Is a variant of this possible with C# Generics?

Question

Am i trying to achieve the impossible or is it simply unnecessary? The code in GetList is the same for all specializations of the class A, apart from the bit to create a new instance of the specialization. There is no default constructor, always the same list of parameters.

At the moment I am using instead public static List<A> GetList(int i, Func<A> createMeAnInstance)

Which feels unclean.

abstract class A
{
  protected A (int i)
  {

  }

  public static List<T> GetList<T>(int i) where T : A
  {
    var list = new List<T>();
    for(int x = 1; x< 100; x++)
    {
     var o = new T(i);
     list.Add(o);
    }
  }
 }   

 public class B : A
 {
    public B (int i) : base(i) {}
 }

 ...

 var list = A.GetList<B>(1);

score 3 · Answer 1 · answered Apr 11 '11 at 14:53

3

Operator new compiles as call Activator.CreateInstance.

Just write

Activator.CreateInstance(typeof(T), new object[] {i})

not perfect but works.

answered Apr 11 '11 at 14:53

gandjustas

1,925
14
12

1

No, it doesn't compile as a call to `Activator.CreateInstance` – Dyppl Apr 11 '11 at 14:57
http://stackoverflow.com/questions/367577/why-does-the-c-compiler-emit-activator-createinstance-when-calling-new-in-with-a – gandjustas Apr 11 '11 at 15:02
I'm tempted to mark this one as correct. Seems the cleanest to me. Sending in a instantiating code as a parameter doesnt seem DRY to me. Any comments as to Func param vs Activator.CreateInstance ? – jenson-button-event Apr 11 '11 at 15:03
Activator.CreateInstance is slow, but with Func you actually specifies creation of object outside of your function. It's an arbitary decision depending on your problem. – gandjustas Apr 11 '11 at 15:06
@gandjustas: ok, you were right and I apologize. Thanks for the link, it was new for me – Dyppl Apr 11 '11 at 15:07
I've marked this up as its an option and not incorrect, but I've chosen the delegate approach - it might not be dry but Its safer IMO. – jenson-button-event Apr 11 '11 at 15:15

score 2 · Accepted Answer · answered Apr 11 '11 at 14:54

2

You can't call parameterized constructors on generic types. Your Func<> version is the best approach here.

answered Apr 11 '11 at 14:54

Sean

60,939
11
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Daniel A. White · Answer 3 · 2011-04-11T15:06:51.460

0

You can't do that. You only can specify new() as a type constraint for constructors.

Why don't you take the B instance directly in GetList?

List<B> list = A.GetList(new B(i));

Use this code:

public static List<T> GetList<T>(T o) where T : A
{
    var list = new List<T>();
    list.Add(o);
    return list;
}

edited Apr 11 '11 at 15:06

answered Apr 11 '11 at 14:48

Daniel A. White

187,200
47
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well are u saying GetList(Func code) or GetList(A a) ? The latter doesnt work for me – jenson-button-event Apr 11 '11 at 15:05
added a for loop which hopefully highlights why i need a delegate and not a literal. – jenson-button-event Apr 11 '11 at 15:13

score 0 · Answer 4 · answered Apr 11 '11 at 14:53

0

If I understand you correctly... You can create an instance of T by reflecting it's constructors, or using the Activator.

answered Apr 11 '11 at 14:53

M.A. Hanin

8,044
33
51

score 0 · Answer 5 · answered Apr 11 '11 at 15:03

0

I think it's better to use default constructor and move i to the base class (A) as a field. But in some situations it could be more appropriate to use reflection

answered Apr 11 '11 at 15:03

Archeg

8,364
7
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Is a variant of this possible with C# Generics?

5 Answers5