Question

I have a strange issue when binding a combobox in a WPF window.

I am loading a List object, to populate the combobox, through it’s .ItemsSource, on Window Load, however, it does not populate.

At least, not until the arrow is clicked on. Once the arrow is clicked on, the window freezes up, and after a minute or so, the 4000+ records are displayed in it.

Now, the list gets loaded, however, when I click on the combobox it takes awhile for it to render, so how can I show a waitcursor while it is rendering the list?

Here’s the Xaml for it:

<ComboBox Grid.Column="1" 
                      HorizontalAlignment="Stretch" Margin="3" Name="tUser" VerticalAlignment="Stretch"
                      DisplayMemberPath="UsersName" SelectedValuePath="UserID" SelectedValue="0"/>

And the code-behind is:

Private Sub CreateTask_Loaded(sender As Object, e As System.Windows.RoutedEventArgs) Handles Me.Loaded
    Using New WaitCursor
        _LA.Show()
        Me.tOrder.ItemsSource = GetOrderList()
        Me.tUser.ItemsSource = GetUserList()
        _LA.Hide()
    End Using
End Sub

Private Function GetUserList() As List(Of UserTyping)
    Dim _Qry As New List(Of UserTyping)
    Using db As New DataAccess
        With db
            .QueryType = CmdType.InlineSQL
            .Query = "Select 0 As UserID, '-Select User-' As UsersName Union All Select userID As UserID, userFullName As UsersName From vwSelectUser"
            Using _Results = .GetResults()
                If _Results IsNot Nothing Then
                    If _Results IsNot Nothing Then
                        _Qry = (From row In _Results.Cast(Of DbDataRecord)()
                                    Select New UserTyping() With {
                                        .UserID = Common.IsNull(Of Long)(row, 0, 0),
                                        .UsersName = Common.IsNull(Of String)(row, 1, String.Empty)
                                        }).ToList()
                    Else
                        _Qry = New List(Of UserTyping)
                    End If
                End If
            End Using
        End With
    End Using
    Return _Qry
    _Qry.Clear()
End Function
Partial Public Class UserTyping
    Public Property UserID As Long
    Public Property UsersName As String
End Class

Answer 1

I’m not sure if there’s a better way, but…

If you want to avoid waiting when you open the ComboBox for the first time when it has many items, you can set ComboBox.IsDropDownOpen to True and back to False.

Edit:
A little bit of research turned up VirtualizingStackPanel. Basically, your XAML will look like this:

<ComboBox>
    <ComboBox.ItemsPanel>
        <ItemsPanelTemplate>
            <VirtualizingStackPanel />
        </ItemsPanelTemplate>
    </ComboBox.ItemsPanel>
</ComboBox>

I was able to view a ComboBox with 40k numbers instantly.