为什么这样UWP组合框可以初始化后的空白，但可供选择正常工作？ [英] Why this UWP ComboBox can be blank after initialization but works fine for selection?

问题描述

我有这样一个组合框

<ComboBox
    Grid.Column="1"
    Padding="5,0,0,0"
    DisplayMemberPath="Description" 
    SelectedItem="{Binding MaxXXAge, Mode=TwoWay, Converter={StaticResource MaxXXAgeToMaxXXAgeMemberConverter}}"
    ItemsSource="{Binding ElementName=SettingsXXScrollViewer, Path=DataContext.MaxXXAgeMemberGroup, Mode=OneWay}" />

不过，在初始化之后，组合框为空。它实际上是在此之后正常工作。我可以选择并显示所选择的项目预期。它只是乍一看是行不通的。不过，我已经初始化MaxXXAge和转换器被触发。这是该集团

However, after initialization, the combobox is blank. It actually works fine after this. I can select and show the selected item as expected. It's just the first glance doesn't work. However, I already initialized MaxXXAge and the converter has been triggered. Here is the group

public IReadOnlyList<MaxXXAgeMembers> MaxXXAgeMemberGroup { get { return MaxXXAgeMembers.Options; } }

这是MaxXXAgeMembers

And this is the definition for MaxXXAgeMembers

public class MaxXXAgeMembers
        {
            public MaxXXAge MaxXXAge { get; private set; }
            public string Description { get; private set; }

            public static readonly IReadOnlyList<MaxXXAgeMembers> Options = new ReadOnlyCollection<MaxXXAgeMembers>(new[]
            {
                new MaxXXAgeMembers { MaxXXAge =  MaxXXAge.OneDay, Description = Strings.SettingSync_OneDay},
.......
            });

            public static MaxXXAgeMembers FromMaxXXAge(MaxXXAge maxXXAge)
            {
                return Options.First(option => option.MaxXXAge == maxXXAge);
            }
        }

//添加了重写等于以后

//Added the Overriding Equals later

public override bool Equals(object obj)
{
     if (obj == null || !(obj is MaxEmailAgeMembers))
          return false;
     return ((MaxEmailAgeMembers)obj).Description.Equals(this.Description);
}

public override int GetHashCode()
{
     return this.Description.GetHashCode();
}

该转换器是这样的。

The converter is like this

public sealed class MaxEmailAgeToMaxEmailAgeMemberConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, string language)
    {
        return WPSettingsEmailViewModel.MaxEmailAgeMembers.FromMaxEmailAge((MaxEmailAge)value);
    }

    public object ConvertBack(object value, Type targetType, object parameter, string language)
    {
        return ((WPSettingsEmailViewModel.MaxEmailAgeMembers)value).MaxEmailAge;
    }
}

任何想法？

Any idea?

推荐答案

因为你没有在第一时间选择什么它是空白。如果我没有记错的话，你必须要么使用的SelectedItem 绑定您的选择或的SelectedValue 与 SelectedValuePath 。

It's blank because you don't have anything selected in the first place. If I'm not mistaken, you have to either use SelectedItem to bind your selection or SelectedValue with SelectedValuePath.

其实，我从来没有使用的SelectedValue 与 SelectedValuePath 我自己，所以初始化这ComboBox.ItemSource将被绑定到项目收集后 - 例如的ObservableCollection<&人GT;人{搞定;设置;} - 我还设置所选的项目属性人员SelectedPerson {搞定;设置;} 从集合中的一个值。然后我绑定ComboBox.SelectedItem这个属性，所以在初始化它显示预定义的选择的值。

I actually never use SelectedValue with SelectedValuePath myself, so after initializing collection of items which ComboBox.ItemSource will be binded to - for example ObservableCollection<Person> Persons {get; set;} - I also set selected item property Person SelectedPerson {get; set;} to one of the values from collection. Then I bind ComboBox.SelectedItem to this property, so on initialization it shows predefined selected value.

我想你可以达到同样的用的SelectedValue 和 SelectedValuePath ，但你必须一起使用它们这里描述。

I guess you can achieve the same with SelectedValue and SelectedValuePath, but you have to use them together as described here.

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