MonthCalendar控制选择范围与EnableVisualStyles？

Question

我正在使用MonthCalendar控件，并希望以编程方式选择日期范围。当我这样做时，如果已经调用了Application.EnableVisualStyles()，控件就不会正确地绘制。根据MSDN，这是一个已知的问题。

在启用视觉样式的情况下使用MonthCalendar将导致MonthCalendar控件的选择范围不能正确绘制(from：http://msdn.microsoft.com/en-us/library/system.windows.forms.monthcalendar.aspx)

除了不调用EnableVisualStyles之外，真的没有其他的解决办法吗？从我的角度来看，这似乎使得这个特殊的控制对于一系列的应用程序和一个相当明显的疏忽完全无用。

Answer 1

在寻找相同问题的解决方案时，我首先在这里遇到了这个问题，但后来我发现了安德松的一个博客条目。我觉得这很有帮助。下面是我从Nicke的例子中得出的结论：

public class MonthCalendarEx : System.Windows.Forms.MonthCalendar
{
    private int _offsetX;
    private int _offsetY;
    private int _dayBoxWidth;
    private int _dayBoxHeight;

    private bool _repaintSelectedDays = false;

    public MonthCalendarEx() : base()
    {
        OnSizeChanged(null, null);
        this.SizeChanged += OnSizeChanged;
        this.DateChanged += OnSelectionChanged;
        this.DateSelected += OnSelectionChanged;
    }

    protected static int WM_PAINT = 0x000F;

    protected override void WndProc(ref System.Windows.Forms.Message m)
    {
        base.WndProc(ref m);
        if (m.Msg == WM_PAINT)
        {
            Graphics graphics = Graphics.FromHwnd(this.Handle);
            PaintEventArgs pe = new PaintEventArgs(
                graphics, new Rectangle(0, 0, this.Width, this.Height));
            OnPaint(pe);
        }
    }

    private void OnSelectionChanged(object sender, EventArgs e)
    {
        _repaintSelectedDays = true;
    }

    private void OnSizeChanged(object sender, EventArgs e)
    {                         
        _offsetX = 0;
        _offsetY = 0;

        // determine Y offset of days area 
        while (
            HitTest(Width / 2, _offsetY).HitArea != HitArea.PrevMonthDate &&
            HitTest(Width / 2, _offsetY).HitArea != HitArea.Date)
        {
            _offsetY++;
        }

        // determine X offset of days area 
        while (HitTest(_offsetX, Height / 2).HitArea != HitArea.Date)
        {
            _offsetX++;
        }

        // determine width of a single day box
        _dayBoxWidth = 0;
        DateTime dt1 = HitTest(Width / 2, _offsetY).Time;

        while (HitTest(Width / 2, _offsetY + _dayBoxHeight).Time == dt1)
        {
            _dayBoxHeight++;
        }

        // determine height of a single day box
        _dayBoxWidth = 0;
        DateTime dt2 = HitTest(_offsetX, Height / 2).Time;

        while (HitTest(_offsetX + _dayBoxWidth, Height / 2).Time == dt2)
        {
            _dayBoxWidth++;
        }
    }

    protected override void OnPaint(PaintEventArgs e)
    { 
        base.OnPaint(e);

        if (_repaintSelectedDays)
        {
            Graphics graphics = e.Graphics;
            SelectionRange calendarRange = GetDisplayRange(false);
            Rectangle currentDayFrame = new Rectangle(
                -1, -1, _dayBoxWidth, _dayBoxHeight);

            DateTime current = SelectionStart;
            while (current <= SelectionEnd)                
            {
                Rectangle currentDayRectangle;

                using (Brush selectionBrush = new SolidBrush(
                    Color.FromArgb(
                        255, System.Drawing.SystemColors.ActiveCaption))) 
                {                    
                    TimeSpan span = current.Subtract(calendarRange.Start); 
                    int row = span.Days / 7; 
                    int col = span.Days % 7; 

                    currentDayRectangle = new Rectangle(
                        _offsetX + (col + (ShowWeekNumbers ? 1 : 0)) * _dayBoxWidth, 
                        _offsetY + row * _dayBoxHeight, 
                        _dayBoxWidth, 
                        _dayBoxHeight);

                    graphics.FillRectangle(selectionBrush, currentDayRectangle); 
                }

                TextRenderer.DrawText(
                    graphics, 
                    current.Day.ToString(), 
                    Font, 
                    currentDayRectangle, 
                    System.Drawing.SystemColors.ActiveCaptionText, 
                    TextFormatFlags.HorizontalCenter | TextFormatFlags.VerticalCenter);

                if (current == this.TodayDate)
                {
                    currentDayFrame = currentDayRectangle;
                }

                current = current.AddDays(1);
            }

            if (currentDayFrame.X > 0)
            {
                graphics.DrawRectangle(new Pen(
                    new SolidBrush(Color.Red)), currentDayFrame);
            }

