我有两个实现IEnumerable的类 A 和 B 。
A 使用 B 中的GetEnumerator。
B 是 A 类的成员。
但是成员 B 可以为空，因此我添加了空检查。
该问题也在代码示例的注释中说明。应该在另一个分支中放什么以停止foreach空异常？
在以下示例中：
rootNode是 B 。
所示功能是 A 的功能。
B 是 A 的成员。
public IEnumerator<BVHNode<BoundingVolumeClass>> GetEnumerator()
{
if (rootNode != null)
{
return rootNode.GetEnumerator();
}
else
{
return null;
//return null cause foreach null exception
//what can be put here to stop it?
}
}
答案 0 :(得分：5)
不返回 dtLoan = CDate("2019-01-30")
dtProcess = CDate ("2020-02-28")
'dtLoan = CDate("2019-01-31")
'dtProcess = CDate ("2020-02-29")
'dtLoan = CDate("2019-02-28")
'dtProcess = CDate ("2020-02-29")
if LastDateOfMonth(dtLoan) = dtLoan AND dtProcess = LastDateOfMonth(dtProcess) then
response.write " this mean that the Loan date is end of the month, say 31 Jan, 28, 29 of Feb, 31 Feb "
response.write " and Process Date is also end of the month " & "<br>"
response.write " **** End of the month Loan Date : " & dtLoan & "<br>"
response.write " **** End of the month Process Date : " & dtProcess & "<br>"
elseif LastDateOfMonth(dtLoan) <> dtLoan AND dtProcess <> LastDateOfMonth(dtProcess) then
daysFromEndOfLoanMth = DateDiff("d",LastDateOfMonth(dtLoan),dtLoan)
response.write " How many days from end of Loan month: " & daysFromEndOfLoanMth & "<br>"
daysFromEndOfProcessMth = DateAdd("d",daysFromEndOfLoanMth,LastDateOfMonth(dtProcess))
response.write " From end of the month Add " & daysFromEndOfLoanMth & " Days = " & daysFromEndOfProcessMth & "<br>"
response.write " The date of process : " & dtProcess & "<br>"
dtShouldProcess = day(dtLoan) & "/" & Month(dtProcess) & "/" & Year(dtProcess)
if isDate(dtShouldProcess) then
dtShouldProcess=CDate(dtShouldProcess)
else
dtShouldProcess=daysFromEndOfProcessMth
end if
response.write " ** The date of should Process : ** " & dtShouldProcess & "<br>"
if dtProcess = dtShouldProcess then
'if dtProcess = daysFromEndOfProcessMth then
response.write " **** Loan Date : " & dtLoan & "<br>"
response.write " **** Process Date : " & dtProcess & "<br>"
end if
'daysFromEndOfProcessMth = DateDiff("d",LastDateOfMonth(dtProcess1),dtProcess1)
'response.write " How many days from Process Date end of the month: " & daysFromEndOfProcessMth & "<br>"
end if
，而是使用空集合：
null