.net – 警告：“在lambda表达式中使用迭代变量可能会产生意外结果”

Dim numbers1 As New List(Of Int32)({1, 2, 3})
Dim numbers2 As New List(Of Int32)({3, 4, 5})
For Each n1 In numbers1
    ' no warning '
    Dim contains = numbers2.Contains(n1)
Next
For Each n1 In numbers1
    ' warning on n1'
    Dim contains = (From num In numbers2 Where num = n1).Any
Next

所以我仍然不明白为什么编译器认为我可能在第二次迭代中得到意想不到的结果,而我对第一次安全是安全的.我不认为@ ee-m的interesting link提供了这种行为的原因,(这不是for-each问题,对于n1 As Int32 = 1 To 3也会导致编译器警告).

我真的不相信以下应该是“最佳实践”：

For Each n1 In numbers1
    Dim number1 = n1
    ' no warning'
    Dim contains = (From num In numbers2 Where num = number1).Any
Next

局部变量number1是多余的,并且使得代码的可读性降低,正如@ Meta-Knight已经强调的那样.注意：这三种方式都是安全的,并给出了正确的结果.

我建议采用复制到局部var(Dim exc = excel)作为标准’最佳实践’