C# WPF - MVVM - ComboBox SelectedItem

WPF - MVVM - ComboBox SelectedItem

提问by Bip

I have ViewModel(implemented INotifyPropertyChanged) in the background and class Categorywhich has only one property of type string. My ComboBox SelectedItem is bind to an instance of a Category. When i change the value of instance, SelectedItem is not being updated and Combobox is not changed.

我在后台和类中有ViewModel（实现INotifyPropertyChanged）Category只有一个类型的属性string。我的 ComboBox SelectedItem 绑定到一个类别的实例。当我更改实例的值时，SelectedItem 不会更新，组合框也不会更改。

EDIT: code

编辑：代码

Combobox:

组合框：

<ComboBox x:Name="categoryComboBox" Grid.Column="1"  Grid.Row="3" Grid.ColumnSpan="2" 
          Margin="10" ItemsSource="{Binding Categories}"
          DisplayMemberPath="Name" SelectedValue="{Binding NodeCategory, Mode=TwoWay}"/>

Property:

财产：

private Category _NodeCategory;
public Category NodeCategory
{
    get
    {
        return _NodeCategory;
    }
    set
    {
        _NodeCategory = value;
        OnPropertyChanged("NodeCategory");
    }
}

[Serializable]
public class Category : INotifyPropertyChanged
{
    private string _Name;
    [XmlAttribute("Name")]
    public string Name
    {
        get
        {
            return _Name;
        }
        set
        {
            _Name = value;
            OnPropertyChanged("Name");
        }
    }

    public void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    [field:NonSerialized]
    public event PropertyChangedEventHandler PropertyChanged;
}

and what I am trying is: when I set

我正在尝试的是：当我设置

NodeCategory = some_list_of_other_objects.Category;

to have that item selected in Comboboxwith appropriate DisplayMemberPath

以Combobox适当的方式选择该项目DisplayMemberPath

采纳答案by akjoshi

The category you are setting in this line -

您在此行中设置的类别 -

NodeCategory = some_list_of_other_objects.Category;

and one present in your Categories collection(ItemsSource="{Binding Categories}") should be referring to same object. If they are not then SelectedItemwon't work.

并且存在于您的 Categories 集合中的一个（ItemsSource="{Binding Categories}"）应该指的是同一个对象。如果他们不是那么SelectedItem将无法工作。

Solution 1 -

解决方案 1 -

You can also try to use SelectedValuePathlike this -

你也可以尝试这样使用SelectedValuePath——

<ComboBox x:Name="categoryComboBox" 
          ItemsSource="{Binding Categories}"
          DisplayMemberPath="Name" 
          SelectedValuePath="Name" 
          SelectedValue="{Binding NodeCategory, Mode=TwoWay}" />

and in code you can do something like this -

在代码中你可以做这样的事情 -

private string _NodeCategory;
public string NodeCategory
{
    get
    {
        return _NodeCategory;
    }
    set
    {
        _NodeCategory = value;
        OnPropertyChanged("NodeCategory");
    }
}

and set selected item like this -

并像这样设置所选项目 -

NodeCategory = some_list_of_other_objects.Category.Name;

and use selected value like this -

并使用这样的选定值 -

Category selectedCategory = 
   some_list_of_other_objects.FirstOrDefault(cat=> cat.Name == NodeCategory);

or

或者

Category selectedCategory = 
   Categories.FirstOrDefault(cat=> cat.Name == NodeCategory);

---

---

Solution 2 -

解决方案 2 -

Another possible solution can be -

另一种可能的解决方案是 -

NodeCategory = 
  Categories.FirstOrDefault(cat=> cat.Name == some_list_of_other_objects.Category.Name);

this way your NodeCategoryproperty will have the reference of an object in Categoriescollection and SelectedItemwill work.

这样您的NodeCategory属性将具有Categories集合中对象的引用并且SelectedItem将起作用。

回答by Benjamin Gale

Your XAML needs a couple of modifications but I think the real problem is with the code you have posted which I don't think is telling the full story. For starters, your combobox ItemSourceis bound to a property called Categories but you do not show how this property is coded or how your NodeCategoryproperty is initially synced with the item.

您的 XAML 需要进行一些修改，但我认为真正的问题在于您发布的代码，我认为这并不能说明全部情况。首先，您的组合框ItemSource绑定到一个名为 Categories 的属性，但您没有显示此属性的编码方式或您的NodeCategory属性最初如何与项目同步。

Try using the following code and you will see that the selected item is kept in sync as the user changes the value in the combobox.