            _repaintSelectedDays = false;
        }
    }
}

Answer 2

下面是一个显示超过一个月的版本(CalendarDimensions != (1,1))，并修复了其他一些问题：

/// <summary>
/// When Visual Styles are enabled on Windows XP, the MonthCalendar.SelectionRange
/// does not paint correctly when more than one date is selected.
/// See: http://msdn.microsoft.com/en-us/library/5d1acks5(VS.80).aspx
/// "Additionally, if you enable visual styles on some controls, the control might display incorrectly
/// in certain situations. These include the MonthCalendar control with a selection range set...
/// This class fixes that problem.
/// </summary>
/// <remarks>Author: Mark Cranness</remarks>
public class FixVisualStylesMonthCalendar : System.Windows.Forms.MonthCalendar {

    /// <summary>
    /// The width of a single cell (date) in the calendar.
    /// </summary>
    private int dayCellWidth;
    /// <summary>
    /// The height of a single cell (date) in the calendar.
    /// </summary>
    private int dayCellHeight;

    /// <summary>
    /// The calendar first day of the week actually used.
    /// </summary>
    private DayOfWeek calendarFirstDayOfWeek;

    /// <summary>
    /// Only repaint when VisualStyles enabled on Windows XP.
    /// </summary>
    private bool repaintSelectionRange = false;

    /// <summary>
    /// A MonthCalendar class that fixes SelectionRange painting problems 
    /// on Windows XP when Visual Styles is enabled.
    /// </summary>
    public FixVisualStylesMonthCalendar() {

        if (Application.RenderWithVisualStyles
                && Environment.OSVersion.Version < new Version(6, 0)) {

            // If Visual Styles are enabled, and XP, then fix-up the painting of SelectionRange
            this.repaintSelectionRange = true;
            this.OnSizeChanged(this, EventArgs.Empty);
            this.SizeChanged += new EventHandler(this.OnSizeChanged);

        }
    }

    /// <summary>
    /// The WM_PAINT message is sent to make a request to paint a portion of a window.
    /// </summary>
    public const int WM_PAINT = 0x000F;

    /// <summary>
    /// Override WM_PAINT to repaint the selection range.
    /// </summary>
    [System.Diagnostics.DebuggerStepThroughAttribute()]
    protected override void WndProc(ref Message m)
    {
        base.WndProc(ref m);
        if (m.Msg == WM_PAINT
                && !this.DesignMode
                && this.repaintSelectionRange) {
            // MonthCalendar is ControlStyles.UserPaint=false => Paint event is not raised
            this.RepaintSelectionRange(ref m);
        }
    }

    /// <summary>
    /// Repaint the SelectionRange.
    /// </summary>
    private void RepaintSelectionRange(ref Message m) {

        using (Graphics graphics = this.CreateGraphics())
        using (Brush backBrush
                = new SolidBrush(graphics.GetNearestColor(this.BackColor)))
        using (Brush selectionBrush
                = new SolidBrush(graphics.GetNearestColor(SystemColors.ActiveCaption))) {

            Rectangle todayFrame = Rectangle.Empty;

            // For each day in SelectionRange...
            for (DateTime selectionDate = this.SelectionStart;
                    selectionDate <= this.SelectionEnd;
                    selectionDate = selectionDate.AddDays(1)) {

                Rectangle selectionDayRectangle = this.GetSelectionDayRectangle(selectionDate);
                if (selectionDayRectangle.IsEmpty) continue;

                if (selectionDate.Date == this.TodayDate) {
                    todayFrame = selectionDayRectangle;
                }

                // Paint as 'selected' a little smaller than the whole rectangle
                Rectangle highlightRectangle = Rectangle.Inflate(selectionDayRectangle, 0, -2);
                if (selectionDate == this.SelectionStart) {
                    highlightRectangle.X += 2;
                    highlightRectangle.Width -= 2;
                }
                if (selectionDate == this.SelectionEnd) {
                    highlightRectangle.Width -= 2;
                }

