刚学WPF不久，以下是测试的，老是选不到
<UserControl x:Class="HelloMvvmLight.ComboBoxWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:HelloMvvmLight"
DataContext="{Binding Source={StaticResource Locator},Path=Combo}"
mc:Ignorable="d"
Height="450" Width="800">
<StackPanel>
<Button x:Name="x" Command="{Binding GetDic}" Width="100" Height="30"> </Button>
<ComboBox Name="bo" Width="120" ItemsSource="{Binding Dic}" SelectedValue="{Binding SeValue,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay}" SelectedValuePath="Key" DisplayMemberPath="Value"></ComboBox>
</StackPanel>
</UserControl>

public class ComboBoxViewModel:ViewModelBase
{
public ComboBoxViewModel()
{
Dic = new Dictionary<string, string>();
selectValue = "0";
}

    private Dictionary<string,string> dic;

    public Dictionary<string,string> Dic
    {
        get { return dic; } 
        set
        {
            dic = value;
            this.RaisePropertyChanged("Dic");
        }
    }

    private string selectValue;

    public string SeValue
    {
        get { return selectValue; }
        set
        {
            selectValue = value;
            this.RaisePropertyChanged("SeValue");
        }
    }



    public RelayCommand GetDic => new Lazy<RelayCommand>(() => new RelayCommand(() => 
    {
        Dic.Add("0", "--请选择--");
        Dic.Add("A", "A");
        Dic.Add("B", "B");
        Dic.Add("C", "C");
        SeValue = "0";

    })).Value;

  
}