Casting an Item Collection from a listbox to a generic list

Try this (using System.Linq):

OfType() is an extension method, so you need to use System.Linq

List<string> selectedFields = new List<string>();
selectedFields.AddRange(chkDFMFieldList.CheckedItems.OfType<string>());

Or just do it in one line:

List<string> selectedFields = chkDFMFieldList.CheckedItems.OfType<string>().ToList();

This is not exactly the answer to your requirement, but posting a more general answer. You could do it in a variety of ways:

T[] items = new T[lb.Items.Count];
lb.Items.CopyTo(items, 0);
var lst = new List<T>(items);

2) looping and adding using foreach as you mentioned.

3) using Linq

var lst = lb.Items.Cast<T>().ToList();

4) or

var lst = lb.Items.OfType<T>().ToList();

When I did some performance testing like below, I found copying to array method the fastest while the Linq methods slower. Of course in real world scenarios these wouldnt matter. I prefer the 3rd method (Linq) for readability.

DateTime d = DateTime.Now;
for (int i = 0; i < 10000; i++)
{
    Action();
}
MessageBox.Show((DateTime.Now - d).TotalMilliseconds.ToString());

For an iteration of 10000 times run multiple times with about 300 items in list box,

1) ~100ms

2) ~150ms

3) ~250ms

4) ~260ms

If you don't have access to LINQ then there isn't a more elegant way since you're performing a second operation on the list items (calling ToString()) in addition to populating the selectedFields collection.

Casting an Item Collection from a listbox to a generic list

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