                // Paint background, selection and day-of-month text
                graphics.FillRectangle(backBrush, selectionDayRectangle);
                graphics.FillRectangle(selectionBrush, highlightRectangle);
                TextRenderer.DrawText(
                    graphics,
                    selectionDate.Day.ToString(),
                    this.Font,
                    selectionDayRectangle,
                    SystemColors.ActiveCaptionText,
                    TextFormatFlags.HorizontalCenter | TextFormatFlags.VerticalCenter);

            }

            if (this.ShowTodayCircle && !todayFrame.IsEmpty) {
                // Redraw the ShowTodayCircle (square) that we painted over above
                using (Pen redPen = new Pen(Color.Red)) {
                    todayFrame.Width--;
                    todayFrame.Height--;
                    graphics.DrawRectangle(redPen, todayFrame);
                }
            }

        }
    }

    /// <summary>
    /// When displayed dates changed, clear the cached month locations.
    /// </summary>
    private SelectionRange previousDisplayedDates = new SelectionRange();

    /// <summary>
    /// Gets a graphics Rectangle for the area corresponding to a single date on the calendar.
    /// </summary>
    private Rectangle GetSelectionDayRectangle(DateTime selectionDateTime) {

        // Handle the leading and trailing dates from the previous and next months
        SelectionRange allDisplayedDates = this.GetDisplayRange(false);
        SelectionRange fullMonthDates = this.GetDisplayRange(true);
        int adjust1Week;
        DateTime selectionDate = selectionDateTime.Date;
        if (selectionDate < allDisplayedDates.Start 
                || selectionDate > allDisplayedDates.End) {
            // Selection Date is not displayed on calendar
            return Rectangle.Empty;
        } else if (selectionDate < fullMonthDates.Start) {
            // Selection Date is trailing from the previous partial month
            selectionDate = selectionDate.AddDays(7);
            adjust1Week = -1;
        } else if (selectionDate > fullMonthDates.End) {
            // Selection Date is leading from the next partial month
            selectionDate = selectionDate.AddDays(-14);
            adjust1Week = +2;
        } else {
            // A mainline date
            adjust1Week = 0;
        }

        // Discard cached month locations when calendar moves
        if (this.previousDisplayedDates.Start != allDisplayedDates.Start
                || this.previousDisplayedDates.End != allDisplayedDates.End) {
            this.DiscardCachedMonthDateAreaLocations();
            this.previousDisplayedDates.Start = allDisplayedDates.Start;
            this.previousDisplayedDates.End = allDisplayedDates.End;
        }

        Point monthDateAreaLocation = this.GetMonthDateAreaLocation(selectionDate);
        if (monthDateAreaLocation.IsEmpty) return Rectangle.Empty;

        DayOfWeek monthFirstDayOfWeek = (new DateTime(selectionDate.Year, selectionDate.Month, 1)).DayOfWeek;
        int dayOfWeekAdjust = (int)monthFirstDayOfWeek - (int)this.calendarFirstDayOfWeek;
        if (dayOfWeekAdjust < 0) dayOfWeekAdjust += 7;
        int row = (selectionDate.Day - 1 + dayOfWeekAdjust) / 7;
        int col = (selectionDate.Day - 1 + dayOfWeekAdjust) % 7;
        row += adjust1Week;

        return new Rectangle(
            monthDateAreaLocation.X + col * this.dayCellWidth,
            monthDateAreaLocation.Y + row * this.dayCellHeight,
            this.dayCellWidth,
            this.dayCellHeight);

    }

    /// <summary>
    /// Cached calendar location from the last lookup.
    /// </summary>
    private Point[] cachedMonthDateAreaLocation = new Point[13];

    /// <summary>
    /// Discard the cached month locations when calendar moves.
    /// </summary>
    private void DiscardCachedMonthDateAreaLocations() {
        for (int i = 0; i < 13; i++) this.cachedMonthDateAreaLocation[i] = Point.Empty;
    }

    /// <summary>
    /// Gets the graphics location (x,y point) of the top left of the
    /// calendar date area for the month containing the specified date.
    /// </summary>
    private Point GetMonthDateAreaLocation(DateTime selectionDate) {

