Filtering whole of hierarchical list with LINQ

I have a simple hiearchical list of objects "ProductGroups" (IQueryable). Each "ProductGroup" has a collection of ProductGroups called "Children",an integer ParentId field and a boolean "IsEnabled" field.

public class ProductGroup
{
    public int Id;
    public int ParentId;
    public ICollection<ProductGroup> Children;
    public bool IsEnabled;
}

I want to be able to return a tree of ProductGroups where "IsEnabled" is true.

At the moment, if I do

ProductGroups.Where(x => x.IsEnabled)

This returns the enabled products. If I do

ProductGroups.Where(x => x.ParentId == null)

This returns the roots. I want to be able to return a full tree excluding the disabled items in the neatest way possible (i.e. not using for loops after querying the collection).

ProductGroup1 (IsEnabled == true)
    |
    --------- ProductGroup2 (IsEnabled == true)
    |               |
    |               ----------- ProductGroup4 (IsEnabled == false)
    |
    --------- ProductGroup4 (IsEnabled == false)

i.e. Return开发者_如何学Python ProductGroup1 with 1 child ProductGroup2

Thanks

---

I'm not sure of your exact needs with the filtered collection. I'm looking at it as a View-Model. And a recursive extension method provides you with a neat way to achieve this filter:

public static class Helper
{
    public static ICollection<ProductGroup> EnabledWithChildren(this ICollection<ProductGroup> source)
    {
        var result = new List<ProductGroup>();
        foreach (var ele in source)
        {
            if(ele.IsEnabled == true)
            {
                var productGroup = new ProductGroup{Id = ele.Id, IsEnabled = ele.IsEnabled, ParentId = ele.ParentId};
                if(ele.Children != null && ele.Children.Count() > 0) 
                    productGroup.Children = ele.Children.EnabledWithChildren();
                result.Add(productGroup);
            }
        }
        return result;
    }    
}

And usage:

public class ProductGroup
{
    public int Id;
    public int? ParentId;
    public ICollection<ProductGroup> Children;
    public bool IsEnabled;

    static void Main()
    {
        var filteredProductGroups = ProductsGroups.EnabledWithChildren();
    }
}