c# - 为什么我的 comboBox 的 SelectedValuePath 属性行为不正常？

Question

我正在努力寻找为什么我的 SelectedValuePath 没有导致我的组合框将 double 值传递到我的 View 模型属性 DelayLength。当我在执行期间更改组合框选择时，组合框变为红色并给出错误:

ConvertBack cannot convert value '[2 Seconds, 2]' (type 'KeyValuePair`2'). BindingExpression:Path=DelayLength; DataItem='TestViewModel' (HashCode=62605785); target element is 'ComboBox' (Name=''); target property is 'SelectedItem' (type 'Object') NotSupportedException:'System.NotSupportedException: DoubleConverter cannot convert from System.Collections.Generic.KeyValuePair`2[[System.String, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Double, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].

是否因为我相信我正确地遵循了文档而遗漏了一些简单的东西？

窗口.xaml

    <ComboBox Width="120" Height="24" HorizontalAlignment="Left" Margin="5,0"
              DisplayMemberPath="Key"
              SelectedValuePath="Value"
              ItemsSource="{Binding AvailableLengths}"
              SelectedItem="{Binding DelayLength}"/>

    <TextBox Text="{Binding DelayLength, Mode=OneWay}" IsReadOnly="True"></TextBox>

</StackPanel>

Window.xaml.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new TestViewModel();
    }
}

TestViewModel.cs

class TestViewModel : GalaSoft.MvvmLight.ViewModelBase
{
    public Collection<KeyValuePair<string, double>> AvailableLengths
    {
        get
        {
            if (_availableLengths == null)
            {
                _availableLengths = new Collection<KeyValuePair<string, double>>()
                {
                    new KeyValuePair<string, double>("None", 0),
                    new KeyValuePair<string, double>("0.5 Seconds", 0.5),
                    new KeyValuePair<string, double>("1 Second", 1),
                    new KeyValuePair<string, double>("2 Seconds", 2),
                    new KeyValuePair<string, double>("3 Seconds", 3)
                };
            }

            return _availableLengths;
        }
    }

    private double _delayLength;

    public double DelayLength
    {
        get { return _delayLength; }
        set
        {
            _delayLength = value;
            RaisePropertyChanged(nameof(DelayLength));
        }
    }

    private Collection<KeyValuePair<string, double>> _availableLengths;
}

Answer 1

SelectedValuePath 不能与 SelectedItem 一起使用。您必须使用 SelectedValue。

来自 MSDN 的自定义摘要:

The SelectedValuePath property provides a way to specify a SelectedValue for the SelectedItem. The SelectedItem represents an object in the Items collection and Control displays the value of a single property of the selected item. The SelectedValuePath property specifies the path to the property that is used to determine the value of the SelectedValue property.