C# 如何初始化为空的 IEnumerable<Object> 并允许连接到它？
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How to initialize IEnumerable<Object> that be empty and allow to Concat to it?
提问by Majid
I tried this code for adding bto books:
我尝试使用此代码添加b到books：
IEnumerable<Book> books =null;
foreach (Book b in context.Books.AsEnumerable())
if (someConditions)
books = books.Concat(new[] {b});
but gives me this error on last line of code:
但在最后一行代码中给了我这个错误：
System.ArgumentNullException: Value cannot be null. Parameter name: first
System.ArgumentNullException：值不能为空。参数名称：第一个
it seems that nullCollectioncould not concatenated. I use EF,so how should I initialize my Collectionthat have no thing in it and I could concatenate to it?
似乎nullCollection无法串联。我使用 EF，那么我应该如何初始化我的Collection那些没有东西的东西，我可以连接到它吗？
采纳答案by Rodrigo López
It seams all you want to do is filter your context.Books by some criteria.
它接缝您想要做的就是按某些标准过滤您的 context.Books。
IEnumerable<Book> books = context.Books.Where(b => someConditions);
If you still need the empty IEnumerable you can just call Enumerable.Empty():
如果您仍然需要空的 IEnumerable，您可以调用 Enumerable.Empty()：
IEnumerable<Book> books = Enumerable.Empty<Book>();
回答by chris.ellis
IEnumerable<Book> books = new List<Book>();
回答by Jakub Dropia
You need create books as a IEnumerable empty object like List, but need remember to call, after loop, ToList() on books. For example:
您需要将书籍创建为一个 IEnumerable 空对象，如 List，但需要记住在循环后调用 ToList() 对书籍。例如：
IEnumerable<int> books = new List<int>(); IEnumerable<int> books2 = new int[] { 1, 2, 3, 4 }; foreach (int b in books2) if (b > 2) books = (new[] { b }).Concat(books); books = books.ToList();
IEnumerable<int> books = new List<int>(); IEnumerable<int> books2 = new int[] { 1, 2, 3, 4 }; foreach (int b in books2) if (b > 2) books = (new[] { b }).Concat(books); books = books.ToList();
回答by Matthew Watson
Personally I'd just go with:
就我个人而言，我只会选择：
IEnumerable<Book> books = new Book[0];
rather than using a List.
而不是使用列表。
回答by AnorZaken
This is what you are trying to do:
这就是你想要做的：
IEnumerable<Book> books = Enumerable.Empty<Book>();
books = books.Concat(context.Books.AsEnumerable().Where(b => someCondition));
Alternatively you can do this if you like to start from null:
或者，如果您想从 null 开始，也可以这样做：
IEnumerable<Book> books = null;
var moreBooks = context.Books.AsEnumerable().Where(b => someCondition);
books = books == null ? moreBooks : books.Concat(moreBooks);
...although I have several question as to why you want / need to do things this way.
...虽然我有几个问题，为什么您想要/需要以这种方式做事。
