Use nested generics without a lot of code

Question

I would like to create a linq like extension method to get all items from a hierarchical structure like a tree.

This is my extension

public static List<T> GetAllRecursive<T, TU>(this IList<T> list, Func<T, TU> func) where TU : IEnumerable<T> {
    var allList = new List<T>();
    var toAdd = list.ToList();
    while(true) {
        allList.AddRange(toAdd);
        var childs = toAdd.SelectMany(x => func(x)).ToList();
        if(childs.Count == 0) {
            return allList;
        }
        toAdd = childs;
    }
}

I call it like this

var allGuidelines = Guidelines.GetAllRecursive(x => (IEnumerable<MachineGuidelineTreeItemViewModel>)x.Children);

How can I improve this method so I don't need to cast every time I use this?

Thanks in advance

and why do you need to cast it to IEnumerable at all? Isn't x.Children an IEnumerable<> already? Is it a more generic type, for example, IEnumerable<object> or something like that? If so, how do you know that it's safe to cast it to IE<MachineGuidelineTreeItemViewModel> ? — quetzalcoatl
– quetzalcoatl, Commented Oct 13, 2017 at 7:17
I need to cast, because the compiler does not understand BindableCollection<T> is a IEnumerable<T> the problem is the generics. — horotab
– horotab, Commented Oct 13, 2017 at 7:25
If Children is BindableCollection<TreeItemViewModel> it means it can contain any descendant of TreeItemViewModel, not just MachineGuidelineTreeItemViewModel, so it's not safe to cast it to IEnumerable<MachineGuidelineTreeItemViewModel>. You might know it's safe at runtime but compiler sure cannot. — Evk
– Evk, Commented Oct 13, 2017 at 7:32

Sergey Kalinichenko · Accepted Answer · 2017-10-13 11:42:58Z

1

You can remove TU type parameter, and use OfType on IEnumerable instead:

public static List<T> GetAllRecursive<T>(this IList<T> list, Func<T, IEnumerable> func) {
    var allList = new List<T>();
    var toAdd = list.ToList();
    while(true) {
        allList.AddRange(toAdd);
        var childs = toAdd.SelectMany(x => func(x).OfType<T>()).ToList();
        if(childs.Count == 0) {
            return allList;
        }
        toAdd = childs;
    }
}

This approach is more lenient, because it lets you pass functions that return a mixed bag of items, with the method filtering it by type.

answered Oct 13, 2017 at 11:42

Sergey Kalinichenko

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3 Comments

BurnsBA Over a year ago

I think maybe the toAdd variable and childs variable, while required, do not necessarily have to be Lists. Probably won't make much of a difference, but I'm thinking it's possible to avoid that ToList() call when assigning childs...

Sergey Kalinichenko Over a year ago

@BurnsBA I agree, there's definitely some room for making this method even more general. I went with OP's implementation to address one specific shortcoming, while keeping the rest of his code in place.

horotab Over a year ago

thats exactly what i was looking for now i have a small call: var allGuidelines = Guidelines.GetAllRecursive(x => x.Children); - Thanks

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