Generics and Delegates

Question

Hi

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

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

make, object[] params = null)

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

this way?

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

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

specific closed type instead of <T> 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?

Vulpes · 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.

Vulpes · Answer

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.

S · Answer

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?

Vulpes · Answer

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.

S · Answer

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?