        Point monthDateAreaLocation = this.cachedMonthDateAreaLocation[selectionDate.Month];
        HitTestInfo hitInfo;
        if (!monthDateAreaLocation.IsEmpty
                && (hitInfo = this.HitTest(monthDateAreaLocation.X, monthDateAreaLocation.Y + this.dayCellHeight))
                    .HitArea == HitArea.Date
                && hitInfo.Time.Year == selectionDate.Year
                && hitInfo.Time.Month == selectionDate.Month) {

            // Use previously cached lookup
            return monthDateAreaLocation;

        } else {

            // Assume the worst (Error: empty)
            monthDateAreaLocation = this.cachedMonthDateAreaLocation[selectionDate.Month] = Point.Empty;

            Point monthDataAreaPoint = this.GetMonthDateAreaMiddle(selectionDate);
            if (monthDataAreaPoint.IsEmpty) return Point.Empty;

            // Move left from the middle to find the left edge of the Date area
            monthDateAreaLocation.X = monthDataAreaPoint.X--;
            HitTestInfo hitInfo1, hitInfo2;
            while ((hitInfo1 = this.HitTest(monthDataAreaPoint.X, monthDataAreaPoint.Y))
                    .HitArea == HitArea.Date
                    && hitInfo1.Time.Month == selectionDate.Month
                || (hitInfo2 = this.HitTest(monthDataAreaPoint.X, monthDataAreaPoint.Y + this.dayCellHeight))
                    .HitArea == HitArea.Date
                    && hitInfo2.Time.Month == selectionDate.Month) {
                monthDateAreaLocation.X = monthDataAreaPoint.X--;
                if (monthDateAreaLocation.X < 0) return Point.Empty; // Error: bail
            }

            // Move up from the last column to find the top edge of the Date area
            int monthLastDayOfWeekX = monthDateAreaLocation.X + (this.dayCellWidth * 7 * 13) / 14;
            monthDateAreaLocation.Y = monthDataAreaPoint.Y--;
            while (this.HitTest(monthLastDayOfWeekX, monthDataAreaPoint.Y).HitArea == HitArea.Date) {
                monthDateAreaLocation.Y = monthDataAreaPoint.Y--;
                if (monthDateAreaLocation.Y < 0) return Point.Empty; // Error: bail
            }

            // Got it
            this.cachedMonthDateAreaLocation[selectionDate.Month] = monthDateAreaLocation;
            return monthDateAreaLocation;

        }
    }

    /// <summary>
    /// Paranoid fudge/wobble of the GetMonthDateAreaMiddle in case 
    /// our first estimate to hit the month misses.
    /// (Needed? perhaps not.)
    /// </summary>
    private static Point[] searchSpiral = {
        new Point( 0, 0),
        new Point(-1,+1), new Point(+1,+1), new Point(+1,-1), new Point(-1,-1), 
        new Point(-2,+2), new Point(+2,+2), new Point(+2,-2), new Point(-2,-2)
    };

    /// <summary>
    /// Gets a point somewhere inside the calendar date area of
    /// the month containing the given selection date.
    /// </summary>
    /// <remarks>The point returned will be HitArea.Date, and match the year and
    /// month of the selection date; otherwise it will be Point.Empty.</remarks>
    private Point GetMonthDateAreaMiddle(DateTime selectionDate) {

        // Iterate over all displayed months, and a search spiral (needed? perhaps not)
        for (int dimX = 1; dimX <= this.CalendarDimensions.Width; dimX++) {
            for (int dimY = 1; dimY <= this.CalendarDimensions.Height; dimY++) {
                foreach (Point search in searchSpiral) {

                    Point monthDateAreaMiddle = new Point(
                        ((dimX - 1) * 2 + 1) * this.Width / (2 * this.CalendarDimensions.Width)
                            + this.dayCellWidth * search.X,
                        ((dimY - 1) * 2 + 1) * this.Height / (2 * this.CalendarDimensions.Height)
                            + this.dayCellHeight * search.Y);
                    HitTestInfo hitInfo = this.HitTest(monthDateAreaMiddle);
                    if (hitInfo.HitArea == HitArea.Date) {
                        // Got the Date Area of the month
                        if (hitInfo.Time.Year == selectionDate.Year
                                && hitInfo.Time.Month == selectionDate.Month) {
                            // For the correct month
                            return monthDateAreaMiddle;
                        } else {
                            // Keep looking in the other months
                            break;
                        }
                    }

                }
            }
        }
        return Point.Empty; // Error: not found

    }

    /// <summary>
    /// When this MonthCalendar is resized, recalculate the size of a day cell.
    /// </summary>
    private void OnSizeChanged(object sender, EventArgs e) {

