I'm quite new to C#, and have two questions regarding generic lists and extension methods. Sorry if the questions are a bit stupid..
What is the difference between:
public static IObjectWithTempID FindByTempID
(this ObservableCollection<IObjectWithTempID > list, long tempID)
and
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 :$
I have run into a strange issue. When I declare an ObservableCollection
, such as this
ObservableCollection<TestObjectWithTempID> a =
new ObservableCollection<TestObjectWithTempID>();
it is possible to call extension method
public static long FindByTempID
(this IEnumerable< IObjectWithTempID > list, long tempID){}
on the list. It is surprisingly not possible to call extension method
public static long FindByTempID
(this ObservableCollection< IObjectWithTempI D> list, long tempID){}
though. What have I missed here?
For 2, it needs to be:
TheMethod<T>(ObservableCollection<T> list)
where T : ISomeInterface
Otherwise you could attempt to add any ISomeInterface instance to a more-specific list.
For your first question:
In the second method declaration the E in FindByTempID<E>
is a type parameter . In other words it is a placeholder for a type, not an actual type. E
can be used in the method body as if it were a real type. The general term for this type of programming is generics .
Having these extension methods
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
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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