MVVMLight -当属性改变时UI不更新

我正在使用c#和MVVMLight开发一个小型数据驱动应用程序，当我试图添加/编辑新项目时，我遇到了一个问题。

基本上，我可以在构造函数中将SelectedMember属性设置为数据库中已经存在的Member对象，没有问题:

public MemberDetailViewModel()
{
    _context = new DatabaseEntities();
     GetMembers(); // This just loads the Member objects into an ObservableCollection.
     SelectedMember = _members.FirstOrDefault(); // UI updates as expected.
}

SelectedMember的实现如下:

public Member SelectedMember
{
    get { return _selectedMember; }
    set { Set(ref _selectedMember, value); }
}

现在这很好，因为当我编辑对象时，所有的数据验证工作。

但是，当我使用下面的代码添加一个新对象时:

private void NewMember()
{
    var member = new Member();
    SelectedMember = member; // UI doesn't update, is still on previous object.
}

UI没有更新，并且SelectedMember中的Member对象似乎与构造函数中加载的成员对象相同。

另外，当我从构造函数中加载一个空对象到SelectedMember属性时，应用程序似乎不知道它的存在，并且当我编辑控件中的值时没有数据验证例程:

public MemberDetailViewModel()
{
    _context = new DatabaseEntities();
     SelectedMember = new Member(); // Binding appears to fail, data validation code not firing when control values changed.
}

我做错了什么?

编辑:

当视图模型被加载的时候我看到了这个

System.Windows.Data Error: 40 : BindingExpression path error: 'Centres' property not found on 'object' ''Member_B31A44ECE333F4E7E07BE25FA5BDC79874BAE6C64589F9765ACA373D89BBBD6B' (HashCode=45485186)'. BindingExpression:Path=Centres; DataItem='Member_B31A44ECE333F4E7E07BE25FA5BDC79874BAE6C64589F9765ACA373D89BBBD6B' (HashCode=45485186); target element is 'ComboBox' (Name=''); target property is 'ItemsSource' (type 'IEnumerable')
System.Windows.Data Warning: 40 : BindingExpression path error: 'SelectedMember' property not found on 'object' ''Member_B31A44ECE333F4E7E07BE25FA5BDC79874BAE6C64589F9765ACA373D89BBBD6B' (HashCode=45485186)'. BindingExpression:Path=SelectedMember.LocalCentre; DataItem='Member_B31A44ECE333F4E7E07BE25FA5BDC79874BAE6C64589F9765ACA373D89BBBD6B' (HashCode=45485186); target element is 'ComboBox' (Name=''); target property is 'SelectedValue' (type 'Object')

我觉得我做了什么蠢事…但是什么?

编辑2:上面的错误是固定的，但UI仍然不更新，当SelectedMember已经改变，即使显式调用RaisePropertyChanged。

Edit3: 希望这是你需要的XAML:

<UserControl x:Class="Project.Views.MemberDetailView"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:models="clr-namespace:Project.Client.Models"
         mc:Ignorable="d" 
         d:DesignHeight="600" d:DesignWidth="800">
<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/Assets/ResourceDictionaries/DataEntryScreens.xaml"/>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</UserControl.Resources>
<WrapPanel>
    <Expander Header="Personal Information">
        <StackPanel Orientation="Vertical">
            <StackPanel Orientation="Vertical">
                <TextBlock Text="Forename:" />
                <TextBox Text="{Binding SelectedMember.Forename, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}"/>
            </StackPanel>
            <!-- Other controls omitted. -->
        </Expander>
    </WrapPanel>
</UserControl>
<!--MemberDetailView.xaml.cs-->
public partial class MemberDetailView : UserControl
{
    public MemberDetailView()
    {
        InitializeComponent();
        this.DataContext = new MemberDetailViewModel();
    }
}

public MemberDetailView()
{
    InitializeComponent();
    this.DataContext = new MemberDetailViewModel() { Header = "Members" };
}

public MemberDetailView()
{
    InitializeComponent();
    //this.DataContext = new MemberDetailViewModel() { Header = "Members" };
}