How to search an item and get its index in Observable Collection

Question

public struct PLU
{ 
    public int ID { get; set; } 
    public string name { get; set; } 
    public double price { get; set; } 
    public int quantity {get;set;}
}

public static ObservableCollection<PLU> PLUList = new ObservableCollection<PLU>();

I have the ObservableCollection as above. Now I want to search the ID in the PLUList and get its index like this:

int index = PLUList.indexOf();
if (index > -1)
{
    // Do something here
}
else
{
    // Do sth else here..
}

What's the quick fix?

EDIT:

Let's assume that some items were added to PLUList and I want to add another new item. But before adding I want to check if the ID already exists in the list. If it does then I would like to add +1 to the quantity.

Mithrandir · Accepted Answer · 2012-02-17T13:15:30.803

25

Use LINQ :-)

var q =  PLUList.Where(X => X.ID == 13).FirstOrDefault();
if(q != null) 
{
   // do stuff
}
else 
{
   // do other stuff
}

Use this, if you want to keep it a struct:

var q =  PLUList.IndexOf( PLUList.Where(X => X.ID == 13).FirstOrDefault() );
if(q > -1) 
{
   // do stuff
}
else 
{
   // do other stuff
}

edited Feb 17 '12 at 13:15

answered Feb 17 '12 at 13:07

Mithrandir

24,869
6
50
66

It's a struct. You can't test it for null. – A.R. Feb 17 '12 at 13:12
I needed to replace struct to class. Thanks :) – user995387 Feb 17 '12 at 13:13
whoa.. what shud I do now? what do you think is the best? struct? or class? – user995387 Feb 17 '12 at 14:35
@user995387 : i would prefer to use a class. – Mithrandir Feb 17 '12 at 14:39
@Mithrandir : as far as i am getting the index value, I will stick with struct. Because I want to modify the collection item as well... Do you have any better idea to do it with class? – user995387 Feb 17 '12 at 14:48
@user995387: you can modify the collection item, if its a reference type as well. MS says structs should have no bigger playload than 16 bytes. The field/properties in your objects have definetly more than 16 bytes. – Mithrandir Feb 17 '12 at 14:51
@Mithrandir re: 16 bytes limit - citation needed – Jakub Konecki Sep 09 '15 at 07:37
@JakubKonecki Really, does it? Here you go then: https://msdn.microsoft.com/en-us/library/ms229017(v=vs.110).aspx – Mithrandir Sep 09 '15 at 14:47
1

Getting the index by first finding the item and then walking the whole list again is going to be slow for large lists or if the code runs in a busy execution path – Sten Petrov Feb 16 '20 at 17:50

Eliahu Aaron · Answer 2 · 2020-02-16T16:49:22.653

Although this post is old and already answered, it can still be helpful to others so I here is my answer.

You can create extension methods similar to List<T>.FindIndex(...) methods:

public static class ObservableCollectionExtensions
{
    public static int FindIndex<T>(this ObservableCollection<T> ts, Predicate<T> match)
    {
        return ts.FindIndex(0, ts.Count, match);
    }

    public static int FindIndex<T>(this ObservableCollection<T> ts, int startIndex, Predicate<T> match)
    {
        return ts.FindIndex(startIndex, ts.Count, match);
    }

    public static int FindIndex<T>(this ObservableCollection<T> ts, int startIndex, int count, Predicate<T> match)
    {
        if (startIndex < 0) startIndex = 0;
        if (count > ts.Count) count = ts.Count;

        for (int i = startIndex; i < count; i++)
        {
            if (match(ts[i])) return i;
        }

        return -1;
    }
}

Usage:

int index = PLUList.FindIndex(x => x.ID == 13);
if (index > -1)
{
    // Do something here...
}
else
{
    // Do something else here...
}

`ObservableCollection`s tend to change asynchronously, these extensions are not thread-safe. If threading is not a concern, this answer performs better than the accepted answer — Sten Petrov, Feb 16 '20 at 17:51
Any other way of finding the index is equally not thread safe, it is up to the programmer to take care of thread safety while searching for the index. — Eliahu Aaron, Feb 16 '20 at 17:56
yes, but the thread-safety should be considered in your code. `col.FindIndex(match)` itself can break — Sten Petrov, Feb 17 '20 at 08:12

score 3 · Answer 3 · edited Feb 16 '20 at 16:30

If you want to retrieve the item from your list, just use LINQ:

PLU item = PLUList.Where(z => z.ID == 12).FirstOrDefault();

But this will return the item itself, not its index. Why do you want the index?

Also, you should use class instead of struct if possible. Then you could test item against null to see if the ID was found in the collection.

if (item != null)
{
    // Then the item was found
}
else
{
    // No item found !
}

A.R. · Answer 4 · 2012-02-17T13:10:10.943

2

Here is a quick fix.

int findID = 3;
int foundID=  -1;
for (int i = 0; i< PLUList.Count; i++)
{
  if (PLUList[i].ID == findID)
  {
    foundID = i;
    break;
  }
}

// Your code.
if (foundID > -1) {
// Do something here
...

edited Feb 17 '12 at 13:10

answered Feb 17 '12 at 13:04

A.R.

15,405
19
77
123

There are several issues with this answer: 1) `<=` in the loop guard should be `<` as when the loop will come to the last iteration you will get a index out of bounds exception and 2) Need to be checking `PLUList[i].ID == findID`. – Strelok Feb 17 '12 at 13:07
@Strelok: See, this is why intellisense is evil! I can't just type into a textbox anymore =). Anyway, thanks for the tip. – A.R. Feb 17 '12 at 13:11
1

@Strelok: My real mistake was rushing to post the trivial answer! – A.R. Feb 17 '12 at 13:27

score 1 · Answer 5 · edited Feb 16 '20 at 17:02

It's just a normal collection. You can just iterate over it, check the ID and return the index of the object.

int index = -1;

for(int i=0;i<PLUList.Count;i++) {
 PLU plu = PLUList[i];
 if (plu.ID == yourId) {
   index = i;
   break;
 }
}

if (index > -1) {
// Do something here
}
else {
// Do sth else here..
}

LINQ VERSION:

private void getIndexForID(PLUListint idToFind,ObservableCollection<PLU> PLUList) {
   PLU target = PLUList.Where( z => z.ID == yourID ).FirstOrDefault();
   return target == null ? -1 : PLUList.IndexOf ( target );
}

How to search an item and get its index in Observable Collection

5 Answers5