IEnumerable任务

发布于 2024-12-02 20:58:18 字数 859 浏览 5 评论 0原文

我有两个对象声明为 IEnumerable xVal 和 IEnumerable yVal。然后我的程序在哪里：

var xVals = from p in result where p.chemical == "Benzene" select p.SampDate_Name;
var yVals = from p in result where p.chemical == "Benzene" select p.PCL;

然后我尝试执行以下分配：

 xVal = xVals as IEnumerable<DateTime>;
 yVal = yVals as IEnumerable<double>;

上面的代码将xVal和yVal设置为null。

有人能解释一下这里出了什么问题吗？我将非常感谢您的帮助。

谢谢。

有一点是确定的：输入不为空。因此，根据我的调试，xVals 和 yVals 是非空的。我还可以循环遍历它们以获取每个内容的内容。当我回来并发布我的发现时，我会尝试其他建议。非常感谢大家的回复。

这里的问题是 LINQ 是惰性执行的，当你执行 foreach 或 ToList/ToArray/ToDictionary 时，它将被执行。我的代码中的两个 linq 查询在分配完成之前没有执行。因此，xVal 和 yVal 被设置为 null。感谢您的所有帮助。

原文

I have two objects declared as IEnumerable<DateTime> xVal and IEnumerable<double> yVal.
Then some where is my program I have:

var xVals = from p in result where p.chemical == "Benzene" select p.SampDate_Name;
var yVals = from p in result where p.chemical == "Benzene" select p.PCL;

Then I am trying to the following assignment:

 xVal = xVals as IEnumerable<DateTime>;
 yVal = yVals as IEnumerable<double>;

And the above code sets xVal and yVal to null.

Could anybody explain what is wrong here? I would very much appreciate the help.

Thanks.

One thing is certain: The input is not null. So xVals and yVals are non-null as debugged by me. I am also able to loop through them to get the content of each.
I will try the other suggestions when I get back and post my findings. Thanks a lot folks for your replies.

The problem here was that LINQ is executed lazy, It will be executed when you do either foreach or ToList/ToArray/ToDictionary. The two linq queries in my code were not executed before the assignment was done. Consequently xVal and yVal were set to null.
Thanks for all the help.

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夏日浅笑〃 2024-12-09 20:58:18

如果输入为 null 或无法执行强制转换，as 关键字会将输出设置为 null。

如果您使用 xVal = (IEnumerable)xVals 您可能会收到一个异常，指出无法进行转换。

as 关键字的 MSDN 说明

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坏尐絯℡ 2024-12-09 20:58:18

如果您有一个通用集合，要将其获取到 IEnumerable，请尝试 .AsEnumerable()。所以你的新代码...

xVal = xVals.AsEnumerable();
yVal = yVals.AsEnumerable();

If you have a generic collection, to get it to IEnumerable, try .AsEnumerable(). So your new code...

xVal = xVals.AsEnumerable();
yVal = yVals.AsEnumerable();

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柠檬色的秋千 2024-12-09 20:58:18

as 关键字实际上并不执行强制转换，而是执行相关引用类型之间的转换。因此，如果您有一个具体类型，您可以使用 as 将其分配给具有基类类型的新变量。如果类型不兼容，则 as 将返回 null。这是进行类型检查而不必担心异常的好方法。相反，您可以对转换结果执行空检查。

很难说出 p.SampDate_Name 和 p.PCL 是什么类型，但我猜字符串是什么类型？如果是这样，您可以将强制转换添加到 select 语句中。

var xVals = from p in result where p.chemical == "Benzene" select DateTime.Parse(p.SampDate_Name);
var yVals = from p in result where p.chemical == "Benzene" select Double.Parse(p.PCL);

The as keyword doesn't actually do a cast, but does conversions between related reference types. So if you have a concrete type, you could treat assign it to a new variable with a type of the base class using as. If the types are not compatible, then as will return null. This is a good way to do type checking without having to worry about exceptions. Instead you can perform a null check on the result of the conversion.

It's hard to tell what types p.SampDate_Name and p.PCL are, but I would guess strings? If so you can add the cast to the select statement.

var xVals = from p in result where p.chemical == "Benzene" select DateTime.Parse(p.SampDate_Name);
var yVals = from p in result where p.chemical == "Benzene" select Double.Parse(p.PCL);

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