命名动态生成的 combobox 项目

Question

I am currently working on a WPF project in .NET Core 3.1. During the execution I have to read some file names to a combobox. I do that by first adding them to a list. After that I iterate through that list and add them to the combobox.

Problem Because they should only be accessible if an Item from another combobox is selected, I need to disable the items I just added. Because of that, and to later enable them, I need to name them. But I could not figure our how to do that.

Here is my code:

        string[] allAircraft;
        allAircraft = Directory.GetFiles(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData) + @"\ApplicationName\Aircraft\", "*.lxml", SearchOption.TopDirectoryOnly);
        foreach (var currentAircraft in allAircraft)
        {
            aircraft.Add(System.IO.Path.GetFileNameWithoutExtension(currentAircraft));
        }

        foreach (string currentListItem in aircraft)
        {
            comboAircraft.Items.Add(currentListItem);
        }

        foreach (ComboBoxItem currentComboItems in comboAircraft.Items)
        {
            currentComboItems.Name = "comboAircraftItems" + currentComboItems.Content;
        }

At the line foreach (ComboBoxItem currentItems in comboAircraft.Items) I get the Exception:

System.InvalidCastException: 'Unable to cast object of type 'System.String' to type 'System.Windows.Controls.ComboBoxItem'.'

I am really running out of ideas. I googled one entire day but could not find anything.

Have a nice day everyone!

Answer 1

@ t-kldw, Your task is very difficult to solve when working directly with UI elements. But it is very simple to solve, if you separate the Data and their Presentation, as it should be in MVVM.

Unfortunately, what you wrote contains few details for an example with full implementation. I can give only partial advice. It is necessary to create an additional container-class for the data needed in one combo box. This class MUST have an INotifyPropertyChanged implementation.

To simplify, take implementation from the topic OnPropertyChanged and instance property

/// <summary>Base class implementing INotifyPropertyChanged.</summary>
public abstract class BaseINPC : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    /// <summary>Called AFTER the property value changes.</summary>
    /// <param name="propertyName">The name of the property.
    /// In the property setter, the parameter is not specified. </param>
    public void RaisePropertyChanged([CallerMemberName] string propertyName = "")
        => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}

An example class with two properties: File Name and IsEnabled to allow selection.

public class FileItem : BaseINPC
{
    private string _fileName;
    private bool _isEnabled;

    public string FileName { get => _fileName; set { _fileName = value; RaisePropertyChanged(); } }
    public bool IsEnabled { get => _isEnabled; set { _isEnabled = value; RaisePropertyChanged(); } }
}

So that I can give further advice, you need to provide more details of your task. For example, I don't even have a clue why currentComboItems.Name might be needed.... I think that you are clearly doing something wrong.

Answer 2

You are trying to convert String to ComboBoxItem . When you populate your combobox with data, you add String , not ComboBoxItem . So you should use String in the last foreach loop.

 foreach (string currentComboItems in comboAircraft.Items) { currentComboItems.Name = "comboAircraftItems" + currentComboItems; }

Another solution is to add ComboBoxItem to your combobox instead of String .

foreach (string currentListItem in aircraft)
{
    ComboBoxItem item = new ComboBoxItem()
    {
        Content = currentListItem,
        Name = "comboAircraftItems" + currentListItem
    };
    comboAircraft.Items.Add(item);
}

Answer 3

ItemsControl.Items only holds data objects. In order to render this items, each is wrapped into a container eg, ComboBoxItem . These UI containers are generated by a ItemContainerGenerator , which is associated with the current ItemsControl . The ItemContainerGenerator is also responsible to track the mapping between the data item and its UI container.
To obtain the container of an item you use the ItemsControl.ItemContainerGenerator :

DependencyObject itemContainer = ItemContainerGenerator.ContainerFromItem(item);

Your simplified code becomes:

List<string> allAircrafts = Directory.EnumerateFiles(
    Path.Combine(
      Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
      @"\ApplicationName\Aircraft\"),
    "*.lxml",
    SearchOption.TopDirectoryOnly)
  .Select(Path.GetFileNameWithoutExtension)
  .ToList();

comboAircraft.ItemsSource = allAircrafts;

foreach (string item in allAircrafts)
{
  var itemContainer = comboAircraft.ItemContainerGenerator.ContainerFromItem(item) as ComboBoxItem;
  itemContainer.Name = "comboAircraftItems" + itemContainer.Content;
}

But note, that the approach you chose has issues. When UI virtualization is enabled for ItemsCopntrol , then only the visible items have their containers generated. Items out side the viewport of the scroll viewer are not generated and can't be returned by the ItemContainerGenerator .
Since UI virtualization is not enabled for the ComboBox by default, this still can work, as long you don't enable this performance optimization feature.

The optimal solution would need you to create a filtered source collection for the ComboBox that displays the filtered items. This way you won't clutter the control with items that are not of interest for the user and are not selectable anyway.

Create a collection of type ObservableCollection<string> which contains the selected items of the primary ComboBox you named comboAircraft .
Set this collection as ItemsSource of the second filtered ComboBox .

Alternatively create a CollectionViewSource of the original source collection and bind it to the secondary ComboBox . ICollectionView supports filtering. The proper solution

Using either of the two recommended solutions eliminates the requirement to care about item containers. This adds flexibility and simplifies your code.