如何避免“对不可为空的实体进行可能的“空”分配”
抱歉这个尴尬的例子，但我想尽可能保持它的重点
可以说，我已经解析了一个 json 文件，它给了我
var customers = myObj.SubObj?.CustomerList.EmptyIfNull();
var plusCustomers = customers.Where(...) // Resharper warning
myobj 下面的层次结构不能为空 有时 SubObject 可以为 null
我有一个扩展方法
IEnumerable<T> EmptyIfNull<T>(this IEnumerable<T> enumerable) => enumerable ?? Enumerable.Empty<T>();
所以我希望 plusCustomers 行是安全的（空列表而不是 null）。但我收到警告
可能对不可为 null 的实体进行“null”赋值
如果我将第一行替换为，
var customers = myObj.SubObj.CustomerList.EmptyIfNull();
我就会消除警告，但这是不正确的。我怎样才能让 Resharper 相信这没问题？...或者我错了？
Sorry for this awkward example, but I want to keep it as to the point as possible
Lets say, I have parsed a json-file, which gives me the hierarchy below
var customers = myObj.SubObj?.CustomerList.EmptyIfNull();
var plusCustomers = customers.Where(...) // Resharper warning
myobj cannot be null
SubObject can be null sometimes
I have an extension method
IEnumerable<T> EmptyIfNull<T>(this IEnumerable<T> enumerable) => enumerable ?? Enumerable.Empty<T>();
So I expect the plusCustomers-line to be safe(empty list instead of null). But I get a warning
possible "null"assignment to a non-nullable entity
If I replace the first line with
var customers = myObj.SubObj.CustomerList.EmptyIfNull();
I get rid of the warning, but that is not correct. How can I convince Resharper that this is ok?...or I'm I mistaken?
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您可以添加括号：
如果 LHS 操作数为 null，则空链接 (
?.) 的作用是将成员访问的整个链评估为 null，而不是仅将该单个成员访问评估为 null。例如，考虑一长串成员访问abcdef。如果只有a可为空，则只需在a之后编写?即可确保安全。如果a为 null，它不仅仅将a?.b计算为 null，因为在这种情况下，您必须在之后编写?为了安全起见，每个成员都可以访问，这有点烦人。因此，如果没有括号，由于
?.，整个成员访问都可以为空，无论您最后调用什么。通过添加括号，您可以打破成员访问链，以便
myObj.SubObj?.CustomerList属于可为 null 的类型，并按预期调用EmptyIfNull-返回EmptyIfNull应该返回的任何类型。You can add brackets:
The effect of the null chaining (
?.) is to evaluate the whole chain of member accesses to null, if the LHS operand is null, rather than evaluating just that single member access to null. For example, consider a long chain of member accessesa.b.c.d.e.f. If onlyais nullable, you would only need to write?afterafor this to be safe. It doesn't just evaluatea?.bto null ifais null, because in that case you'd have to write?after every member access for this to be safe, and that's a bit annoying.So without the brackets, the whole member access is made nullable due to
?., regardless of what you call at the end.By adding brackets, you break up the chain of member accesses, so that
myObj.SubObj?.CustomerListis of a nullable type, and callingEmptyIfNullon that works as expected - returning whatever typeEmptyIfNullshould return.