        // Discard previous cached Month Area Location
        DiscardCachedMonthDateAreaLocations();
        this.dayCellWidth = this.dayCellHeight = 0;

        // Without this, the repaint sometimes does not happen...
        this.Invalidate();

        // Determine Y offset of days area
        int middle = this.Width / (2 * this.CalendarDimensions.Width);
        int dateAreaTop = 0;
        while (this.HitTest(middle, dateAreaTop).HitArea != HitArea.PrevMonthDate
                && this.HitTest(middle, dateAreaTop).HitArea != HitArea.Date) {
            dateAreaTop++;
            if (dateAreaTop > this.ClientSize.Height) return; // Error: bail
        }

        // Determine height of a single day box
        int dayCellHeight = 1;
        DateTime dayCellTime = this.HitTest(middle, dateAreaTop).Time;
        while (this.HitTest(middle, dateAreaTop + dayCellHeight).Time == dayCellTime) {
            dayCellHeight++;
        }

        // Determine X offset of days area
        middle = this.Height / (2 * this.CalendarDimensions.Height);
        int dateAreaLeft = 0;
        while (this.HitTest(dateAreaLeft, middle).HitArea != HitArea.Date) {
            dateAreaLeft++;
            if (dateAreaLeft > this.ClientSize.Width) return; // Error: bail
        }

        // Determine width of a single day box
        int dayCellWidth = 1;
        dayCellTime = this.HitTest(dateAreaLeft, middle).Time;
        while (this.HitTest(dateAreaLeft + dayCellWidth, middle).Time == dayCellTime) {
            dayCellWidth++;
        }

        // Record day box size and actual first day of the month used
        this.calendarFirstDayOfWeek = dayCellTime.DayOfWeek;
        this.dayCellWidth = dayCellWidth;
        this.dayCellHeight = dayCellHeight;

    }

}

我的测试表明Windows 7没有画图问题，我希望Vista也没有，所以这只是尝试对Windows进行修复。

Answer 3

我在上面的Mark代码中发现了一个小问题:在完全禁用视觉样式的XP系统上，即使调用了Application.RenderWithVisualStyles ()，也会将Application.EnableVisualStyles设置为False。

因此，在这种情况下，自定义画图代码根本不运行。为了修复它，我将FixVisualStylesMonthCalendar构造函数的第一行更改为

if (Application.VisualStyleState != System.Windows.Forms.VisualStyles.VisualStyleState.NoneEnabled && 
            Environment.OSVersion.Version < new Version(6, 0))

整个代码都在这个答案的底部。

我找不到任何办法来评论答案本身。以下代码的学分属于原始作者-(如果他或任何人能够验证并更新这个答案，我很乐意删除这个答案)

/// <summary>
/// When Visual Styles are enabled on Windows XP, the MonthCalendar.SelectionRange
/// does not paint correctly when more than one date is selected.
/// See: http://msdn.microsoft.com/en-us/library/5d1acks5(VS.80).aspx
/// "Additionally, if you enable visual styles on some controls, the control might display incorrectly
/// in certain situations. These include the MonthCalendar control with a selection range set...
/// This class fixes that problem.
/// </summary>
/// <remarks>Author: Mark Cranness - PatronBase Limited.</remarks>
public class FixVisualStylesMonthCalendar : System.Windows.Forms.MonthCalendar
{

    /// <summary>
    /// The width of a single cell (date) in the calendar.
    /// </summary>
    private int dayCellWidth;
    /// <summary>
    /// The height of a single cell (date) in the calendar.
    /// </summary>
    private int dayCellHeight;

    /// <summary>
    /// The calendar first day of the week actually used.
    /// </summary>
    private DayOfWeek calendarFirstDayOfWeek;

    /// <summary>
    /// Only repaint when VisualStyles enabled on Windows XP.
    /// </summary>
    private bool repaintSelectionRange = false;

    /// <summary>
    /// A MonthCalendar class that fixes SelectionRange painting problems 
    /// on Windows XP when Visual Styles is enabled.
    /// </summary>
    public FixVisualStylesMonthCalendar()
    {

        if (Application.VisualStyleState != System.Windows.Forms.VisualStyles.VisualStyleState.NoneEnabled && //Application.RenderWithVisualStyles && 
            Environment.OSVersion.Version < new Version(6, 0))
        {
            // If Visual Styles are enabled, and XP, then fix-up the painting of SelectionRange
            this.repaintSelectionRange = true;
            this.OnSizeChanged(this, EventArgs.Empty);
            this.SizeChanged += new EventHandler(this.OnSizeChanged);
        }
    }

