Extract derived elements from a list of base elements

Question

I have an interface list which stores a variety of objects derived from that interface. I would like to write a function where the caller specifies the type to extract. I've tried this:

List<IParts> PartList;

...

public List<IPart> Fetch(Type PartType)
{
   return this.PartList.OfType<PartType>().Cast<IPart>().ToList();
}

But it doesn't like a passed type. Any suggestions?

Answer 1

Specify it as a type parameter instead:

public List<IPart> Fetch<T>() where T : IPart
{
    return this.PartList.OfType<T>().Cast<IPart>().ToList();
}

You'd call it like this:

List<IPart> foo = parts.Fetch<Exhaust>();

That's fine if you know the part type at compile-time. If you don't, you'll need to use reflection to basically get the OfType() method in Enumerable<T> and create the write generic method from it. Something like:

private static readonly MethodInfo OfTypeMethod = 
    typeof(Enumerable).GetMethod("OfType");

public List<IPart> Fetch(Type partType)
{
    MethodInfo method = OfTypeMethod.MakeGenericMethod(partType);
    IEnumerable enumerable = (IEnumerable) method.Invoke
        (null, new object[] { PartList });
    return enumerable.Cast<IPart>().ToList();
}

Answer 2

If you have to support various types at runtime (rather than at compile time with generics):

public List<IPart> Fetch(Type PartType)
{
    if (!typeof(IPart).IsAssignableFrom(PartType))
    {
        throw new ArgumentOutOfRangeException("PartType", "Must derive from IPart");
    }
    return this.PartList.Where(i => i != null && PartType.IsAssignableFrom(i.GetType())).ToList();
}