创建实现给定接口的对象的实例

Question

I am trying to construct an object with Activator.CreateInstance(), however i am receiving null for some unknown to me reason.

public class SpawnManager
{

    public void CreateSpawnable<T>()
    {
        Type type = typeof(T);

        ISpawnable<SpawnableParameters> spawnable = Activator.CreateInstance(type) as ISpawnable<SpawnableParameters>;

        // the spawnable object always returns null
    }

    public void Start()
    {
        CreateSpawnable<SpawnableCollectible>();
    }

}


public class SpawnableCollectible : ISpawnable<ParametersCollectible>
{

    public void Spawn(ParametersCollectible parameters)
    {
    }

}

Can somebody explain why i can't create an object instance which implements the given interface like that and write the correct approach for instantiating such an object?

Answer 1

Your types do not match. SpawnableCollectible implements ISpawnable<ParametersCollectible> not ISpawnable<SpawnableParameters> so the cast fails and as operator returns null .

Answer 2

To add to what Selman22 said:

You'll probably want to constrain T to be a type that derives from ISpawnable<SpawnableParameters> . This will fix your problem:

public void CreateSpawnable<T>() where T: ISpawnable<SpawnableParameters>
{
    //...
}

This way, the client code cannot invoke your method with an invalid type T , and the as cast will always succeed.