DataGridView でセル毎に色分けして表示

CustomDataGridViewTextBoxColumn & CustomDataGridViewTextBoxCell

要点は CustomDataGridViewTextBoxCell の Paint() の オーバーライドです

CustomDataGridViewTextBoxColumn.cs

using System.Drawing.Drawing2D;

namespace System.Windows.Forms
{
    public class CustomDataGridViewTextBoxColumn : DataGridViewColumn
    {
        public CustomDataGridViewTextBoxColumn() : base(new CustomDataGridViewTextBoxCell())
        {
        }

        public override DataGridViewCell CellTemplate
        {
            get
            {
                return base.CellTemplate;
            }
            set
            {
                if (!(value is CustomDataGridViewTextBoxCell))
                {
                    throw new InvalidCastException("CustomDataGridViewTextBoxCellオブジェクトを指定してください。");
                }
                base.CellTemplate = value;
            }
        }
    }

    public class CustomDataGridViewTextBoxCell : DataGridViewTextBoxCell
    {
        public static DateTime dt;

        static DateTime GetNowDate()
        {
            long ticks = DateTime.Now.Ticks;

            return new DateTime(ticks - (ticks % TimeSpan.TicksPerDay));
        }

        static CustomDataGridViewTextBoxCell()
        {
            dt = GetNowDate();
        }

        public static void UpdateNow()
        {
            dt = GetNowDate();
        }

        public CustomDataGridViewTextBoxCell()
        {
        }

        public static HatchBrush brush1 = new HatchBrush(HatchStyle.Percent50, Color.DarkGray, Color.White);

        protected override void Paint(Graphics graphics,
            Rectangle clipBounds,
            Rectangle cellBounds,
            int rowIndex,
            DataGridViewElementStates cellState,
            object value,
            object formattedValue,
            string errorText,
            DataGridViewCellStyle cellStyle,
            DataGridViewAdvancedBorderStyle advancedBorderStyle,
            DataGridViewPaintParts paintParts)
        {
            Date? date = value as Date;

            if (date != null)
            {
                long days = (date.dt.Ticks - CustomDataGridViewTextBoxCell.dt.Ticks) / TimeSpan.TicksPerDay;

                if (days < 0)
                {
                    if (days >= -30)
                    {
                        cellStyle.ForeColor = Color.Red;
                    }
                    else
                    {
                        cellStyle.BackColor = Color.LightPink;
                    }
                }
                else
                {
                    if (days <= 30)
                    {
                        cellStyle.ForeColor = Color.Blue;
                    }
                    else
                    {
                        cellStyle.BackColor = Color.LightCyan;
                    }
                }
            }
            else
            {
                graphics.FillRectangle(brush1, cellBounds);

                paintParts &= ~DataGridViewPaintParts.Background;
            }

            base.Paint(graphics, clipBounds, cellBounds, rowIndex, cellState, value, formattedValue, errorText, cellStyle, advancedBorderStyle, paintParts);
        }
    }

    public class Date : IComparable<Date>
    {
        public DateTime dt;

        public string? text;

        public Date(DateTime dt)
        {
            this.dt = dt;
            this.text = null;
        }

        public Date(long ticks) : this(new DateTime(TrimDay(ticks))) { }

        public Date(int year, int month, int day)
        {
            Debug.Assert(year > 0 && month >= 1 && month <= 12 && day >= 1 && day <= 31);

            this.dt = new DateTime(year, month, day);

            this.text = null;
        }

        public Date(Date date) : this(date.dt, date.text) { }

        public Date(string date)
        {
            this.dt = DateTime.Parse(date);

            this.text = null;
        }

        protected Date(DateTime dt, string? text)
        {
            this.dt = dt;
            this.text = text;
        }

        public static long TrimDay(long tick)
        {
            return tick - (tick % TimeSpan.TicksPerDay);
        }

        static readonly string[] day_of_weeks = { "日", "月", "火", "水", "木", "金", "土" };

        protected string GetText()
        {
            return dt.ToString("yyyy/MM/dd") + "(" + day_of_weeks[(int)dt.DayOfWeek] + ")";
        }

        public void SetText()
        {
            text = GetText();
        }

        public override string ToString()
        {
            if (string.IsNullOrEmpty(text))
            {
                text = GetText();
            }

            return text;
        }

        public int CompareTo(Date? other)
        {
            if (other == null)
            {
                return 1;
            }

            return Comparer<DateTime>.Default.Compare(dt, other.dt);
        }
    }
}