public class Container
{
    protected Container()
    {
        this.Foos = new List<IFooBuilder>();
    }
 
    public IList<IFooBuilder> Foos { get; set; }
 
    public FooBuilder<T> Foo<T>() where T : IFoo, new()
    {
        var builder =
            new FooBuilder<T>();
 
        this.Foos.Add(builder);
 
        return builder;
    }
 
    public IEnumerable<IFoo> GetFoos()
    {
        var foos =
            from builder in this.Foos
            select builder.GetBuiltInstance();
 
        return foos;
    }
}
 
/// <summary>
/// This interface makes it possible since it gives me a non-generic type to
/// cast all the generic types into when they are stored.
/// </summary>
public interface IFooBuilder
{
    IFoo GetBuiltInstance();
}
 
public class IFooBuilder<T> : IFooBuilder where T : IFoo, new()
{
    public IFoo GetBuiltInstance()
    {
        throw new NotImplementedException();
    }
}