Converting a generic collection of a concrete type to a collection of a base type

Question

I've got a number of classes that implement a specific interface (ISearchable) and I'd like to return an IEnumerable of the base type (ISearchable) from a static method, but I'm not sure how to convert it without making intermediate collections.

The code is pretty simple, one of the domain objects' implementations is like so:

public class account : ISearchable
{
    public static IEnumerable<ISearchable> Search(string keyword)
    {
        // ORMVendorCollection<T> implements IQueryable<T>
        ORMVendorCollection<account> results = /* linq query */

        // this works if I change the return type to IEnumerable<account>
        // but it uglifies client code a fair bit
        return results.AsEnumerable<account>();

        // this doesn't work, but it's what I'd like to achieve            
        return results.AsEnumerable<ISearchable>(); 
    }
}

The client code, ideally looks like this:

public static IEnumerable<ISearchable> Search(string keyword)
{
    return account.Search(keyword)
        .Concat<ISearchable>(order.Search(keyword))
        .Concat<ISearchable>(otherDomainClass.Search(keyword));
}

Answer 1

Use the Cast<T> extension method

return results.Cast<ISearchable>();

Answer 2

Couldn't you just do something like?

public class account : ISearchable 
{ 
    public static IEnumerable<ISearchable> Search(string keyword) 
    { 
        var results = /* linq query */ 
        foreach(var result in results)
            yeild return result;
    } 
} 

Answer 3

For C# 4.0, you may be able to cast directly, since IEnumerable<> is covariant

return (IEnumerable<ISearchable>)results.AsEnumerable();