    /// <summary>
    /// The WM_PAINT message is sent to make a request to paint a portion of a window.
    /// </summary>
    public const int WM_PAINT = 0x000F;

    /// <summary>
    /// Override WM_PAINT to repaint the selection range.
    /// </summary>
    [System.Diagnostics.DebuggerStepThroughAttribute()]
    protected override void WndProc(ref Message m)
    {
        base.WndProc(ref m);
        if (m.Msg == WM_PAINT
                && !this.DesignMode
                && this.repaintSelectionRange)
        {
            // MonthCalendar is ControlStyles.UserPaint=false => Paint event is not raised
            this.RepaintSelectionRange(ref m);
        }
    }

    /// <summary>
    /// Repaint the SelectionRange.
    /// </summary>
    private void RepaintSelectionRange(ref Message m)
    {

        using (Graphics graphics = this.CreateGraphics())
        using (Brush backBrush
                = new SolidBrush(graphics.GetNearestColor(this.BackColor)))
        using (Brush selectionBrush
                = new SolidBrush(graphics.GetNearestColor(SystemColors.ActiveCaption)))
        {

            Rectangle todayFrame = Rectangle.Empty;

            // For each day in SelectionRange...
            for (DateTime selectionDate = this.SelectionStart;
                    selectionDate <= this.SelectionEnd;
                    selectionDate = selectionDate.AddDays(1))
            {

                Rectangle selectionDayRectangle = this.GetSelectionDayRectangle(selectionDate);
                if (selectionDayRectangle.IsEmpty) continue;

                if (selectionDate.Date == this.TodayDate)
                {
                    todayFrame = selectionDayRectangle;
                }

                // Paint as 'selected' a little smaller than the whole rectangle
                Rectangle highlightRectangle = Rectangle.Inflate(selectionDayRectangle, 0, -2);
                if (selectionDate == this.SelectionStart)
                {
                    highlightRectangle.X += 2;
                    highlightRectangle.Width -= 2;
                }
                if (selectionDate == this.SelectionEnd)
                {
                    highlightRectangle.Width -= 2;
                }

                // Paint background, selection and day-of-month text
                graphics.FillRectangle(backBrush, selectionDayRectangle);
                graphics.FillRectangle(selectionBrush, highlightRectangle);
                TextRenderer.DrawText(
                    graphics,
                    selectionDate.Day.ToString(),
                    this.Font,
                    selectionDayRectangle,
                    SystemColors.ActiveCaptionText,
                    TextFormatFlags.HorizontalCenter | TextFormatFlags.VerticalCenter);

            }

            if (this.ShowTodayCircle && !todayFrame.IsEmpty)
            {
                // Redraw the ShowTodayCircle (square) that we painted over above
                using (Pen redPen = new Pen(Color.Red))
                {
                    todayFrame.Width--;
                    todayFrame.Height--;
                    graphics.DrawRectangle(redPen, todayFrame);
                }
            }

        }
    }

    /// <summary>
    /// When displayed dates changed, clear the cached month locations.
    /// </summary>
    private SelectionRange previousDisplayedDates = new SelectionRange();

    /// <summary>
    /// Gets a graphics Rectangle for the area corresponding to a single date on the calendar.
    /// </summary>
    private Rectangle GetSelectionDayRectangle(DateTime selectionDateTime)
    {

        // Handle the leading and trailing dates from the previous and next months
        SelectionRange allDisplayedDates = this.GetDisplayRange(false);
        SelectionRange fullMonthDates = this.GetDisplayRange(true);
        int adjust1Week;
        DateTime selectionDate = selectionDateTime.Date;
        if (selectionDate < allDisplayedDates.Start
                || selectionDate > allDisplayedDates.End)
        {
            // Selection Date is not displayed on calendar
            return Rectangle.Empty;
        }
        else if (selectionDate < fullMonthDates.Start)
        {
            // Selection Date is trailing from the previous partial month
            selectionDate = selectionDate.AddDays(7);
            adjust1Week = -1;
        }
        else if (selectionDate > fullMonthDates.End)
        {
            // Selection Date is leading from the next partial month
            selectionDate = selectionDate.AddDays(-14);
            adjust1Week = +2;
        }
        else
        {
            // A mainline date
            adjust1Week = 0;
        }

