Generics and Delegates

Guest User

Hi

Can anyone be so kind to explain (step by step) the below peice of code I stumbled across

public static List ReadList(string procedure, Func, T>

make, object[] params = null)

Ive never seen anything like this using generics  and Func<> delegates in

this way?

 From my understanding of the below method name ReadList which returns a

generic List when invoked, ReadList needs to be implemented with a

specific closed type instead of  and accepts 4 input parameters, the

first of type string, the second a datareader, the third a make, and fourth

a list of objects that return null?

5 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes14y ago
Accepted answer
I'm not surprised that you can't make any sense of this method declaration as it's defective and won't compile.

Firstly, there is one too many right angle brackets.

Secondly, 'params' is a keyword and can't be used as the name of a parameter unless 'escaped' with an @.

If it had been written like this, then it would compile:

public static List ReadList(string procedure, Func make, object[] @params = null)

The method would then take three parameters:

a) A string named 'procedure'

b) A Func delegate named 'make', representing a method which takes a single parameter of type IDataReader and produces a result of type T.

c) An object array named @params which, if not present, has a default value of null.
Vulpes14y ago
I haven't actually implemented it - just a 'return null;' in my simulation :)

All I was doing was demonstrating how you would call the method and how the compiler could infer the type of T.

It will, of course, work just as well if you specify T explicitly and/or specify null for the third argument.
Guest User14y ago
Thanks Vulpes,

Forgive my denseness but I dont see how you have implemented ReadList in your example. I can only see how youve supplied arguements to the parenthesis, ie

List list = ReadList("whatever", idr => 6.0); // Your copy

List list = ReadList("whatever", idr => 6.0); // my copy with after ReadList with no implementation?

Should ReadList have originally been T before the implementation to ReadList?
Vulpes14y ago
That's right, Steven.

As there are no constraints on T, then it could be replaced by any class or struct.

The method would return a reference to a List object.

Also, you shouldn't need to specify T when calling the method as the compiler should be able to deduce it from the delegate return type. To take a (rather silly) example, this should compile fine:

List list = ReadList("whatever", idr => 6.0);

As 6.0 is a double literal, the compiler should be able to figure out that T is double.

@params would, of course, be given its default value of null.
Guest User14y ago
Thats brilliant thanks Vuples, I got the code from DoFactory documentation believe it or not!

Could you explain how List the Return Type and ReadList the method name would work, I assume when implemented the placeholder would be replaced with strongly typed types\classes?