C# 如何确保 FirstOrDefault<KeyValuePair> 已返回值

How can I make sure that FirstOrDefault<KeyValuePair> has returned a value

提问by desautelsj

Here's a simplified version of what I'm trying to do:

这是我正在尝试做的简化版本：

var days = new Dictionary<int, string>();
days.Add(1, "Monday");
days.Add(2, "Tuesday");
...
days.Add(7, "Sunday");

var sampleText = "My favorite day of the week is 'xyz'";
var day = days.FirstOrDefault(x => sampleText.Contains(x.Value));

Since 'xyz' is not present in the dictionary, the FirstOrDefault method will not return a valid value. I want to be able to check for this situation but I realize that I can't compare the result to "null" because KeyValuePair is a struc. The following code is invalid:

由于字典中不存在“xyz”，因此 FirstOrDefault 方法将不会返回有效值。我希望能够检查这种情况，但我意识到我无法将结果与“null”进行比较，因为 KeyValuePair 是一个结构体。以下代码无效：

if (day == null) {
    System.Diagnotics.Debug.Write("Couldn't find day of week");
}

We you attempt to compile the code, Visual Studio throws the following error:

我们尝试编译代码，Visual Studio 抛出以下错误：

Operator '==' cannot be applied to operands of type 'System.Collections.Generic.KeyValuePair<int,string>' and '<null>'

How can I check that FirstOrDefault has returned a valid value?

如何检查 FirstOrDefault 是否返回了有效值？

采纳答案by Kobi

FirstOrDefaultdoesn't return null, it returns default(T).
You should check for:

FirstOrDefault不返回 null，它返回default(T).
您应该检查：

var defaultDay = default(KeyValuePair<int, string>);
bool b = day.Equals(defaultDay);

From MSDN - Enumerable.FirstOrDefault<TSource>:

从MSDN -Enumerable.FirstOrDefault<TSource>：

default(TSource) if source is empty; otherwise, the first element in source.

默认( TSource) 如果源为空；否则，source 中的第一个元素。

Notes:

笔记：

• If your code is generic it is better to use EqualityComparer<T>.Default.Equals(day, defaultDay), becuase .Equalsmay be overridden or daycould be a null.
• In C# 7.1 you will be able to use var defaultDay = default;, see Target-typed "default" literal.
• See also: Reference Source - FirstOrDefault

• 如果您的代码是通用的，最好使用EqualityComparer<T>.Default.Equals(day, defaultDay)，因为.Equals可能会被覆盖或day可能是null.
• 在 C# 7.1 中，您将能够使用var defaultDay = default;，请参阅目标类型的“默认”文字。
• 另见：参考来源 -FirstOrDefault

回答by peaceoutside

This is the most clear and concise way in my opinion:

这是我认为最清晰简洁的方式：

var matchedDays = days.Where(x => sampleText.Contains(x.Value));
if (!matchedDays.Any())
{
    // Nothing matched
}
else
{
    // Get the first match
    var day = matchedDays.First();
}

This completely gets around using weird default value stuff for structs.

这完全绕过了对结构使用奇怪的默认值的东西。

回答by Jocelyn Marcotte

You can do this instead :

你可以这样做：

var days = new Dictionary<int?, string>();   // replace int by int?
days.Add(1, "Monday");
days.Add(2, "Tuesday");
...
days.Add(7, "Sunday");

var sampleText = "My favorite day of the week is 'xyz'";
var day = days.FirstOrDefault(x => sampleText.Contains(x.Value));

and then :

进而 ：

if (day.Key == null) {
    System.Diagnotics.Debug.Write("Couldn't find day of week");
}