尝试使用以下代码，您将看到所选项目在用户更改组合框中的值时保持同步。

XAML

XAML

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
<StackPanel>
    <ComboBox x:Name="categoryComboBox"
              Grid.Column="1"
              Grid.Row="3"
              Grid.ColumnSpan="2"
              Margin="10"
              ItemsSource="{Binding Categories}"
              DisplayMemberPath="Name"
              SelectedItem="{Binding NodeCategory}" />
    <Label Content="{Binding NodeCategory.Name}" />
</StackPanel>

Code-behind

代码隐藏

public partial class MainWindow : Window, INotifyPropertyChanged
{
    private ObservableCollection<Category> _categories = new ObservableCollection<Category>
    {
        new Category { Name = "Squares"},
        new Category { Name = "Triangles"},
        new Category { Name = "Circles"},
    };

    public MainWindow()
    {
        InitializeComponent();
        NodeCategory = _categories.First();
        this.DataContext = this;
    }

    public IEnumerable<Category> Categories
    {
        get { return _categories; }
    }

    private Category _NodeCategory;
    public Category NodeCategory
    {
        get
        {
            return _NodeCategory;
        }
        set
        {
            _NodeCategory = value;
            OnPropertyChanged("NodeCategory");
        }
    }

    public void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

[Serializable]
public class Category : INotifyPropertyChanged
{
    private string _Name;
    [XmlAttribute("Name")]
    public string Name
    {
        get
        {
            return _Name;
        }
        set
        {
            _Name = value;
            OnPropertyChanged("Name");
        }
    }

    public void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    [field: NonSerialized]
    public event PropertyChangedEventHandler PropertyChanged;
}

回答by Mare Infinitus

From my little example:

从我的小例子：

Note: This is setting just a string (or a category from another list), but the basics should be same here:

注意：这只是设置一个字符串（或另一个列表中的类别），但这里的基础知识应该是相同的：

Basically this is done:

基本上是这样做的：

private void button1_Click(object sender, RoutedEventArgs e)
{
    (this.DataContext as ComboBoxSampleViewModel).SelectCategory("Categorie 4");
}

Here is my XAML:

这是我的 XAML：

<Grid>
    <ComboBox Height="23" HorizontalAlignment="Left" Margin="76,59,0,0"   
              Name="comboBox1" VerticalAlignment="Top" Width="120" 
              ItemsSource="{Binding List.Categories}" 
              DisplayMemberPath="Name" 
              SelectedValue="{Binding NodeCategory, Mode=TwoWay}" />
    <Button Content="Button" Height="27" HorizontalAlignment="Left" 
            Margin="76,110,0,0" Name="button1" VerticalAlignment="Top" 
            Width="120" Click="button1_Click" />
</Grid>

and in the ViewModel of the Window

并在窗口的 ViewModel 中

class ComboBoxSampleViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(string info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }

    public CategoryList List { get; set; }

    public ComboBoxSampleViewModel()
    {
        this.List = new CategoryList();
        NodeCategory = List.Selected;
    }

    private ComboBoxSampleItemViewModel nodeCategory;
    public ComboBoxSampleItemViewModel NodeCategory
    {
        get
        {
            return nodeCategory;
        }
        set
        {
            nodeCategory = value;
            NotifyPropertyChanged("NodeCategory");
        }
    }

    internal void SelectCategory(string p)
    {
        this.List.SelectByName(p);
        this.NodeCategory = this.List.Selected;
    }
}

With the help of this little class:

在这个小班级的帮助下：

public class CategoryList
{
    public ObservableCollection<ComboBoxSampleItemViewModel> Categories { get; set; }
    public ComboBoxSampleItemViewModel Selected { get; set; }
    public CategoryList()
    {
        Categories = new ObservableCollection<ComboBoxSampleItemViewModel>();

        var cat1 = new ComboBoxSampleItemViewModel() { Name = "Categorie 1" };
        var cat2 = new ComboBoxSampleItemViewModel() { Name = "Categorie 2" };
        var cat3 = new ComboBoxSampleItemViewModel() { Name = "Categorie 3" };
        var cat4 = new ComboBoxSampleItemViewModel() { Name = "Categorie 4" };

        Categories.Add(cat1);
        Categories.Add(cat2);
        Categories.Add(cat3);
        Categories.Add(cat4);

        this.Selected = cat3;
    }

    internal void SelectByName(string p)
    {
        this.Selected = this.Categories.Where(s => s.Name.Equals(p)).FirstOrDefault();
    }
}

And this Item ViewModel

而这个 Item ViewModel

public class ComboBoxSampleItemViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(string info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }

    private string name;

    public string Name 
    { 
        get
        {
            return name;
        }
        set
        {
            name = value;
            NotifyPropertyChanged("Name");
        }
    }
}

回答by stéphane Boutinet

If Combobox is bound to object class of the View Model, while the SelectionBoxItemof the sender object (in the SelectionChangedin code behind) has not that type, it means it's still loading.

如果 Combobox 绑定到 View Model 的对象类，而SelectionBoxItem发送方对象的（在SelectionChanged后面的代码中）没有该类型，则表示它仍在加载。

ComboBox combo = sender as ComboBox;
if (combo.SelectionBoxItem.GetType() == typeof(BindedClass))
{
            // Not loading
}