c# - 对于条件 VB.NET 与 C#

Question

C#:

static class Module1
{     
    public static void Main()
    {    
        for (index = 1; index <= GetCount(); index++) {
            Console.WriteLine("For {0}", index);
        }
        Console.ReadKey();
    }

    public static int GetCount()
    {
        Console.WriteLine("GetCount");
        return 10;
    }
}

结果(C# 重新检查条件):

GetCount
For 1
GetCount
For 2
GetCount
For 3
GetCount
For 4
GetCount
For 5
GetCount
For 6
GetCount
For 7
GetCount
For 8
GetCount
For 9
GetCount
For 10
GetCount

VB.NET

Module Module1    
  Sub Main()
    For index = 1 To GetCount()
      Console.WriteLine("For {0}", index)
    Next
    Console.ReadKey()
  End Sub

  Public Function GetCount() As Integer
    Console.WriteLine("GetCount")
    Return 10
  End Function
End Module

结果(VB.NET 不重新检查条件):

GetCount
For 1
For 2
For 3
For 4
For 5
For 6
For 7
For 8
For 9
For 10

a) 为什么 VB.NET 不遵守在每次迭代时重新检查 For 条件的“规则”？
b) 是否有办法强制 VB 重新检查此条件？

Answer 1

C#和VB.NET是不同的语言，相似的关键字可以有不同的语义。

来自 the docs for For ... Next (我的重点):

When a For...Next loop starts, Visual Basic evaluates start, end, and step. This is the only time it evaluates these values.

来自 C# 规范的第 8.8.3 节(同上):

When and if control reaches the end point of the embedded statement (possibly from execution of a continue statement), the expressions of the for-iterator, if any, are evaluated in sequence, and then another iteration is performed, starting with evaluation of the for-condition in the step above.

如果您确实希望每次都强制检查条件，VB.NET 提供了极其灵活的 Do ... Loop，它可以有一个 While 或 一个 Until 条件，在开始时运行或循环结束。使用该语句，循环条件每次都被评估。