linq里的非泛型集合的强制类型转换

Question

有如下的类，

class SunPlant

{

public string Name{get;set;}

public string Tag{get;set;}

}

然后在main方法里，

static void Main()

{

//定义一个非泛型集合

ArrayList plants = new ArrayList

{

new SunPlant{Name="a",Tag="red"},

new SunPlant{Name="b",Tag="red"},

new SunPlant{Name="c",Tag="green"},

}

//然后新建一个Linq查询

var query = from SunPlant p in plants where p.Tag =="red" select p;

}

想问下，这个query语句里的强制转换 SunPlant p是什么作用？

Answer 1

var query = select SunPlant p from plants where p.Tag =="red" select p; 这不是linq吧，这么写会报错的。而且要实现IEnumerable接口的才能使用linq，ArrayList集合用不了linq。

 var plants = new List<SunPlant>

            {

            new SunPlant{Name="a",Tag="red"},

            new SunPlant{Name="b",Tag="red"},

            new SunPlant{Name="c",Tag="green"},

            };
            var query = from p in plants
                        where p.Tag == "red"
                        select p;

Answer 2

刚才的代码有错误，已更正了，是这样的: var query = from SunPlant p in plants where p.Tag =="red" select p;

这样写是可以的，已经验证过了。就是不明白为什么。我也很困惑，它确实没有实现IEnumerable<T>这个泛型接口，所有想问问下。这里的from 后面，它强制转换了一下：

SunPlant p

Answer 3

Hi imeya,

你这样写的确是可以的，我这边测试也是没问题的，arraylist 也的确没有继承IEnumerable<T>。

问题在于你手动定义了这个变量。 这样非泛型的arraylist 也可以使用LINQ语句。

When using LINQ to query non-generic IEnumerable collections such as ArrayList, you must explicitly declare the type of the range variable to reflect the specific type of the objects in the collection.

详细的信息，看这个文档。

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/how-to-query-an-arraylist-with-linq

Best Regards,

Hart

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