Dilemma in calling constructor of generic class

Question

I have this generic singleton that looks like this:

public class Cache<T>
{
    private Dictionary<Guid, T> cachedBlocks;

    // Constructors and stuff, to mention this is a singleton

    public T GetCache(Guid id)
    {
        if (!cachedBlocks.ContainsKey(id))
            cachedBlocks.Add(id, LoadFromSharePoint(id))
        return cachedBlocks[id];
    }

    public T LoadFromSharePoint(Guid id)
    {
        return new T(id)    // Here is the problem.
    }
}

The error message is:

Cannot create an instance of type T because it does not have the new() constraint.

I have to mention that I must pass that id parameter, and there is no other way to do so. Any ideas on how to solve this would be highly appreciated.

Answer 1

Normally you would constrain the type T to a type that has a default constructor and call that. Then you'd have to add a method or property to be able to provide the value of id to the instance.

public static T LoadFromSharePoint<T>(Guid id)
    where T : new()     // <-- Constrain to types with a default constructor
{
    T value = new T();
    value.ID = id;
    return value;
}

---

Alternatively since you specify that you have to provide the id parameter through the constructor, you can invoke a parameterized constructor using reflection. You must be sure the type defines the constructor you want to invoke. You cannot constrain the generic type T to types that have a particular constructor other than the default constructor. (E.g. where T : new(Guid) does not work.)

For example, I know there is a constructor new List<string>(int capacity) on List<T>, which can be invoked like this:

var type = typeof(List<String>);
object list = Activator.CreateInstance(type, /* capacity */ 20);

Of course, you might want to do some casting (to T) afterwards.

Answer 2

To do this you should specify what T is. Your Cache<T> can hold anything? Tiger, Fridge and int as well? That is not a sound design. You should constrain it. You need an instance of T which will take a Guid to construct the instance. That's not a generic T. Its a very specific T. Change your code to:

public class Cache<T> where T : Cacheable, new()
{
    private Dictionary<Guid, T> cachedBlocks;

    // Constructors and stuff, to mention this is a singleton

    public T GetCache(Guid id)
    {
        if (!cachedBlocks.ContainsKey(id))
            cachedBlocks.Add(id, LoadFromSharePoint(id))
        return cachedBlocks[id];

       //you're first checking for presence, and then adding to it
       //which does the same checking again, and then returns the
       //value of key again which will have to see for it again. 
       //Instead if its ok you can directly return

       //return cachedBlocks[id] = LoadFromSharePoint(id);

       //if your LoadFromSharePoint is not that expensive.
       //mind you this is little different from your original 
       //approach as to what it does.
    }

    public T LoadFromSharePoint(Guid id)
    {
        return new T { Key = id };    // Here is no more problem.
    }
}

public interface Cacheable
{
    Guid Key { get; set; }
}

Now derive all the cacheables (whatever Ts that you will pass it for Cache<T>) from the interface Cacheable.