Retaining index of an item

Chris Copeland

I've been working on a project for the past week or so, and it's come to my attention that I need a method of being able to 'track' the index of an item within a collection. A prime example of what I'm trying to achieve is the System.Windows.Forms.ListViewItem class, which retains an Index property which is always matched to the actual index within the collection of a System.Windows.Forms.ListView. A method I've been trying is iterating through the collection any time an update ends and updating each item individually. However, I have noticed that this takes a ridiculously long time when we begin achieving millions of rows. Items are removed from the collection, but then after the update has finished, it has to iterate through the collection to synchronize the indices. Other than using LINQ on the collection from the individual item, can anyone suggest a better way to maintain the index of the item within the list?

Ravi Bhavnani

How about just using IndexOf()[^]? :confused: /ravi

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Marco Bertschi

I'm with Ravi on that, what keeps you from adding / updating the information of an item at the time you access it by using IndexOf()?

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How to ask a question

Chris Copeland

I believe an issue with using IndexOf is that if the same item is added more than once to the collection, the value returned by IndexOf will be the first index only. The item I'm storing in the collection could be added multiple times. For instance:

public class MyClass
{
public string Text { get; set; }
public int Index { get; set; }
};

private void ExampleList()
{
// create the list
List<MyClass> list = new List<MyClass>();

// create an instance
MyClass first = new MyClass() { Text = "Hello" };

// append this item into the list twice
list.Add(first);
list.Add(first);

// create a second instance
MyClass second = new MyClass() { Text = "Goodbye" };

// append this item into the list
list.Add(second);

// if the Index property were to use the IndexOf method from the parent list,
// below would be the results:
System.Diagnostics.Debug.WriteLine(list\[0\].Index); // 0
System.Diagnostics.Debug.WriteLine(list\[1\].Index); // 0
System.Diagnostics.Debug.WriteLine(list\[2\].Index); // 1

}

Obviously if multiple instances of the same object is going to be an issue, I could re-factor my code to prevent multiple additions, but it'd be preferable if the developer had the option to add the same item twice. Otherwise, am I restricted entirely to updating the Index property on each collection change? I just wondered if there was some work-around (ie, creating an index array or using some sort of indexing engine built into the framework.) Thanks

Lost User

Chris Copeland wrote:

Obviously if multiple instances of the same object is going to be an issue

You didn't add multiple instances; you added a single object to the list twice. Now, you'd have to loop the list to find ALL the objects if there can be multiple - but then the property "Index" on the object is erroneous - if the object can be added to the list twice, then "Index" should return an array, returning 0 and 1 as the indexes where it is to be found. Alternatively;

public class MyClass: ICloneable
{
public string Id = Guid.NewGuid().ToString();
public string Text { get; set; }

public override string ToString()
{
    return Text;
}

public override int GetHashCode()
{
    return Id.GetHashCode();
}

public object Clone()
{
    return new MyClass() { Text = this.Text };
}

}

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Chris Copeland

I'd not thought about adding a unique identifier property. That makes much more sense than actually indexing the position of the item. I suppose the chances of collision are exceptionally low too when using Guid. Thanks for the assistance, I'll give this a try when I land home!

Ravi Bhavnani

Chris Copeland wrote:

The item I'm storing in the collection could be added multiple times.

This wasn't obvious (to me) in your original post. One solution would be to write an extension method IList<T>.IndicesOf(T) that would return an IEnumerable<int> of indices. But clearly, this doesn't scale when you have a large number of items. /ravi

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