在WPF MVVM应用程序中的ComboBox中设置默认选定项目 [英] Setting a default selected item in ComboBox in WPF MVVM application

问题描述

我一直困扰这个问题数小时...我想做的是其实很简单 - 在ComboBox中设置一个默认的选择项（我使用MVVM模式）。

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).

我在我的视图中有ComboBox的XAML：

I have the following XAML for the ComboBox in my View:

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

在我的ViewModel中，我有一个ObservableCollection，Schools：

In my ViewModel, I have an 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： p>

Finally, School is a simple business object:

[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，组合框不会得到默认值。我试过在XAML中将SelectedIndex设置为0，但没有效果。我试过在一个Window_Loaded事件处理程序中设置SelectedIndex代码隐藏（这是工作），但因为我使用MVVM模式感觉脏。我仍然是新的整个WPF / MVVM的东西，所以如果有人可以指出我的方向，我会感激。

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.

推荐答案

您可以像这样设置SelectedSchool：

You can set SelectedSchool like this:

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" });

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：

If you want init ComboBox with the smallest Id you can use this code:

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

而不是分配常量值。

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