首先上图，看下样式

原理

1. 总览：

Calendar本质上是一个6x7的列表，这个列表可以用ItemsControl来实现。其填充的每一个item都是一个自定义样式的Button，Button外面是一个圆形的border。根据Button按钮的IsEnabled状态，设置foreground以达到灰色不点击的效果。

2. ItemsControl内容的生成：

主要是想办法生成第一行第一列的日期，其他的日期只需要DateTime.AddDays(i)即可。代码参考：

private void SetCalendar(int year, int month)
{_days.Clear();//请忽略DateTime datetime = new DateTime(year, month, 1);int week = (int)datetime.DayOfWeek;//获取指定月份的1号是周几datetime = datetime.AddDays(1 - week);for (int i = 0; i < 42; i++){_days.Add(datetime.AddDays(i));}OnPropertyChanged("Days");//请忽略
}

首先，将当前年月传入，获取到当前月份的第一天是周几，然后推算出ItemsControl的第一行第一列的Datetime，最后就是顺次相加即可。

实现

1. 界面的实现

先上代码：

<Grid  Grid.RowSpan="4" Margin="0,54,0,0" Visibility="{Binding IsShowCalendar,Converter={StaticResource BoolToVisibilityCollapseConverter}}" Background="White" Width="350" HorizontalAlignment="Right"><Grid.RowDefinitions><RowDefinition Height="50"></RowDefinition><RowDefinition Height="auto"></RowDefinition><RowDefinition Height="auto"></RowDefinition><RowDefinition Height="*"></RowDefinition></Grid.RowDefinitions><StackPanel HorizontalAlignment="Center" Orientation="Horizontal"><Button VerticalAlignment="Center"  Command="{Binding YearCommand}" CommandParameter="Pre" Style="{StaticResource TransparentButtonStyle}"><Button.Content><TextBlock Text="&lt;" FontSize="20"></TextBlock></Button.Content></Button><TextBlock VerticalAlignment="Center" FontSize="15" Text="{Binding CurrentYear}"></TextBlock><Button VerticalAlignment="Center"  Command="{Binding YearCommand}" CommandParameter="Next" Style="{StaticResource TransparentButtonStyle}"><Button.Content><TextBlock Text="&gt;" FontSize="20"></TextBlock></Button.Content></Button><Button Margin="50,10,10,10" VerticalAlignment="Center" Command="{Binding MonthCommand}" CommandParameter="Pre" Style="{StaticResource TransparentButtonStyle}"><Button.Content><TextBlock Text="&lt;" FontSize="20"></TextBlock></Button.Content></Button><TextBlock VerticalAlignment="Center" Text="{Binding CurrentMonth}" FontSize="15"></TextBlock><Button VerticalAlignment="Center" Command="{Binding MonthCommand}" CommandParameter="Next" Style="{StaticResource TransparentButtonStyle}"><Button.Content><TextBlock Text="&gt;" FontSize="20"></TextBlock></Button.Content></Button></StackPanel><UniformGrid Grid.Row="1" Columns="7"><TextBlock Text="MON" TextAlignment="Center"></TextBlock><TextBlock Text="TUE" TextAlignment="Center"></TextBlock><TextBlock Text="WED" TextAlignment="Center"></TextBlock><TextBlock Text="THU" TextAlignment="Center"></TextBlock><TextBlock Text="FRI" TextAlignment="Center"></TextBlock><TextBlock Text="SAT" TextAlignment="Center"></TextBlock><TextBlock Text="SUN" TextAlignment="Center"></TextBlock></UniformGrid><ItemsControl Grid.Row="2" ScrollViewer.HorizontalScrollBarVisibility="Auto"ScrollViewer.VerticalScrollBarVisibility="Auto"ItemsSource="{Binding Days}"><ItemsControl.ItemTemplate><DataTemplate><Button Background="{Binding Converter={StaticResource BACKUP_DateTimeToColorConverter}}"  Style="{StaticResource CalendarTransparentButtonStyle}"Content="{Binding Converter={StaticResource BACKUP_DateTimeToDayConverter}}" Command="{Binding DataContext.ChooseDateCommand,RelativeSource={RelativeSource FindAncestor,AncestorType={x:Type Window}}}" CommandParameter="{Binding}"MouseDoubleClick="Button_MouseDoubleClick"><Button.Foreground><MultiBinding Converter="{StaticResource BACKUP_DateTimeToForegroundMultiConverter}"><Binding></Binding><Binding Path="DataContext.CurrentMonth" RelativeSource="{RelativeSource FindAncestor,AncestorType={x:Type Window}}"></Binding></MultiBinding></Button.Foreground><Button.IsEnabled><MultiBinding Converter="{StaticResource BACKUP_DateTimeToEnableMultiConverter}"><Binding></Binding><Binding Path="DataContext.CutoffDayBegin" RelativeSource="{RelativeSource FindAncestor,AncestorType={x:Type Window}}"></Binding><Binding Path="DataContext.CutoffDayEnd" RelativeSource="{RelativeSource FindAncestor,AncestorType={x:Type Window}}"></Binding></MultiBinding></Button.IsEnabled></Button></DataTemplate></ItemsControl.ItemTemplate><ItemsControl.ItemsPanel><ItemsPanelTemplate><UniformGrid Columns="7"></UniformGrid></ItemsPanelTemplate></ItemsControl.ItemsPanel></ItemsControl><StackPanel Grid.Row="3" HorizontalAlignment="Center" VerticalAlignment="Bottom" Margin="0,0,0,31" Orientation="Horizontal"><Button Content="Cancel" Style="{StaticResource btnSecondaryStyle}" Grid.Row="2" Command="{Binding HideCalendarCommand}"></Button><Button Content="{Binding CalendarSetText}" Style="{StaticResource btnPrimaryStyle}" Grid.Row="2" Command="{Binding HideCalendarCommand}"></Button></StackPanel></Grid>

