[英]Extension method on generic list
I'm quite new to C#, and have two questions regarding generic lists and extension methods. 我对C#相当陌生,对于泛型列表和扩展方法有两个问题。 Sorry if the questions are a bit stupid.. 对不起,如果问题有点愚蠢。
What is the difference between: 之间有什么区别?
public static IObjectWithTempID FindByTempID
(this ObservableCollection<IObjectWithTempID > list, long tempID)
and 和
public static IObjectWithTempID FindByTempID< E >
(this ObservableCollection< IObjectWithTempID > list, long tempID)
I have tried to read up on the subject, but still don't understand :$ 我已经尝试阅读有关该主题的内容,但仍然不了解:$
I have run into a strange issue. 我遇到了一个奇怪的问题。 When I declare an ObservableCollection
, such as this 当我声明一个ObservableCollection
,例如这样
ObservableCollection<TestObjectWithTempID> a =
new ObservableCollection<TestObjectWithTempID>();
it is possible to call extension method 可以调用扩展方法
public static long FindByTempID
(this IEnumerable< IObjectWithTempID > list, long tempID){}
on the list. 在清单上。 It is surprisingly not possible to call extension method 令人惊讶的是不可能调用扩展方法
public static long FindByTempID
(this ObservableCollection< IObjectWithTempI D> list, long tempID){}
though. 虽然。 What have I missed here? 我在这里错过了什么?
For 2, it needs to be: 对于2,它必须为:
TheMethod<T>(ObservableCollection<T> list)
where T : ISomeInterface
Otherwise you could attempt to add any ISomeInterface instance to a more-specific list. 否则,您可以尝试将任何 ISomeInterface实例添加到更特定的列表中。
For your first question: 对于第一个问题:
In the second method declaration the E in FindByTempID<E>
is a type parameter . 在第二个方法声明中, FindByTempID<E>
中的FindByTempID<E>
是类型参数 。 In other words it is a placeholder for a type, not an actual type. 换句话说,它是类型的占位符 ,而不是实际类型。 E
can be used in the method body as if it were a real type. E
可以在方法主体中使用,就像它是实类型一样。 The general term for this type of programming is generics . 这类编程的总称是泛型 。
Having these extension methods 具有这些扩展方法
public static class Extenders
{
public static void FindByTempID(this ObservableCollection<IObjectWithTempID> e, long tempID)
{
}
public static void FindByTempID2(this IEnumerable<IObjectWithTempID> e, long tempID)
{
}
public static void FindByTempID3(this ObservableCollection<TestObjectWithTempID> e, long tempID)
{
}
public static void FindByTempID4<T>(this ObservableCollection<T> e, long tempID)
{
}
}
Then 然后
var a1 = new ObservableCollection<TestObjectWithTempID>();
var a2 = new ObservableCollection<IObjectWithTempID>();
//a1.FindByTempID(1); /* Is not a collection of interface */
a1.FindByTempID2(1); // IEnumerable<T> is a generic list
a1.FindByTempID3(1); // It is a collection of TestObjectWithTempID
a1.FindByTempID4(1); // Identify the collection as ObservableCollection<TestObjectWithTempID>
a2.FindByTempID(1); // Is a collection of interface
a2.FindByTempID2(1); // IEnumerable<T> is a generic list
//a2.FindByTempID3(1); /* Is not a collection of TestObjectWithTempID */
a2.FindByTempID4(1); // Identify the collection as ObservableCollection<IObjectWithTempID>
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