c# - WPF - Restore Previous SelectedItem when ComboBox ItemSource Changes

Question

I'm implementing a ComboBox that can be refreshed by users using a button. I'm trying to make it so that the previously selected item is automatically reselected if still present inside the ComboBox after a refresh.

MainWindow.xaml:

<ComboBox Canvas.Left="10" Canvas.Top="10" DisplayMemberPath="Name" IsEnabled="{Binding Path=Enabled}" ItemsSource="{Binding Path=Items}" SelectedItem="{Binding Mode=TwoWay, Path=SelectedItem}" Width="379"/>
<Button Content="{x:Static p:Resources.TextRefresh}" Canvas.Right="10" Canvas.Top="10" Click="OnClickButtonRefresh" Width="75"/>

MainWindow.xaml.cs:

public MainWindow()
{
    InitializeComponent();
    DataContext = m_BrowserInstances = new BrowserInstancesViewModel();
}

private void OnClickButtonRefresh(Object sender, RoutedEventArgs e)
{
    m_BrowserInstances.Populate();
}

[EDITED TO CURRENT VERSION] BrowserInstancesViewModel.cs:

public sealed class BrowserInstancesViewModel : ViewModel
{
    private Boolean m_Enabled;
    public Boolean Enabled
    {
        get { return m_Enabled; }
    }

    private BrowserInstance m_SelectedItem;
    public BrowserInstance SelectedItem
    {
        get { return m_SelectedItem; }
        set
        {
            if (m_SelectedItem != value)
            {
                m_SelectedItem = value;
                NotifyPropertyChanged("SelectedItem");
            }
        }
    }

    private ObservableCollection<BrowserInstance> m_Items;
    public ObservableCollection<BrowserInstance> Items
    {
        get { return m_Items; }
    }

    public BrowserInstancesViewModel()
    {
        Populate();
    }

    private static Func<BrowserInstance, Boolean> Recover(BrowserInstance selectedItem)
    {
        return x =>
        {
            Process currentProcess = x.Process;
            Process selectedProcess = selectedItem.Process;

            if (currentProcess.Id != selectedProcess.Id)
                return false;

            if (currentProcess.MainModule.BaseAddress != selectedProcess.MainModule.BaseAddress)
                return false;

            if (currentProcess.MainWindowTitle != selectedProcess.MainWindowTitle)
                return false;

            return true;
        };
    }

    public void Populate()
    {
        BrowserInstance item = m_SelectedItem;
        List<BrowserInstance> items = new List<BrowserInstance>();

        foreach (Process process in Process.GetProcessesByName("chrome"))
            items.Add(new BrowserInstance(process));

        if (items.Count > 0)
        {
            m_Enabled = true;

            m_Items = new ObservableCollection<BrowserInstance>(items.OrderBy(x => x.Process.Id));

            if (item != null)
                m_SelectedItem = m_Items.SingleOrDefault(Recover(item));

            if (m_SelectedItem == null)
                m_SelectedItem = m_Items[0];
        }
        else
        {
            m_Enabled = false;

            m_Items = new ObservableCollection<BrowserInstance>();
            m_Items.Add(new BrowserInstance());

            m_SelectedItem = m_Items[0];
        }

        NotifyPropertyChanged("Enabled");
        NotifyPropertyChanged("Items");
        NotifyPropertyChanged("SelectedItem");
    }
}

I can get back the previously selected item, but only sometimes. Looks like the code is not working properly when I need to select a default value (Index 0) if the previous selected item cannot be recovered.

score 3 · Accepted Answer

您需要设置m_SelectedItem为找到的项目SingleOrDefault(Recover(...))。

目前，您将其设置为旧实例。该实例不再存在于列表中，显然您的BrowserInstance类没有实现任何相等成员。

根据您当前的代码正确代码：

if(selectedItem != null)
    m_SelectedItem = m_Items.SingleOrDefault(Recover(selectedItem));
if(m_SelectedItem == null)
    m_SelectedItem = m_Items[0];

更新：

您上传的代码有两个问题。

如果没有进程，您添加Process的默认对象的属性值为. 这导致. 通过将前面更改为来修复它BrowserInstancenullNullReferenceExceptionSingleOrDefault
if
```
if(selectedItem != null && selectedItem.Process != null)
```

在Populate方法结束时，您引发PropertyChanged事件Items- 更新组合框中的值 - 以及SelectedItem- 将所选项目设置为用户先前选择的项目。
这里的问题是 WPF 将在引发时更新SelectedItem，因为它在新项目列表中找不到先前选择的项目。这有效地覆盖了您在方法中计算的新选定项。通过不将新选定的项目分配给但在引发事件后将该值分配给来修复它：nullPropertyChangedItemsPopulate
m_SelectedItemselectedItemSelectedItemPropertyChangedItems

public void Populate()
{
    BrowserInstance selectedItem = m_SelectedItem;
    List<BrowserInstance> items = new List<BrowserInstance>();

    foreach (Process process in Process.GetProcessesByName("chrome"))
        items.Add(new BrowserInstance(process));

    if (items.Count > 0)
    {
        m_Enabled = true;

        m_Items = new ObservableCollection<BrowserInstance>(items.OrderBy(x => x.Process.Id));

        if (selectedItem != null && selectedItem.Process != null)
            selectedItem = m_Items.SingleOrDefault(x => (x.Process.Id == selectedItem.Process.Id) && (x.Process.MainModule.BaseAddress == selectedItem.Process.MainModule.BaseAddress));

        if (selectedItem == null)
            selectedItem = m_Items[0];
    }
    else
    {
        m_Enabled = false;

        m_Items = new ObservableCollection<BrowserInstance>();
        m_Items.Add(new BrowserInstance());

        selectedItem = m_Items[0];
    }

    NotifyPropertyChanged("Enabled");
    NotifyPropertyChanged("Items");
    SelectedItem = selectedItem;
}

如果您能正确实现相等，BrowserInstance则可以使用 WPF 功能来保留当前选定的项目。
的代码Populate可以简化如下：

public void Populate()
{
    BrowserInstance selectedItem = m_SelectedItem;
    List<BrowserInstance> items = new List<BrowserInstance>();

    foreach (Process process in Process.GetProcessesByName("chrome"))
        items.Add(new BrowserInstance(process));

    m_Enabled = items.Any();
    m_Items = new ObservableCollection<BrowserInstance>(items.OrderBy(x => x.Process.Id));
    if(!m_Enabled)
        m_Items.Add(new BrowserInstance());

    NotifyPropertyChanged("Enabled");
    NotifyPropertyChanged("Items");
    if (SelectedItem == null)
        SelectedItem = m_Items[0];
}

的相等实现BrowserInstance如下所示：

public sealed class BrowserInstance : IEquatable<BrowserInstance>
{

    // ...

    public bool Equals(BrowserInstance other)
    {
        if (ReferenceEquals(null, other))
            return false;
        if (ReferenceEquals(this, other))
            return true;
        if (m_Process == null)
        {
            if (other.m_Process == null)
                return true;
            return false;
        }

        if (other.m_Process == null)
            return false;

        return m_Process.Id == other.m_Process.Id && m_Process.MainModule.BaseAddress == other.m_Process.MainModule.BaseAddress;
    }

    public override bool Equals(object obj)
    {
        return Equals(obj as BrowserInstance);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            return m_Process != null ? ((m_Process.Id.GetHashCode() * 397) ^ m_Process.MainModule.BaseAddress.GetHashCode()) : 0;
        }
    }
}

c# - WPF - Restore Previous SelectedItem when ComboBox ItemSource Changes

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