Datagrid Cell没有显示值

Question

我有简单的DataGrid，有3列。两个是日期，第三个是只读列，用于计算日期之间的时间段。它工作正常，但Period单元格仅在被选中后显示值。无法弄清楚原因。

这是我的DataGrid

<DataGrid ItemsSource="{Binding ClientList}" SelectedItem="{Binding SelectedClient, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" AutoGenerateColumns="False" Margin="10,39,585,10">
    <DataGrid.Columns>
        <DataGridTextColumn Header="id" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Name" Width="*" Binding="{Binding Name}"></DataGridTextColumn>
    </DataGrid.Columns>
</DataGrid>

<DataGrid Margin="585,39,10,10"
            AutoGenerateColumns="False"
            ItemsSource="{Binding PeriodTable}">
    <DataGrid.Columns>
        <DataGridTextColumn Header="Period" 
                            Width="*"
                            IsReadOnly="True"
                            Binding="{Binding Path=Period, Mode=TwoWay, UpdateSourceTrigger=Default}"></DataGridTextColumn>
        <DataGridTemplateColumn Header="DateFrom" 
                                Width="*">
            <DataGridTemplateColumn.CellEditingTemplate>
                <DataTemplate>
                    <DatePicker SelectedDate="{Binding DateFrom, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"></DatePicker>
                </DataTemplate>
            </DataGridTemplateColumn.CellEditingTemplate>
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Path=DateFrom, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"></TextBlock>
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>
        <DataGridTemplateColumn Header="DateTO" Width="*">
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Path=DateTo, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"></TextBlock>
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
            <DataGridTemplateColumn.CellEditingTemplate>
                <DataTemplate>
                    <DatePicker SelectedDate="{Binding Path=DateTo, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"></DatePicker>
                </DataTemplate>
            </DataGridTemplateColumn.CellEditingTemplate>
        </DataGridTemplateColumn>
    </DataGrid.Columns>
</DataGrid>

和代码

private ObservableCollection<PeriodClass> _periodTable = new ObservableCollection<PeriodClass>();
    public ObservableCollection<PeriodClass> PeriodTable
    {
        get { return _periodTable; }
        set
        {
            if (_periodTable != value)
            {
                _periodTable = value;
                RaisePropertyChanged("PeriodTable");
            }
        }
    }

    public class PeriodClass
    {
        private string _period;
        private DateTime? _dateFrom;
        private DateTime? _dateTo;

        public string Period
        {
            get { return _period; }
            set
            {
                if (_period != value)
                {
                    _period = value;
                }
            }
        }

        public DateTime? DateFrom
        {
            get { return _dateFrom; }
            set
            {
                if (_dateFrom != value)
                {
                    _dateFrom = value;
                }
            }
        }

        public DateTime? DateTo
        {
            get { return _dateTo; }
            set
            {
                if (_dateTo != value)
                {
                    _dateTo = value;
                    if (DateTo > DateFrom)
                    {
                        int days = DateTo.Value.DayOfYear - DateFrom.Value.DayOfYear;

                        if ((days > 7) && (days < 14) && (days % 7) != 0)
                        {
                            if ((days % 7) > 1)
                            {
                                Period = "1 week and " + (days - 7).ToString() + "days";
                            }
                            else
                            {
                                Period = "1 week and " + (days - 7).ToString() + "day";
                            }
                        }
                        else
                        {
                            if ((days % 7) > 1)
                            {
                                Period = (days / 7).ToString() + " weeks and " + (days % 7) + " days";
                            }
                            else
                            {
                                Period = (days / 7).ToString() + " weeks and " + (days % 7) + " day";
                            }

                        }

                        if (days < 7)
                        {
                            Period = days.ToString() + " days";
                        }

                        if ((days % 7) == 0)
                        {
                            if ((days / 7) == 1)
                            {
                                Period = "1 week";
                            }
                            else
                            {
                                Period = (days / 7).ToString() + " weeks";
                            }
                        }
                    }
                }
            }

        }
    }