在WPF MVVM应用程序中的ComboBox中设置默认选定项

Question

我一直坚持这个问题好几个小时......我想要做的事实上很简单 - 在ComboBox中设置默认的选定项目(我使用的是MVVM模式).

我在视图中为ComboBox提供了以下XAML:

<ComboBox ItemsSource="{Binding Schools}" 
          DisplayMemberPath="Acronym" 
          SelectedValue="{Binding SelectedSchool}" 
          SelectedValuePath="Id" 
/>

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");
        }
    }

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; }
}

问题是,当应用程序启动时,组合框不会获得默认值.我已经尝试在XAML中将SelectedIndex设置为0,但无济于事.我已经尝试在代码隐藏(可以工作)中的Window_Loaded事件处理程序中设置SelectedIndex,但是因为我使用的MVVM模式感觉有点脏.我对这整个WPF/MVVM的东西还是新手,所以如果有人能指出我正确的方向,我将不胜感激.

Answer 1

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)
        {
            // ...
        }
    }

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" });

并且您将获得所选项目"c".

如果你想使用最小的Id的init ComboBox,你可以使用这个代码:

SelectedSchool = Schools.Min(x => x.Id);

而不是分配常数值.