扩展方法ConvertAll
ConverAll 的正确用法是什么？它将一种类型转换为另一种类型吗？
像
List<int> intList = new List<int>();
intList.Add(10);
intList.Add(20);
intList.Add(30);
intList.Add(33);
var query= intList.ConvertAll(x=>(double)x);
这样我可以使用cast或OfType<>。
What is the proper use of ConverAll ? Will it convert one type to another type?
like
List<int> intList = new List<int>();
intList.Add(10);
intList.Add(20);
intList.Add(30);
intList.Add(33);
var query= intList.ConvertAll(x=>(double)x);
for this i can use cast or OfType<>.
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ConvertAll不是扩展方法，它是List本身的一个真实方法。它返回一个包含转换后的元素的新列表。因此，在您的示例中，
query变量实际上并不是一个查询，它是一个List。Cast和OfType是扩展方法对IEnumerable进行操作并返回IEnumerable。但是它们不适合您声明的目的：Cast可以转换引用类型，但不能转换值类型，只能将它们拆箱。OfType不执行任何转换，它只是返回已经属于指定类型的任何元素。ConvertAllisn't an extension method, it's a real method onList<T>itself.It returns a new list containing the converted elements. So in your example, the
queryvariable isn't actually a query, it's aList<double>.CastandOfTypeare extension methods that operate onIEnumerableand return anIEnumerable<T>. However they're not suitable for your stated purpose:Castcan convert reference types but cannot convert value types, only unbox them.OfTypedoesn't perform any conversion, it just returns any elements that are already of the specified type.ConvertAll只会调用您的委托/列表中每个元素的匿名方法。这完全取决于你的作用。在您发布的示例代码中，它将尝试将每个元素转换为 double 并返回该元素，这意味着您将得到一个
List作为回报。您不应使用
OfType，因为这将根据类型过滤元素，并且仅当由于继承或接口实现而类型兼容时才会返回与原始类型不同的类型。换句话说，
.OfType将不返回任何元素，因为没有一个整数也是双精度数。ConvertAllwill just call your delegate/anonymous method for each element of the list. What this does is entirely up to you.In the example code you posted, it will attempt to cast each element to a double and return that, which means you'll get a
List<Double>in return.You should not use
OfType<T>, since this will filter the elements based on the type, and will only return a different type than the original if it is type compatible due to inheritance or interface implementation.In other words,
.OfType<Double>will return no elements, since none of the ints are also doubles.ConvertAll是一个投影运算符，与 LINQ 的Select映射最为接近。 LINQ 的Cast是一个特定的投影运算符，代表做你所做的事情[通过投影] - 或者它会（如卢克的回答[和评论]中指出的，我+1）如果你是不转换为值类型。一般来说，LINQ 有一组更完整且经过深思熟虑的运算符，这使得像
ConvertAll这样的旧东西有时看起来有点傻[像这样]。 （或 @stoopid :D）。ConvertAllis a projection operator and maps most closely to LINQ'sSelect. LINQ'sCastis a specific projection operator and represents doing what you did [via projection] - or it would (as pointed out in Luke's answer [and comment], which I +1'd) if you weren't converting to a value type.In general, LINQ has a more complete and well-thought-through set of operators, which makes older stuff like
ConvertAlllook a bit silly at times [like this]. (or @stoopid :D).据我所知，
OfType只会返回集合中指定类型 T 的元素。ConvertAll 允许您将元素转换为另一种类型。
to my knowledge,
OfType<T>will only return the elements in the collection that are of the specified type T.ConvertAll allows you to convert the elements to another type.