C# Linq：连接中的 == 和 equals 有什么区别？
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Linq: What is the difference between == and equals in a join?
提问by Michael Klement
I always wondered why there's an equalskeyword in linq joins rather than using the == operator.
我一直想知道为什么equalslinq 连接中有一个关键字而不是使用 == 运算符。
Property deadline =
(from p in properties
join w in widgets
on p.WidgetID equals w.ID
select p).First();
Instead of
代替
Property deadline =
(from p in properties
join w in widgets
on p.WidgetID == w.ID
select p).First();
[EDIT]Rephrased the question and revised the examples.
[编辑] 重新表述问题并修改示例。
采纳答案by Konrad Rudolph
There's a nice explanation by Matt Warren at The Moth:
The Moth 的Matt Warren 有一个很好的解释：
"The reason C# has the word ‘equals' instead of the ‘==' operator was to make it clear that the ‘on' clause needs you to supply two separate expressions that are compared for equality not a single predicate expression. The from-join pattern maps to the Enumerable.Join() standard query operator that specifies two separate delegates that are used to compute values that can then be compared. It needs them as separate delegates in order to build a lookup table with one and probe into the lookup table with the other. A full query processor like SQL is free to examine a single predicate expression and choose how it is going to process it. Yet, to make LINQ operate similar to SQL would require that the join condition be always specified as an expression tree, a significant overhead for the simple in-memory object case."
“C# 使用‘equals’这个词而不是‘==’运算符的原因是为了明确‘on’子句需要你提供两个单独的表达式来比较相等性，而不是单个谓词表达式。来自- join 模式映射到 Enumerable.Join() 标准查询运算符，该运算符指定两个单独的委托，用于计算然后可以比较的值。它需要它们作为单独的委托，以便使用一个构建查找表并探索查找一个完整的查询处理器，如 SQL 可以自由地检查单个谓词表达式并选择它将如何处理它。然而，要使 LINQ 操作类似于 SQL 需要始终将连接条件指定为表达式树，对于简单的内存对象情况来说是一个很大的开销。”
However, this concerns join. I'm not sure equalsshould be used in your code example (does it even compile?).
然而，这令人担忧join。我不确定是否equals应该在您的代码示例中使用（它甚至可以编译吗？）。
回答by Jon Skeet
Your first version doesn't compile. You onlyuse equalsin joins, to make the separate halves of the equijoin clear to the compiler.
您的第一个版本无法编译。您只使用equalsin 连接，以使编译器清楚等连接的单独部分。
