[C#] new() constraint. Is it possible to add parameters?

No. The implementation of the constraint mechanism in the CLR metadata essentially uses a bit to indicate that the new-constraint exists.

You can usually achieve what you are probably trying to achieve via the use of interfaces providing factory or allocator methods.

Nope, there is no ability to do that, though it would be useful. Interfaces would be an option, except they don't allow you specify constructor arguments. Having a base class would be the only way to ensure you can call a constructor of a particular type, and that isn't always possible.

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You can usually achieve what you are probably trying to achieve via the use of interfaces providing factory or allocator methods.

When using interfaces I have the problem of class conversion and lists.

For some reason Bar:

class Bar : IPool

cannot be converted to IPool for T

public class ResourcePool<T> where T : class, IPool

when using a list:

List<ResourcePool<IPool>> list = new List<ResourcePool<IPool>>();list.Add(ResourcePool<Bar>);

You want to use the interface to provide a means of allocating for the pool, i.e., the interface is the allocator and provided to the pool; its not the pool that uses an interface.

Could you please explain how?

I don't use IPool with the Pool then I can't use IPool specific properties inside the Pool, which is necessary for the pool to work.

I think (correct me if I'm wrong) I see where your problem is.
Generic collections in C# don't work that way, e.g. ResourcePool is different from ResourcePool, which is why you cannot add ResourcePool to the list (or even cast to ResourcePool for that matter).

You need to redesign your class and don't use generics. Try using an interface that exposes IPool as a property, e.g.

interface IPool{}interface IPoolSupport{    IPool Pool { get; }}interface IResourcePool : IPoolSupport{}class ResourcePool<T> : IResourcePool where T : class, IPool, new(){    public ResourcePool()    {        Pool = new T();    }       public IPool Pool { get; private set; }}class Bar : IPool{}var list = new List<IResourcePool>();list.Add(new ResourcePool<Bar>());

This way you can still access IPool from outside of RessourcePool by simply using the getter:

var resPool = list[0];var pool = resPool.Pool;

The missing parameter constraint with new() is makes perfect sense, btw. since the actual implementation of the constructor must be of no concern for classes that implement the interface.

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Could you please explain how?

Sure thing. What you're wanting to do with a parameterized new constraint is provide a specific signature that must be used to create objects. You can't do that, as we've discussed, with the constraint. But there are other ways to mandate interfaces: interface constraints.

So you can have a factory interface that produces objects of type T:

interface Factory<T> {  T Create( /* any parameters you like */ );

Then your pool can take one of those as a parameter:

class Pool<T> {  public Pool( Factory<T> factory ) {    ...  }}

Then you can use that factory's Create() method to create new instances of T with the appropriate parameters.

What I did for my pool class was simply allow a delegate to be provided that implemented the creation of the objects. In that way I didn't need the pool to know how to allocate the objects at all (it will fallback to a parameterless constructor if no delegate is given). I personally think this is the easiest solution to what you're trying to do.



So in this case I could pass any old function I wanted:

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Original post by NickGravelyn What I did for my pool class was simply allow a delegate to be provided that implemented the creation of the objects. In that way I didn't need the pool to know how to allocate the objects at all (it will fallback to a parameterless constructor if no delegate is given). I personally think this is the easiest solution to what you're trying to do.

I've used delegate instead of Func in order to pass parameters such as Game1 game. How could I create a list of your type of pool when they are class specific?

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Original post by darookie I think (correct me if I'm wrong) I see where your problem is. Generic collections in C# don't work that way, e.g. ResourcePool is different from ResourcePool, which is why you cannot add ResourcePool to the list (or even cast to ResourcePool for that matter)

Using your method gives me the following error:

The type 'IResourcePool' cannot be used as type parameter 'T' in the generic type or method 'ResourcePool<T>'. There is no implicit reference conversion from 'Cthonian.IResourcePool' to 'Cthonian.IPool'.

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Original post by jpetrie So you can have a factory interface that produces objects of type T: Then your pool can take one of those as a parameter: Then you can use that factory's Create() method to create new instances of T with the appropriate parameters.

How can I create a list of pools in this case that will allow adding of a pool using any type of class?

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How can I create a list of pools in this case that will allow adding of a pool using any type of class?

The same way you'd have to do before, with your original implementation. You'll note that my example (and Nick's as well) do not change the type signature of the Pool class. This does mean, of course, that you still can't create List>, because Class is not a parent of Class (in general).

You have two solutions. You can fill the list with Pool

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Original post by jpetrie The other, more common option is to have Pool implement an non-generic interface (IPool) or derive from a nongeneric possibly-abstract base class, if appropriate. Then you can store a list of IPools.

Could you please explain how as I thought I had tried this and it didn't work?

Would I also be able to gain access to the IsValid IPool property using this method?

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Could you please explain how as I thought I had tried this and it didn't work?

interface IPool {  bool IsValid { get; }}class Pool<T> : IPool {  ...}List<IPool> pools = new List<IPool>();

If I use your method then:



can't return null and



the generic T does not contain a property for IsValid.