其中，大量用到了Binding和MVVM，如果对方面还不是很扎实，可以参考下刘铁锰的视频.
这段代码中，主要部分是ItemsControl的ItemsSource绑定。这里绑定的是Days,其类型是

public ObservableCollection<DateTime> _days = new ObservableCollection<DateTime>();public ObservableCollection<DateTime> Days{get{return _days;}}

在设置Button的前景色和IsEnabled状态的时候，你也看到了用的是MultiBinding，并定义了两个Converter：
1. DateTimeToForegroundMultiConverter

 class DateTimeToForegroundMultiConverter : IMultiValueConverter{public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture){DateTime dt=new DateTime();if(values[0] is DateTime){dt = (DateTime)values[0];}int currentMonth;Int32.TryParse(values[1].ToString(), out currentMonth);if (dt.Month == currentMonth){if(dt.ToShortDateString()==DateTime.Now.ToShortDateString()){return new SolidColorBrush(Colors.White);}else{return new SolidColorBrush(Colors.Black);}}else{return new SolidColorBrush(Colors.Gray);}}public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture){return null;}}

2. DateTimeToEnableMultiConverter

 class DateTimeToEnableMultiConverter : IMultiValueConverter{public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture){DateTime currentDay, cutoffDayBegin,cutoffDayEnd;if(values[0] is DateTime&&values[1] is DateTime&&values[2] is DateTime){currentDay = (DateTime)values[0];cutoffDayBegin = (DateTime)values[1];cutoffDayEnd = (DateTime)values[2];if (DateTime.Compare(currentDay, cutoffDayBegin) >= 0 && DateTime.Compare(currentDay, cutoffDayEnd) <= 0){return true;}else{return false;}}else{return false;}}public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture){return null;}}

2. 后台ViewModel的实现

其实后台没有什么代码，有一个SetCalendar方法，在月份或者年份发生变动的时候调用一下即可。

private void SetCalendar(int year, int month){_days.Clear();DateTime datetime = new DateTime(year, month, 1);int week = (int)datetime.DayOfWeek;datetime = datetime.AddDays(1 - week);for (int i = 0; i < 42; i++){_days.Add(datetime.AddDays(i));}OnPropertyChanged("Days");}

还有一个就是Button点击之后的Command命令：

 private void exeChooseDate(object obj){DateTime dt;DateTime.TryParse(obj.ToString(), out dt);CurrentMonth = dt.Month;CurrentYear = dt.Year;}

Button每点击一次，就会执行一次这个方法，CurrentMonth和CurrentYear这两个属性的值就会被更改，一些来决定是否需要重新绘制UI切换的上一个/下一个月份。

CurrentMonth和CurrentYear属性见下：

private int _currentYear = 2010;public int CurrentYear{get{return _currentYear;}set{if (_currentYear != value && value > 1978 && value < 9999){_currentYear = value;OnPropertyChanged("CurrentYear");SetCalendar(_currentYear, CurrentMonth);}}}private int _currentMonth = 1;public int CurrentMonth{get{return _currentMonth;}set{if (_currentMonth != value && value < 13 && value > 0){_currentMonth = value;OnPropertyChanged("CurrentMonth");SetCalendar(CurrentYear, _currentMonth);}}}