        // Discard cached month locations when calendar moves
        if (this.previousDisplayedDates.Start != allDisplayedDates.Start
                || this.previousDisplayedDates.End != allDisplayedDates.End)
        {
            this.DiscardCachedMonthDateAreaLocations();
            this.previousDisplayedDates.Start = allDisplayedDates.Start;
            this.previousDisplayedDates.End = allDisplayedDates.End;
        }

        Point monthDateAreaLocation = this.GetMonthDateAreaLocation(selectionDate);
        if (monthDateAreaLocation.IsEmpty) return Rectangle.Empty;

        DayOfWeek monthFirstDayOfWeek = (new DateTime(selectionDate.Year, selectionDate.Month, 1)).DayOfWeek;
        int dayOfWeekAdjust = (int)monthFirstDayOfWeek - (int)this.calendarFirstDayOfWeek;
        if (dayOfWeekAdjust < 0) dayOfWeekAdjust += 7;
        int row = (selectionDate.Day - 1 + dayOfWeekAdjust) / 7;
        int col = (selectionDate.Day - 1 + dayOfWeekAdjust) % 7;
        row += adjust1Week;

        return new Rectangle(
            monthDateAreaLocation.X + col * this.dayCellWidth,
            monthDateAreaLocation.Y + row * this.dayCellHeight,
            this.dayCellWidth,
            this.dayCellHeight);

    }

    /// <summary>
    /// Cached calendar location from the last lookup.
    /// </summary>
    private Point[] cachedMonthDateAreaLocation = new Point[13];

    /// <summary>
    /// Discard the cached month locations when calendar moves.
    /// </summary>
    private void DiscardCachedMonthDateAreaLocations()
    {
        for (int i = 0; i < 13; i++) this.cachedMonthDateAreaLocation[i] = Point.Empty;
    }

    /// <summary>
    /// Gets the graphics location (x,y point) of the top left of the
    /// calendar date area for the month containing the specified date.
    /// </summary>
    private Point GetMonthDateAreaLocation(DateTime selectionDate)
    {

        Point monthDateAreaLocation = this.cachedMonthDateAreaLocation[selectionDate.Month];
        HitTestInfo hitInfo;
        if (!monthDateAreaLocation.IsEmpty
                && (hitInfo = this.HitTest(monthDateAreaLocation.X, monthDateAreaLocation.Y + this.dayCellHeight))
                    .HitArea == HitArea.Date
                && hitInfo.Time.Year == selectionDate.Year
                && hitInfo.Time.Month == selectionDate.Month)
        {

            // Use previously cached lookup
            return monthDateAreaLocation;

        }
        else
        {

            // Assume the worst (Error: empty)
            monthDateAreaLocation = this.cachedMonthDateAreaLocation[selectionDate.Month] = Point.Empty;

            Point monthDataAreaPoint = this.GetMonthDateAreaMiddle(selectionDate);
            if (monthDataAreaPoint.IsEmpty) return Point.Empty;

            // Move left from the middle to find the left edge of the Date area
            monthDateAreaLocation.X = monthDataAreaPoint.X--;
            HitTestInfo hitInfo1, hitInfo2;
            while ((hitInfo1 = this.HitTest(monthDataAreaPoint.X, monthDataAreaPoint.Y))
                    .HitArea == HitArea.Date
                    && hitInfo1.Time.Month == selectionDate.Month
                || (hitInfo2 = this.HitTest(monthDataAreaPoint.X, monthDataAreaPoint.Y + this.dayCellHeight))
                    .HitArea == HitArea.Date
                    && hitInfo2.Time.Month == selectionDate.Month)
            {
                monthDateAreaLocation.X = monthDataAreaPoint.X--;
                if (monthDateAreaLocation.X < 0) return Point.Empty; // Error: bail
            }

            // Move up from the last column to find the top edge of the Date area
            int monthLastDayOfWeekX = monthDateAreaLocation.X + (this.dayCellWidth * 7 * 13) / 14;
            monthDateAreaLocation.Y = monthDataAreaPoint.Y--;
            while (this.HitTest(monthLastDayOfWeekX, monthDataAreaPoint.Y).HitArea == HitArea.Date)
            {
                monthDateAreaLocation.Y = monthDataAreaPoint.Y--;
                if (monthDateAreaLocation.Y < 0) return Point.Empty; // Error: bail
            }

