如何在DataGridTextColumn SortMemberPath中实现转换器

我希望根据转换器返回的值对列进行排序。我有一个类，它包含两个属性VisitDate和VisitTime，但我在XAML中只有一列Visit Info来显示Visit Date和Time。因此，我创建了一个Converter来合并VisitDate和VisitTime，它按照我的喜好显示值，但我无法对列进行排序。

我的XAML源代码

<DataGrid Name="grdPendingList" ItemsSource="{Binding EmployeeList}">
    <DataGrid.Resources>
        <ui:DateTimeMergingConverter x:Key="DateTimeMergingConverterKey"></ui:DateTimeMergingConverter>
    </DataGrid.Resources>
    <DataGridTextColumn Header="Employee" Binding="{Binding empName}" CanUserSort="True"/>
    <DataGridTextColumn Header="Visit Info" Binding="{Binding Converter={StaticResource DateTimeMergingConverterKey}}" CanUserSort="True" SortMemberPath="{Binding Converter={StaticResource DateTimeMergingConverterKey}}"/>
</DataGrid>

模型和视图模型类源代码是

public class Employee
{
    public string empName { get; set; }
    public string VisitDate { get; set; }
    public string VisitTime { get; set; }
}
public class EmployeeInfo
{
    ObservableCollection<Employee> EmployeeList = new ObservableCollection<Employee>();
    public EmployeeInfo()
    {
        EmployeeList.Add(new Employee { empName = "John", VisitDate = "11/28/2015", VisitTime = "05:12 PM" });
        EmployeeList.Add(new Employee { empName = "Potter", VisitDate = "10/28/2015", VisitTime = "04:33 PM" });
        EmployeeList.Add(new Employee { empName = "James", VisitDate = "11/27/2015", VisitTime = "09:12 AM" });
    }
}

转换器源代码为

public class DateTimeMergingConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        try
        {
            if (value != null)
            {
                Employee pData = value as Employee;
                string dateStr = (string.IsNullOrEmpty(pData.VisitDate) ? string.Empty : ((pData.VisitDate).Split('/')).Count() > 2 ? pData.VisitDate : pData.VisitDate + "/2015").ToString() + " " + (string.IsNullOrEmpty(pData.VisitTime) ? string.Empty : (pData.VisitTime)).ToString();
                DateTime dt = string.IsNullOrEmpty(dateStr.Trim()) ? DateTime.Now : DateTime.Parse(dateStr);
                return dt;
            }
            else
            {
                return string.Empty;
            }
        }
        catch (Exception)
        {
            return string.Empty;
        }
    }
    public object ConvertBack(object o, Type type, object parameter, CultureInfo culture)
    {
        return null;
    }
}

public class Employee
{
    public string empName { get; set; }
    public string VisitDate { get; set; }
    public string VisitTime { get; set; }
    public DateTime VisitInfo
    {
        get { //Do your stuff here }
        set { //Do your stuff here }
    }
}
public class EmployeeInfo
{
    ObservableCollection<Employee> EmployeeList = new ObservableCollection<Employee>();
    public EmployeeInfo()
    {
        EmployeeList.Add(new Employee { empName = "John", VisitDate = "11/28/2015", VisitTime = "05:12 PM" });
        EmployeeList.Add(new Employee { empName = "Potter", VisitDate = "10/28/2015", VisitTime = "04:33 PM" });
        EmployeeList.Add(new Employee { empName = "James", VisitDate = "11/27/2015", VisitTime = "09:12 AM" });
    }
}

<DataGrid Name="grdPendingList" ItemsSource="{Binding EmployeeList}">
    <DataGridTextColumn Header="Employee" Binding="{Binding empName}" CanUserSort="True"/>
    <DataGridTextColumn Header="Visit Info" Binding="{Binding VisitInfo}" CanUserSort="True"/>
</DataGrid>