C# 在泛型列表的 ForEach() 中的 lambda 表达式中使用条件运算符？
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Using conditional operator in lambda expression in ForEach() on a generic List?
提问by Jamezor
Is it not allowed to have a conditional operator in a lambda expression in ForEach?
在 ForEach 的 lambda 表达式中不允许有条件运算符吗？
List<string> items = new List<string>{"Item 1", "Item 2", "Item I Care About"};
string whatICareAbout = "";
// doesn't compile :(
items.ForEach(item => item.Contains("I Care About") ?
whatICareAbout += item + "," : whatICareAbout += "");
Compilation error -> "Only assignment, call, increment, decrement, and new object expressions can be used as a statement"
编译错误->“只能将赋值、调用、递增、递减和新对象表达式用作语句”
Trying to use a normal if doesn't work either:
尝试使用正常的 if 也不起作用：
// :(
items.ForEach(item => if (item.Contains("I Care About")) {whatICareAbout += item + ", ";}
Just not possible?
只是不可能？
采纳答案by SLaks
You're using the shorter form of lambda expressions, which only allow a single expressions.
You need to the long form, which allows multiple statements.
您使用的是更短形式的 lambda 表达式，它只允许一个表达式。
您需要长格式，它允许多个语句。
For example:
例如：
items.ForEach(item => {
if (item.Contains("I Care About"))
whatICareAbout += item + ", ";
});
回答by Justin Grant
Try parentheses:
尝试括号：
items.ForEach(item => item.Contains("I Care About") ? (whatICareAbout += item + ",") : (whatICareAbout += "") );
+= has a higher precedence than ?, that may be why you're getting the error. With parentheses, the error may go away. Not 100% sure of this, though... lambda expressions may have additional restrictions which prevent use of assignment statements.
+= 的优先级高于 ?，这可能是您收到错误的原因。使用括号，错误可能会消失。不是 100% 确定这一点，但是...... lambda 表达式可能有额外的限制，阻止使用赋值语句。
UPDATE:
更新：
Instead of multiple += statements, it's a lot cleaner to put the conditional on the right-hand side of the assignment, like this:
与多个 += 语句不同，将条件放在赋值语句的右侧要简洁得多，如下所示：
List<string> items = new List<string> { "one", "two", "three" };
string whatICareAbout = "";
items.ForEach(item => whatICareAbout += item.Contains("I Care About") ? (item + ",") : "");
UPDATE 2:
更新 2：
But it's even better to just use Aggregate() since it's designed for exactly this scenario. Here's one sample:
但最好只使用 Aggregate() ，因为它专为这种情况而设计。这是一个示例：
string whatICareAbout = items.Aggregate("", (total, item) => item.Contains("I Care About") ? (total + item + ",") : total);
But I think @Matt Breckon'sanswer above (that I just saw as I was about to post this)is even better than my example since it deals with removing the terminal ",". Look at his answer... :-)
但我认为上面@Matt Breckon 的回答（我刚刚看到我正要发布这个）甚至比我的例子更好，因为它涉及删除终端“，”。看看他的回答... :-)
回答by Matt Breckon
What are you trying to acheive? Are you trying to form a string of comma separated items where they contain a particular value? In linq you would achieve this using the following:
你想达到什么目的？您是否试图形成一串逗号分隔的项目，其中包含特定值？在 linq 中，您将使用以下方法实现此目的：
List<string> items = new List<string> { "Item 1", "Item 2", "Item I Care About", "Item I Care About", "Item I Care About" };
string whatICareAbout = items.Where(x => x.Contains("I Care About"))
.Aggregate( (y, z) => y + ", " + z);
The output from this is "Item I Care About, Item I Care About, Item I Care About".
其输出是“我关心的项目，我关心的项目，我关心的项目”。
Note: Aggregate is a great way of ensuring there is no trailing ","
注意：聚合是确保没有尾随“,”的好方法
回答by Roman Boiko
The problem was that expression
问题是那个表达
item.Contains("I Care About") ? whatICareAbout += item + "," : whatICareAbout += ""
is not a statement. It just returns a value which has type string.
不是声明。它只返回一个类型为的值string。
There is a trick to make it work (just for fun):
有一个技巧可以让它工作（只是为了好玩）：
items.ForEach(item => (item.Contains("I Care About") ?
whatICareAbout += item + "," : whatICareAbout += "").GetType());
I simply added call to .GetType()method to create a statement from initial expression, and it compiled.
我只是添加了对.GetType()方法的调用以从初始表达式创建一个语句，并编译它。
