在 WPF MVVM 应用程序的 ComboBox 中设置默认选定项
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Setting a default selected item in ComboBox in WPF MVVM application
提问by William
I've been stuck on this problem for hours... what I want to do is actually quite simple - set a default selected item in a ComboBox (I'm using the MVVM pattern).
我已经被这个问题困扰了好几个小时……我想做的其实很简单——在 ComboBox 中设置一个默认的选定项目（我使用的是 MVVM 模式）。
I have the following XAML for the ComboBox in my View:
我的视图中有 ComboBox 的以下 XAML：
<ComboBox ItemsSource="{Binding Schools}"
DisplayMemberPath="Acronym"
SelectedValue="{Binding SelectedSchool}"
SelectedValuePath="Id"
/>
In my ViewModel, I have an ObservableCollection, Schools:
在我的 ViewModel 中，我有一个 ObservableCollection，Schools：
public ObservableCollection<School> Schools { get; private set; }
public CourseFormViewModel()
{
Schools = new ObservableCollection<School>();
try
{
// Gets schools from a web service and adds them to the Schools ObservableCollection
PopulateSchools();
}
catch (Exception ex)
{
// ...
}
}
public int SelectedSchool
{
get { return schoolId; }
set
{
schoolId = value;
OnPropertyChanged("SelectedSchool");
}
}
Finally, School is a simple business object:
最后，School 是一个简单的业务对象：
[DataContract]
public class School
{
[DataMember]
public int Id { get; set; }
[DataMember]
public string Acronym { get; set; }
[DataMember]
public string Name { get; set; }
}
The problem is, that when the application is started, the combobox does not get a default value. I've tried setting the SelectedIndex to 0 in XAML, but to no avail. I've tried setting the SelectedIndex in a Window_Loaded event handler in the code-behind (which works), but since I'm using the MVVM pattern that feels kind of dirty. I'm still new to this whole WPF/MVVM stuff, so if someone could point me in the right direction I would be grateful.
问题是，当应用程序启动时，组合框没有获得默认值。我尝试在 XAML 中将 SelectedIndex 设置为 0，但无济于事。我已经尝试在代码隐藏（有效）的 Window_Loaded 事件处理程序中设置 SelectedIndex，但因为我使用的 MVVM 模式感觉有点脏。我对整个 WPF/MVVM 的东西还是个新手，所以如果有人能指出我正确的方向，我将不胜感激。
回答by kmatyaszek
You can set SelectedSchool like this:
您可以像这样设置 SelectedSchool：
public void CourseFormViewModel()
{
Schools = new ObservableCollection<School>();
try
{
// Gets schools from a web service and adds them to the Schools ObservableCollection
PopulateSchools();
SelectedSchool = 3;
}
catch (Exception ex)
{
// ...
}
}
Test data:
测试数据：
Schools.Add(new School { Id = 1, Name = "aaa", Acronym = "a" });
Schools.Add(new School { Id = 2, Name = "bbb", Acronym = "b" });
Schools.Add(new School { Id = 3, Name = "ccc", Acronym = "c" });
And you will get selected item "c".
您将获得选定的项目“c”。
If you want init ComboBox with the smallest Id you can use this code:
如果您想要具有最小 Id 的 init ComboBox，您可以使用以下代码：
SelectedSchool = Schools.Min(x => x.Id);
Instead of assign constant value.
而不是分配常数值。
