C# 使用 .DefaultIfEmpty() 而不是 .FirstOrDefault() ?? String.Empty;
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Use .DefaultIfEmpty() instead of .FirstOrDefault() ?? String.Empty;
提问by Elisabeth
How can I integrate the .DefaultIfEmpty()extension method so I have notto use
如何集成.DefaultIfEmpty()扩展方法，以便我不必使用
.FirstOrDefault() ?? String.Empty;
Code:
代码：
(from role in roleList
let roleArray = role.RoleId.Split(new char[] { WorkflowConstants.WorkflowRoleDelimiter })
where roleArray.Length.Equals(_SplittedRoleIdArrayLength) &&
HasAccessToCurrentUnit(roleArray[_UnitIndexInRoleId])
select roleArray[_LevelIndexInRoleId]).FirstOrDefault() ?? String.Empty;
采纳答案by ken2k
You could use:
你可以使用：
var query = ...;
return query.DefaultIfEmpty(string.Empty).First();
But this doesn't reduce the complexity IMO.
但这并没有降低 IMO 的复杂性。
回答by Denys Denysenko
If you're interested in extension method then you could use something like this:
如果你对扩展方法感兴趣，那么你可以使用这样的东西：
public static class Helpers
{
public static string FirstOrEmpty(this IEnumerable<string> source)
{
return source.FirstOrDefault() ?? string.Empty;
}
}
Edit
编辑
This method isn't generic because then we'd have to use default(T)and it'll give us nullinstead of string.Empty.
这个方法不是通用的，因为那样我们就必须使用default(T)它，它会给我们null而不是string.Empty.
回答by Ken Kin
The code:
编码：
var query=(
from role in roleList
let delimiter=WorkflowConstants.WorkflowRoleDelimiter
let roleArray=role.RoleId.Split(new char[] { delimiter })
where roleArray.Length.Equals(_SplittedRoleIdArrayLength)
where HasAccessToCurrentUnit(roleArray[_UnitIndexInRoleId])
select roleArray[_LevelIndexInRoleId]
).DefaultIfEmpty("").FirstOrDefault();
For the suspicion about the semantic meaning of DefaultIfEmptyand FirstOrDefault, following is the code decompiled from the library:
出于对DefaultIfEmptyand语义的怀疑FirstOrDefault，以下是从库中反编译出来的代码：
Code
public static IEnumerable<TSource> DefaultIfEmpty<TSource>(this IEnumerable<TSource> source) { return source.DefaultIfEmpty<TSource>(default(TSource)); } public static IEnumerable<TSource> DefaultIfEmpty<TSource>(this IEnumerable<TSource> source, TSource defaultValue) { if (source == null) { throw Error.ArgumentNull("source"); } return DefaultIfEmptyIterator<TSource>(source, defaultValue); } public static TSource First<TSource>(this IEnumerable<TSource> source) { if (source == null) { throw Error.ArgumentNull("source"); } IList<TSource> list = source as IList<TSource>; if (list != null) { if (list.Count > 0) { return list[0]; } } else { using (IEnumerator<TSource> enumerator = source.GetEnumerator()) { if (enumerator.MoveNext()) { return enumerator.Current; } } } throw Error.NoElements(); } public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source) { if (source == null) { throw Error.ArgumentNull("source"); } IList<TSource> list = source as IList<TSource>; if (list != null) { if (list.Count > 0) { return list[0]; } } else { using (IEnumerator<TSource> enumerator = source.GetEnumerator()) { if (enumerator.MoveNext()) { return enumerator.Current; } } } return default(TSource); }
代码
public static IEnumerable<TSource> DefaultIfEmpty<TSource>(this IEnumerable<TSource> source) { return source.DefaultIfEmpty<TSource>(default(TSource)); } public static IEnumerable<TSource> DefaultIfEmpty<TSource>(this IEnumerable<TSource> source, TSource defaultValue) { if (source == null) { throw Error.ArgumentNull("source"); } return DefaultIfEmptyIterator<TSource>(source, defaultValue); } public static TSource First<TSource>(this IEnumerable<TSource> source) { if (source == null) { throw Error.ArgumentNull("source"); } IList<TSource> list = source as IList<TSource>; if (list != null) { if (list.Count > 0) { return list[0]; } } else { using (IEnumerator<TSource> enumerator = source.GetEnumerator()) { if (enumerator.MoveNext()) { return enumerator.Current; } } } throw Error.NoElements(); } public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source) { if (source == null) { throw Error.ArgumentNull("source"); } IList<TSource> list = source as IList<TSource>; if (list != null) { if (list.Count > 0) { return list[0]; } } else { using (IEnumerator<TSource> enumerator = source.GetEnumerator()) { if (enumerator.MoveNext()) { return enumerator.Current; } } } return default(TSource); }
And here are something to mention:
这里有一些值得一提的：
DefaultIfEmptyhas a parameterless overload, which invokes the parameterized overload withdefault(TSource)and return its result.The only difference between parameterless
FirstOrDefaultandFirst, is the latter would throw when the collection is empty.For more information, see Enumerable.FirstOrDefault<TSource> Methodon MSDN.
FirstOrDefaultsemantically expressedfirst or default, and so called; it's not namedfirst or null. In c#,default(T)for a reference type is null, but for non-reference type, it's not. For example,default(int)is zero.The keyword
defaultwas never said nullsemantically. It's DEFAULT.Also, for more information, default Keywordon MSDN.
DefaultIfEmpty有一个无参数重载，它调用参数化重载default(TSource)并返回其结果。无参数
FirstOrDefault和First,之间的唯一区别是后者会在集合为空时抛出。有关详细信息，请参阅MSDN 上的Enumerable.FirstOrDefault<TSource> 方法。
FirstOrDefault语义上表达的first or default，所谓的；它没有命名first or null。在 c# 中，default(T)对于引用类型是null，但对于非引用类型，它不是。例如，default(int)为零。关键字
default在语义上从未被说为空。这是默认的。此外，有关更多信息，请参阅MSDN 上的默认关键字。
回答by Ian Robertson
I found this course on PluralSight interesting and I remembered it when I saw this question.
我发现 PluralSight 上的这门课程很有趣，当我看到这个问题时我就记住了。
You could watch the whole course, but the Map Reduce and Option<T>strategies especially with DefaultIfEmpty seemed like it could be a good fit to your use case.
您可以观看整个课程，但 Map Reduce 和Option<T>策略，尤其是 DefaultIfEmpty 似乎很适合您的用例。
Tactical Design Patterns in .NET: Control Flow by Zoran Horvathttps://app.pluralsight.com/library/courses/tactical-design-patterns-dot-net-control-flow/table-of-contents
.NET 中的战术设计模式：Zoran Horvat 的控制流https://app.pluralsight.com/library/courses/tactical-design-patterns-dot-net-control-flow/table-of-contents
