RadioButton单选框的绑定问题（MVVM）

<RadioButton GroupName="condition"
             IsChecked="{Binding CurrentOption,Converter={StaticResource OptionConverter},ConverterParameter=全部}">全部</RadioButton>
<RadioButton GroupName="condition"Margin="10,0,0,0" 
             IsChecked="{Binding CurrentOption,Converter={StaticResource OptionConverter},ConverterParameter=内墙}">内墙</RadioButton>
<RadioButton GroupName="condition" Margin="10,0,1,0" 
             IsChecked="{Binding CurrentOption,Converter={StaticResource OptionConverter},ConverterParameter=外墙}">外墙</RadioButton>

public class OptionConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null || parameter == null)
            return false;
        string checkvalue = value.ToString();
        string targetvalue = parameter.ToString();
        bool r = checkvalue.Equals(targetvalue, StringComparison.InvariantCultureIgnoreCase);
        return r;
    }
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null || parameter == null)
            return "全部";
        bool usevalue = (bool)value;
        if (usevalue)
            return parameter.ToString();
        return null;
    }
}

private string currentOption;
public string CurrentOption
{
    get { return currentOption; }
    set
    {
        if (currentOption != value)
        {
            currentOption = value;
            OnPropertyChanged("CurrentOption");
        }
    }
}