            // Got it
            this.cachedMonthDateAreaLocation[selectionDate.Month] = monthDateAreaLocation;
            return monthDateAreaLocation;

        }
    }

    /// <summary>
    /// Paranoid fudge/wobble of the GetMonthDateAreaMiddle in case 
    /// our first estimate to hit the month misses.
    /// (Needed? perhaps not.)
    /// </summary>
    private static Point[] searchSpiral = {
    new Point( 0, 0),
    new Point(-1,+1), new Point(+1,+1), new Point(+1,-1), new Point(-1,-1), 
    new Point(-2,+2), new Point(+2,+2), new Point(+2,-2), new Point(-2,-2)
};

    /// <summary>
    /// Gets a point somewhere inside the calendar date area of
    /// the month containing the given selection date.
    /// </summary>
    /// <remarks>The point returned will be HitArea.Date, and match the year and
    /// month of the selection date; otherwise it will be Point.Empty.</remarks>
    private Point GetMonthDateAreaMiddle(DateTime selectionDate)
    {

        // Iterate over all displayed months, and a search spiral (needed? perhaps not)
        for (int dimX = 1; dimX <= this.CalendarDimensions.Width; dimX++)
        {
            for (int dimY = 1; dimY <= this.CalendarDimensions.Height; dimY++)
            {
                foreach (Point search in searchSpiral)
                {

                    Point monthDateAreaMiddle = new Point(
                        ((dimX - 1) * 2 + 1) * this.Width / (2 * this.CalendarDimensions.Width)
                            + this.dayCellWidth * search.X,
                        ((dimY - 1) * 2 + 1) * this.Height / (2 * this.CalendarDimensions.Height)
                            + this.dayCellHeight * search.Y);
                    HitTestInfo hitInfo = this.HitTest(monthDateAreaMiddle);
                    if (hitInfo.HitArea == HitArea.Date)
                    {
                        // Got the Date Area of the month
                        if (hitInfo.Time.Year == selectionDate.Year
                                && hitInfo.Time.Month == selectionDate.Month)
                        {
                            // For the correct month
                            return monthDateAreaMiddle;
                        }
                        else
                        {
                            // Keep looking in the other months
                            break;
                        }
                    }

                }
            }
        }
        return Point.Empty; // Error: not found

    }

    /// <summary>
    /// When this MonthCalendar is resized, recalculate the size of a day cell.
    /// </summary>
    private void OnSizeChanged(object sender, EventArgs e)
    {

        // Discard previous cached Month Area Location
        DiscardCachedMonthDateAreaLocations();
        this.dayCellWidth = this.dayCellHeight = 0;

        // Without this, the repaint sometimes does not happen...
        this.Invalidate();

        // Determine Y offset of days area
        int middle = this.Width / (2 * this.CalendarDimensions.Width);
        int dateAreaTop = 0;
        while (this.HitTest(middle, dateAreaTop).HitArea != HitArea.PrevMonthDate
                && this.HitTest(middle, dateAreaTop).HitArea != HitArea.Date)
        {
            dateAreaTop++;
            if (dateAreaTop > this.ClientSize.Height) return; // Error: bail
        }

        // Determine height of a single day box
        int dayCellHeight = 1;
        DateTime dayCellTime = this.HitTest(middle, dateAreaTop).Time;
        while (this.HitTest(middle, dateAreaTop + dayCellHeight).Time == dayCellTime)
        {
            dayCellHeight++;
        }

        // Determine X offset of days area
        middle = this.Height / (2 * this.CalendarDimensions.Height);
        int dateAreaLeft = 0;
        while (this.HitTest(dateAreaLeft, middle).HitArea != HitArea.Date)
        {
            dateAreaLeft++;
            if (dateAreaLeft > this.ClientSize.Width) return; // Error: bail
        }

        // Determine width of a single day box
        int dayCellWidth = 1;
        dayCellTime = this.HitTest(dateAreaLeft, middle).Time;
        while (this.HitTest(dateAreaLeft + dayCellWidth, middle).Time == dayCellTime)
        {
            dayCellWidth++;
        }

        // Record day box size and actual first day of the month used
        this.calendarFirstDayOfWeek = dayCellTime.DayOfWeek;
        this.dayCellWidth = dayCellWidth;
        this.dayCellHeight = dayCellHeight;

    }
}