C# Sort list by IDs, but without creating a new list

Question

I have 2 lists, one is the original list and the other is a sorted list. I want to sort the first list by the second list, but my method will not return anything, so it must sort the list in place.

I did this method:

public void SortByList(IEnumerable<JObject> source, IEnumerable<JObject> products)
{
    var ids = source.Select(m => m["_id"].ToString()).ToList();
    products.OrderBy(m => ids.IndexOf(m["_id"].ToString()));
}

But as you know, OrderBy creates a new list. How can I Sort using the ids?

You can sort in place using a Comparison<T> delegate: stackoverflow.com/a/3309292/860585 — Rotem
– Rotem, Commented Jun 25, 2018 at 12:28
Or an IComparer<T>. But that's not going to work while your parameter is IEnumerable<JObject> - can you change it to a List<JObject>? — Jon Skeet
– Jon Skeet, Commented Jun 25, 2018 at 12:28
use the yield syntax public IEnumerable<JObject> ... and inside of the method, you write actual code. But instead of returning a list, you use yield return to return a single entry. C# will do all the magic required to make it an IEnumerable<> to further enhance, you can then make the method a static extension by making the method static and adding a this in front of source — X39
– X39, Commented Jun 25, 2018 at 12:31
The method @X39 suggests is the only option if you need it to remain an IEnumerable. Otherwise in order to sort it you would have to convert it to a concrete type, e.g. List<JObject>. — Rotem
– Rotem, Commented Jun 25, 2018 at 12:32

Tim Schmelter · Accepted Answer · 2018-06-25 12:43:01Z

1

Well, there are no lists, so i assume that you actually pass lists. If you don't want to create a new list you have to cast it to list or change the parameter type to List<T>. Then you can use List.Sort:

public void SortByList(List<JObject> sourceList, IEnumerable<JObject> products)
{
    List<JObject> productList = products as List<JObject> ?? products.ToList();
    sourceList.Sort((x1, x2) => productList
        .FindIndex(p => p["_id"] == x1["_id"])
        .CompareTo(productList.FindIndex(p => p["_id"] == x2["_id"])));
}

answered Jun 25, 2018 at 12:43

Tim Schmelter

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Comments

X39 · Accepted Answer · 2018-06-25 12:48:10Z

1

Use the yield syntax:

//Returns source sorted by products
//Due to usage of the `this` keyword on the `IEnumerable<JObject> source` parameter,
//this method can be called like a member method on any instance imlpementing `IEnumerable<JObject>`.
public static class MyProductExtensionMethods {
    public static IEnumerable<JObject> SortByList(this IEnumerable<JObject> source,
                IEnumerable<JObject> products) {
        foreach(var prod in products) {
            // sorting happens here and assigning the next sorted item
            // to the variable  `sorted_next` (sorting part obviously not provided)
            yield return sorted_next;
        }
    }
}

3 Comments

r3plica Over a year ago

Is this not just returning a list though? I need it to be void. It must sort in place

X39 Over a year ago

sort in place is not working with IEnumerable<> at all (see the IEnumerator interface for more on why it is not, short version: it was never supposed to support something like that). If you need it to sort in-place, you need to pass a valid, sortable list or array. This method gives you a separate IEnumerable<> based on what you fed it.

X39 Over a year ago

If you again, want to feed a List<> implementation, you probably got a duplicate here of stackoverflow.com/questions/3309188/… as CodeNotFound said

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