Count all duplicates in ListView column. C#

So if I understand you want to have another listview with duplicates and the count?

If so you can create a List<> and then add that item into the list in this case will be a string I believe, maybe something like this? This is just a console app for demonstration.

List<string> myList = new List<string>();

myList.Add("One");
myList.Add("Two");
myList.Add("Three");
myList.Add("One");
myList.Add("Two");
myList.Add("Four");

List<string> noDupes = myList.Distinct().ToList();


foreach(string dupe in noDupes)
{
    Console.WriteLine(dupe);
}

Console.WriteLine(noDupes.Count());

Given the next list:

List<myClass> items = new List<myClass>()
{
    new myClass() { Name = "John Doe", Item = "Shoes" },
    new myClass() { Name = "John Doe", Item = "T-Shirt" },
    new myClass() { Name = "John Doe", Item = "Jeans" },
    new myClass() { Name = "Rick Astley", Item = "Baseball bat" },
    new myClass() { Name = "Rick Astley", Item = "Sandwich" },
    new myClass() { Name = "Rick Astley", Item = "T-Shirt" },
    new myClass() { Name = "Rick Astley", Item = "Shoes" },
    new myClass() { Name = "Jane Doe", Item = "Shoes" },
    new myClass() { Name = "Jane Doe", Item = "T-Shirt" },
    new myClass() { Name = "Joe Mm.", Item = "Shoes" },
    new myClass() { Name = "Joe Mm.", Item = "Milk" },
};

You can group and count in this way:

var l = items.GroupBy(group => group.Item)
                .Select(n => new { Item = n.Key, Count = n.Count() })
                .ToList();

The result:

Item: Shoes  Count: 4
Item: T-Shirt  Count: 3
Item: Jeans  Count: 1
Item: Baseball bat  Count: 1
Item: Sandwich  Count: 1
Item: Milk  Count: 1

The outcome you want can be achieved using System.Linq.GroupBy for the logic and DataGridView (Winforms Example) for the views:

The amount of code to write is small! Just use Data Binding Winforms, Xamarin, WPF for two lists:

First make two classes containing the properties you wish to display:

internal class UserPurchase
{       
    public string Name { get; internal set; } // Internal makes item ReadOnly in view
    public string Purchase { get; internal set; }
}

and

internal class PurchasedItem
{
    public string Description { get; internal set; }
    public int Count { get; internal set; }
}

Then declare two BindingList (Winforms example) or ObservableCollection (others).

internal BindingList<UserPurchase> PurchasesByUser { get; } = new BindingList<UserPurchase>();
internal BindingList<PurchasedItem> PurchasedByItem { get; } = new BindingList<PurchasedItem>();

This shows how to attach the bindings in Winforms (for other platforms use the xaml). As a result DataGridView will now generate columns automatically using the properties of the two classes.

protected override void OnHandleCreated(EventArgs e)
{
    base.OnHandleCreated(e);
    if (!DesignMode)
    {
        // Bind the lists to the UI element to display them.
        dataGridView1.DataSource = PurchasesByUser;
        dataGridView2.DataSource = PurchasedByItem;
        initList1();
    }
}

Here is the list initializer which populates List1 PurchasesByUser then applies System.Linq.GroupBy on List1 to calculate List2 PurchasedByItem:

private void initList1()
{
    XElement xsource = XElement.Parse(source);
    XElement[] xusers = xsource.Elements("user").ToArray();
    foreach (XElement xuser in xusers)
    {
        XElement[] xpurchases = xuser.Element("items").Elements("item").ToArray();
        foreach (XElement xpurchase in xpurchases)
        {
            var xname = xuser.Element("name");
            // Every time you add a record to the list, it appears in the View automatically.
            PurchasesByUser.Add(new UserPurchase { Name = (string)xname, Purchase = (string)xpurchase });
        }
    }
    // Make what is essentially a list *of* lists using System.Linq.GroupBy
    var groups = 
        PurchasesByUser.GroupBy(purchase => purchase.Purchase);
    foreach (var group in groups)
    {
        // Decoding the Group:
        Debug.WriteLine(
            $"The group '{group.Key}' holds {group.Count()} items");
        Debug.WriteLine(
            $"The customers are { string.Join(",", group.ToList().Select(item=>item.Name))}");
        var byItem = new PurchasedItem { Description = group.Key, Count = group.Count() };
        PurchasedByItem.Add(byItem);
    }
}

Finally to model your database (with subitems), I used this string in Xml format:

const string source =
    @"<purchases>
        <user>
            <name>John Doe</name>
            <items>
                <item>Shoes</item>
                <item>T-Shirts</item>
                <item>Jeans</item>
            </items>
        </user>
        <user>
            <name>Rick Astley</name>
            <items>
                <item>Baseball bat</item>
                <item>Sandwich</item>
                <item>T-Shirts</item>
                <item>Shoes</item>
            </items>
        </user>
        <user>
            <name>Jane Doe</name>
            <items>
                <item>Shoes</item>
                <item>T-Shirts</item>
            </items>
        </user>
        <user>
            <name>Anonymous</name>
            <items>
                <item>Laptop</item>
            </items>
        </user>
        <user>
            <name>Joe Mm.</name>
            <items>
                <item>Milk</item>
                <item>Shoes</item>
            </items>
        </user>
    </purchases>";

This is just one approach of course but your question is a good way to demonstrate combining concepts from System.Linq System.Xml.Linq and Data Binding. I posted my code on GitHub if you'd like to experiment with it.