WPF - 强制绑定不可见的组合框

Question

我有一个包含多个用户控件的 WPF 窗口，其中一些是不可见的（可见性 = 隐藏）。其中一个控件有一个具有 ItemsSource 绑定的 ComboBox，我想在加载窗口/控件时预设其选定的项目。

但是，在组合框可见之前，似乎不会应用绑定。当我设置 SelectedItem 属性并在调试器中遇到断点时，我注意到 ItemsSource 在那一刻为空。有没有办法强制 WPF 应用数据绑定并在组合框保持不可见时填充组合框？

可重现的例子：

MainWindow.xaml

<Window x:Class="HiddenComboBoxBinding.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:HiddenComboBoxBinding"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Border x:Name="comboboxParent" Visibility="Collapsed">
            <ComboBox x:Name="cmbSearchType" SelectedIndex="0" ItemsSource="{Binding SearchTypeOptions}" DisplayMemberPath="Name" SelectionChanged="cmbSearchType_SelectionChanged" />
        </Border>
    </Grid>
</Window>

MainWindow.xaml.cs

using System.Linq;
using System.Windows;
using System.Windows.Controls;

namespace HiddenComboBoxBinding
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private ViewModel viewModel { get; set; } = new ViewModel();

        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = viewModel;

            // Add some or all of our search types - in the real code, there's some business logic here
            foreach (var searchType in SearchType.AllSearchTypes)
            {
                viewModel.SearchTypeOptions.Add(searchType);
            }

            // Pre-select the last option, which should be "Bar"
            cmbSearchType.SelectedItem = SearchType.AllSearchTypes.Last();
        }

        private void cmbSearchType_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {

        }
    }
}

ViewModel.cs

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace HiddenComboBoxBinding
{
    public class ViewModel : INotifyPropertyChanged
    {
        public ObservableCollection<SearchType> SearchTypeOptions { get; set; } = new ObservableCollection<SearchType>();

        #region INotifyPropertyChanged Members
        private void NotifyPropertyChanged(string propertyName) { PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName)); }
        public event PropertyChangedEventHandler PropertyChanged;
        #endregion
    }

    public class SearchType
    {
        // Source list of Search Types
        private static List<SearchType> _AllSearchTypes;
        public static List<SearchType> AllSearchTypes
        {
            get
            {
                if(_AllSearchTypes == null)
                {
                    _AllSearchTypes = new List<SearchType>();
                    _AllSearchTypes.Add(new SearchType() { Name = "Foo" });
                    _AllSearchTypes.Add(new SearchType() { Name = "Bar" });
                }
                return _AllSearchTypes;
            }
        }

        // Instance properties - for the purposes of a minimal, complete, verifiable example, just one property
        public string Name { get; set; }
    }
}

Answer 1

我能够找出问题所在。设置 SelectedItem 确实有效（即使当时 ItemsSource 为空），但在 XAML 中，ComboBox 具有 SelectedIndex="0" 并且优先于在代码隐藏中设置的 SelectedItem。