Get Unique Selected Items From Multiple ASP.NET ListBox and Merge them using LINQ

I recently did a short article on Get Selected Items From Multiple ASP.NET ListBox and Merge the results using LINQ. A user pointed out what if you wanted only the unique values from each list. Well here are two options to do that.
Add two list boxes and a button control to your page:

<div>
    <asp:ListBox ID="ListBox1" runat="server" SelectionMode="Multiple">
        <asp:ListItem Text="One" Value="One" />
        <asp:ListItem Text="Two" Value="Two" />
        <asp:ListItem Text="Three" Value="Three" />
        <asp:ListItem Text="Four" Value="Four" />
        <asp:ListItem Text="Five" Value="Five" />
        <asp:ListItem Text="Six" Value="Six" />
        <asp:ListItem Text="Seven" Value="Seven" />
    </asp:ListBox>

    <asp:ListBox ID="ListBox2" runat="server" SelectionMode="Multiple">
        <asp:ListItem Text="One" Value="One" />
        <asp:ListItem Text="Two" Value="Two" />
        <asp:ListItem Text="Three" Value="Three" />
        <asp:ListItem Text="Four" Value="Four" />
        <asp:ListItem Text="Five" Value="Five" />
        <asp:ListItem Text="Six" Value="Six" />
        <asp:ListItem Text="Seven" Value="Seven" />
    </asp:ListBox>

    <br />
    <asp:Button ID="Button1" runat="server" Text="Get Selected Items"
        onclick="Button1_Click" />
</div>

You have two options to select unique values from both the listboxes. You can use Distinct or Union. Here’s the code for both.

Distinct

C#

protected void Button1_Click(object sender, EventArgs e)
{
    var distinct = from p in ListBox1.Items.OfType<ListItem>()
                .Concat(ListBox2.Items.OfType<ListItem>())
                .Where(o => o.Selected)
                .Distinct()
                select new
                {
                   Text = p.Text
                };

    foreach (var item in distinct)
    {
        Response.Write(item.Text + "<br/>");
    }
}

VB.NET

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) _
Handles Button1.Click
    Dim distinct = _
        From p In ListBox1.Items.OfType(Of ListItem)() _
            .Concat(ListBox2.Items.OfType(Of ListItem)()) _
            .Where(Function(o) o.Selected) _
            .Distinct() _
            Select New With {Key .Text = p.Text}

    For Each item In distinct
        Response.Write(item.Text & "<br/>")
    Next item
End Sub

Union

C#

protected void Button1_Click(object sender, EventArgs e)
{
    var list1 = from p in ListBox1.Items.OfType<ListItem>()
                .Where(o => o.Selected)
                select new
                {
                    Text = p.Text
                };

    var list2 = from p in ListBox2.Items.OfType<ListItem>()
            .Where(o => o.Selected)
                select new
                {
                    Text = p.Text
                };

    var unique = list1.Union(list2);

    foreach (var item in unique)
    {
        Response.Write(item.Text + "<br/>");
    }
}

VB.NET

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) _
Handles Button1.Click
    Dim list1 = _
         From p In ListBox1.Items.OfType(Of ListItem)() _
         .Where(Function(o) o.Selected) _
          Select New With {Key .Text = p.Text}

    Dim list2 = _
      From p In ListBox2.Items.OfType(Of ListItem)() _
      .Where(Function(o) o.Selected) _
      Select New With {Key .Text = p.Text}

    Dim unique = list1.Union(list2)

    For Each item In unique
        Response.Write(item.Text & "<br/>")
    Next item
End Sub

OUTPUT