如何确保 FirstOrDefault<KeyValuePair> 已返回值

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);

从 MSDN - Enumerable.FirstOrDefault<TSource>：

如果源为空，则默认（TSource）；否则，源中的第一个元素。

笔记：

如果您的代码是通用的，最好使用 EqualityComparer.Default.Equals(day, defaultDay)，因为 .Equals 可能会被覆盖或 day 可能为空。

在 C# 7.1 中，您将能够使用 KeyValuePair defaultDay = default;，请参阅目标类型的“默认”文字。

另请参阅：参考源 - FirstOrDefault

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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();
}

这完全解决了对结构使用奇怪的默认值的东西。

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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));

接着 ：

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

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选择一个有效值列表，然后检查它是否包含您的 day.Value；像这样：

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));

//check that FirstOrDefault has returned a valid value
if (days.Select(x=>x.Value).Contains(day.Value))
{
    //VALID
}
else
{
    System.Diagnostics.Debug.Write("Couldn't find day of week");
}

这是另一个使用键值对列表的示例：

var days = new List<KeyValuePair<int, string>>();
days.Add(new KeyValuePair<int, string>(1, "Monday"));
days.Add(new KeyValuePair<int, string>(1, "Tuesday"));
days.Add(new KeyValuePair<int, string>(1, "Sunday"));
var sampleText = "My favorite day of the week is 'xyz'";
var day = days.FirstOrDefault(x => sampleText.Contains(x.Value));

//check that FirstOrDefault has returned a valid value
if (days.Select(x => x.Value).Contains(day.Value))
{
    //VALID
}
else
{
    System.Diagnostics.Debug.Write("Couldn't find day of week");
}

希望这对某人有帮助！

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