如果FirstOrDefault返回null，请从FirstOrDefault中选择一个属性 [英] selecting a property from FirstOrDefault in case FirstOrDefault returns null

问题描述

我要发表以下声明:

var block = blocksById.FirstOrDefault(X => X.Value == tracResult.ID).Key

我的问题是，在我拥有FirstOrDefault null值的情况下，如何更正确地处理它. 我只是觉得if陈述唯一的解决方案……

My question is how to deal with it more correctly in case I have FirstOrDefault null value. I just not feel that if statments the only solution that can be here...

推荐答案

由于引入了

Since the introduction of the null conditional operators (?), it's as simple as:

var block = blocksById.FirstOrDefault(X => X.Value == tracResult.ID)?.Key;

请记住，在这种情况下，即使Value是int，block的类型也将是Nullable<int>，因此也可能是null.

Keep in mind that in this scenario, even if Value is an int, block will be of type Nullable<int>, and hence, can be null.

但是，如果返回null时要分配默认值，则可以使用

However if what you want to assign a default value if null is returned you can make use of null coalescing operator (??) is this way:

var block = blocksById.FirstOrDefault(X => X.Value == tracResult.ID)?.Key ?? 6;

尽管如此，对于更复杂的Select语句，以下内容仍然有效...

Nevertheless, for more complex Select statements, the following is still valid...

将其拆分为Where和Select:

var block = blocksById.Where(x => x.Value == tracResult.ID)
                      .Select(x => x.Key)
                      .FirstOrDefault();

这样，如果FirstOrDefault返回null，您将不会收到NullRefferenceException.

That way you won't get a NullRefferenceException if the FirstOrDefault returns null.

或者，您可以给它一个默认值，如下所示:

Alternative you can give it a default value like this:

var block = blocksById.Where(x => x.Value == tracResult.ID)
                      .Select(x => x.Key)
                      .FirstOrDefault() ?? somedefaultvalue;

或如@Silvermind所述，用于非可空到可空类型(int):

Or as @Silvermind stated for non nullable to nullable types (int's):

var block = blocksById.Where(x => x.Value == tracResult.ID)
                      .Select(x => (int?) x.Key)
                      .FirstOrDefault() ?? somedefaultvalue;

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更新:有些人似乎怀疑这是否是一个有效的用例，并争辩说必须在程序的稍后部分对null进行检查.

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Update: some people seems to have doubts about this being a valid use case and argue that the check for null has to be done later on in the program.

尽管在很多情况下是这样，但不一定总是必须为true，或者如果是，则这种方式可能更方便.

While in a lot of circumstances this is the case, is doesn't always have to be true, or if it is, it might be more convenient this way.

一些例子:

//one liner, null check delegated (not my favorite): 
return SomeCalculatedValue(collection.Where(condition).Select(selection).FirstOrDefault());

//or a mapping (shown with default value):
var result = collection.Where(condition)
                       .Select(c => 
                           { 
                                Foo = c.Foo,
                                Bar = c.Bar
                           }).FirstOrDefault()) ?? new { Foo = "New", Bar = "Empty"};

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