Creating instance of an object which implements a given interface

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?

You can find such mistakes yourself by using the debugger to look at the runtime values of all computations involved. See the answer. — usr
– usr, Commented Jan 5, 2015 at 11:03

Selman Genç · Accepted Answer · 2015-01-05 11:02:27Z

7

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

answered Jan 5, 2015 at 11:02

Selman Genç

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3 Comments

Mama Tate Over a year ago

Thanks. So how should I correctly approach this certain scenario?

Selman Genç Over a year ago

implement the correct interface in your type. Or, cast it to correct type.

John Over a year ago

Casting to the correct type: var spawnable = (T)Activator.CreateInstance(type);

dcastro · Accepted Answer · 2015-01-05 11:09:42Z

1

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.

answered Jan 5, 2015 at 11:09

dcastro

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