在silverlight中将单选按钮绑定到enum

我使用这个问题/答案作为我当前需要的源代码。它工作良好，但我只能选择我的单选按钮一次。下面是我的代码:

<StackPanel Orientation="Vertical" Grid.Column="0"  >
    <sdk:Label Content="Materiale:" Margin="0,0,0,5" />
    <RadioButton GroupName="Materiale" Margin="0,0,0,2"  
                    Content="All" 
                    IsChecked="{Binding Path=Materiale, 
                                Mode=TwoWay,
                                Converter={StaticResource EnumToVisibilityConverter},
                                ConverterParameter=All}"
                    Command="{Binding CambioMaterialeCommand}" />
    <RadioButton GroupName="Materiale" Margin="0,0,0,2"  
                    Content="RotabiliSingoli" 
                    IsChecked="{Binding Path=Materiale,
                                Mode=TwoWay,
                                Converter={StaticResource EnumToVisibilityConverter},
                                ConverterParameter=RotabiliSingoli}"
                    Command="{Binding CambioMaterialeCommand}" />
    <RadioButton GroupName="Materiale" Margin="0,0,0,2"  
                    Content="Treni" 
                    IsChecked="{Binding Path=Materiale,
                                Mode=TwoWay,
                                Converter={StaticResource EnumToVisibilityConverter},
                                ConverterParameter=Treni}"
                    Command="{Binding CambioMaterialeCommand}" />
    <RadioButton GroupName="Materiale" Margin="0,0,0,2"  
                    Content="CarrozzeLocomotive" 
                    IsChecked="{Binding Path=Materiale, 
                                Mode=TwoWay, 
                                Converter={StaticResource EnumToVisibilityConverter}, 
                                ConverterParameter=CarrozzeLocomotive}"
                    Command="{Binding CambioMaterialeCommand}"/>
</StackPanel>

在ViewModel中，我有:

public void CambioMateriale()
        {
            switch (Materiale)
            {
                case E_Materiale.All:
                    IsVisibleLocomotivaCarozza = false;
                    IsVisibleTrenoPartenza = false;
                    break;
                case E_Materiale.RotabiliSingoli:
                    IsVisibleLocomotivaCarozza = false;
                    IsVisibleTrenoPartenza = false;
                    break;
                case E_Materiale.Treni:
                    IsVisibleLocomotivaCarozza = false;
                    IsVisibleTrenoPartenza = true;
                    break;
                case E_Materiale.CarrozzeLocomotive:
                    IsVisibleLocomotivaCarozza = true;
                    IsVisibleTrenoPartenza = false;
                    break;
            }
        }

当在开关上放置断点时，我可以看到Materiale值在从一个单选按钮到另一个单选按钮时发生变化，但是如果我尝试重新选择一个我已经选择的单选按钮，则此值不会改变并保持之前选择的单选按钮值。

谁能给我解释一下这是怎么回事?

谢谢你的帮助，

[编辑]

这是我的enum转换器:

   public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        string parameterString = parameter as string;
        if (parameterString == null)
            return DependencyProperty.UnsetValue;
        if (Enum.IsDefined(value.GetType(), value) == false)
            return DependencyProperty.UnsetValue;
        object parameterValue = Enum.Parse(value.GetType(), parameterString, true);
        return parameterValue.Equals(value);
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return value.Equals(true) ? parameter : DependencyProperty.UnsetValue;
    }

(/编辑)