Contravariance in C#

Question

I have a problem with contra-variant interfaces and am not sure the best way to solve it. These are my classes:

public interface IObject {
}
public interface IBigObject : IObject {
}
public interface ISmallObject : IObject {
}
public class AnObject : IBigObject {

} 
public class DataWrapper<T> {
    public T Data { get; set; }
} 

I want to be able to pass a DataWrapper<IObject> (which could be a number of derived classes) and get at the item as an IObject, and the only way I can do this as far as I can see is like this:

IObject itemToAdd = null;
if (values[1] is IObject) {
    itemToAdd = values[1] as IObject;
} else if (values[1] is DataWrapper<IObject>) {
    itemToAdd = ((DataWrapper<IObject>)values[1]).Data;
} else if (values[1] is DataWrapper<IBigObject>) {
    itemToAdd = ((DataWrapper<IBigObject>)values[1]).Data;
} else if (values[1] is DataWrapper<BigObjectBase>) {
    itemToAdd = ((DataWrapper<ObservableBigObject>)values[1]).Data;
} else if (values[1] is DataWrapper<ObservableBigObject>) {
    itemToAdd = ((DataWrapper<ObservableBigObject>)values[1]).Data;
} 

Is there a shorter way?

Answer 1

Classes cannot have variance in C#, so you'll have to create an interface e.g.

public interface IObjectWrapper<out T> where T : IObject
{
    T Value { get; }
}

Then you should be able to do

if(values[1] is IObjectWrapper<IObject>)
{
    itemToAdd = ((IObjectWrapper<IObject>)values[1]).Value;
}

If you don't want to do that, you could use reflection:

Type t = values[1].GetType();
if(t.IsGenericType && 
    t.GetGenericTypeDefinition() == typeof(DataWrapper<>) && 
    typeof(IObject).IsAssignableFrom(t.GetGenericArguments()[0]))
{
    itemToAdd = (IObject)t.GetProperty("Data").GetValue(value[0], null)
}