我怎样才能返回一个空的IEnumerable？

本文翻译自：How can I return an empty IEnumerable?

Given the following code and the suggestions given in this question , I've decided to modify this original method and ask if there are any values in the IEnumarable return it, if not return an IEnumerable with no values. 鉴于以下代码和此问题中给出的建议，我决定修改这个原始方法，并询问IEnumarable中是否有任何值返回它，如果没有返回没有值的IEnumerable。

Here is the method: 这是方法：

public IEnumerable<Friend> FindFriends() { //Many thanks to Rex-M for his help with this one. //https://stackoverflow.com/users/67/rex-m return doc.Descendants("user").Select(user => new Friend { ID = user.Element("id").Value, Name = user.Element("name").Value, URL = user.Element("url").Value, Photo = user.Element("photo").Value }); }

Since everything is inside the return statement, I don't know how I could do this. 由于一切都在return语句中，我不知道如何做到这一点。 Would something like this work? 会这样的吗？

public IEnumerable<Friend> FindFriends() { //Many thanks to Rex-M for his help with this one. //https://stackoverflow.com/users/67/rex-m if (userExists) { return doc.Descendants("user").Select(user => new Friend { ID = user.Element("id").Value, Name = user.Element("name").Value, URL = user.Element("url").Value, Photo = user.Element("photo").Value }); } else { return new IEnumerable<Friend>(); } }

The above method doesn't work, and in fact it's not supposed to; 上面的方法不起作用，事实上它不应该; I just feel it illustrates my intentions. 我觉得这说明了我的意图。 I feel I should specify that the code doesn't work because you can't create an instance of an abstract class. 我觉得我应该指定代码不起作用，因为你不能创建一个抽象类的实例。

Here is the calling code, I don't want it to receive a null IEnumerable at any time: 这是调用代码，我不希望它随时接收null IEnumerable：

private void SetUserFriends(IEnumerable<Friend> list) { int x = 40; int y = 3; foreach (Friend friend in list) { FriendControl control = new FriendControl(); control.ID = friend.ID; control.URL = friend.URL; control.SetID(friend.ID); control.SetName(friend.Name); control.SetImage(friend.Photo); control.Location = new Point(x, y); panel2.Controls.Add(control); y = y + control.Height + 4; } }

Thank you for your time. 感谢您的时间。

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#1楼

参考：https://stackoom.com/question/DYBu/我怎样才能返回一个空的IEnumerable

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#2楼

You can use list ?? Enumerable.Empty<Friend>() 你可以使用list ?? Enumerable.Empty<Friend>() list ?? Enumerable.Empty<Friend>() , or have FindFriends return Enumerable.Empty<Friend>() list ?? Enumerable.Empty<Friend>() ，或让FindFriends返回Enumerable.Empty<Friend>()

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#3楼

您可以返回Enumerable.Empty<T>() 。

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#4楼

至于我，最优雅的方式是yield break

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#5楼

I think the simplest way would be 我认为最简单的方法是

return new Friend[0];

The requirements of the return are merely that the method return an object which implements IEnumerable<Friend> . 返回的要求仅仅是该方法返回一个实现IEnumerable<Friend>的对象。 The fact that under different circumstances you return two different kinds of objects is irrelevant, as long as both implement IEnumerable. 事实上，在不同情况下你返回两种不同类型的对象是无关紧要的，只要两者都实现IEnumerable。

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#6楼

That's of course only a matter of personal preference, but I'd write this function using yield return: 这当然只是个人偏好的问题，但我会使用yield return来编写这个函数：

public IEnumerable<Friend> FindFriends() { //Many thanks to Rex-M for his help with this one. //http://stackoverflow.com/users/67/rex-m if (userExists) { foreach(var user in doc.Descendants("user")) { yield return new Friend { ID = user.Element("id").Value, Name = user.Element("name").Value, URL = user.Element("url").Value, Photo = user.Element("photo").Value } } } }