLength; i ++)
{
stringValue = stringValue + " " + i.ToString();
Console.WriteLine(stringValue);
}
输出:
42 1
42 1 2
42 1 2 3
42 1 2 3 4
42 1 2 3 4 5
42 1 2 3 4 5 6
42 1 2 3 4 5 6 7
42 1 2 3 4 5 6 7 8
现在我的问题解决了，即使每次循环发生时，字符串的长度都会改变，Visual Basic在for循环中使用stringValue.Length条件在Visual Basic方面与C#有何不同.在C#中，如果我在for循环条件中使用stringValue.Length，则每次循环发生时它都会更改初始字符串值.为什么会这样?
Now my question resolves around how Visual Basic differs to C# in terms of Visual Basic using the stringValue.Length condition in the for loop even though each time the loop occurs the length of the string changes. Whereas in C# if I use the stringValue.Length in the for loop condition it changes the initial string value each time the loop occurs. Why is this?
推荐答案
在C#中，每次迭代都会评估循环边界条件.在VB.NET中，仅在进入循环时对其进行评估.
In C#, the loop boundary condition is evaluated on each iteration. In VB.NET, it is only evaluated on entry to the loop.
因此，在所涉及的C#版本中，由于stringValue的长度在循环中被更改，因此最终循环变量值将被更改.
So, in the C# version in the question, because the length of stringValue is being changed in the loop, the final loop variable value will be changed.
在VB.NET中，最终条件是包容性的，因此在C#中将使用<=而不是<.
In VB.NET, the final condition is inclusive, so you would use <= instead of < in C#.
C#中的结束条件评估具有一个推论，即即使它没有变化，但计算起来很昂贵，但也应该在循环之前进行一次计算.
The end condition evaluation in C# has the corollary that even if it doesn't vary but it is expensive to calculate, then it should be calculated just once before the loop.
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