spectre.console/src/Spectre.Console/Rendering/Borders/RoundedBorder.cs

using System;

namespace Spectre.Console.Rendering
{
    /// <summary>
    /// Represents a rounded border.
    /// </summary>
    public sealed class RoundedBorder : Border
    {
        /// <inheritdoc/>
        public override Border? SafeBorder { get; } = Border.Square;

        /// <inheritdoc/>
        protected override string GetBoxPart(BorderPart part)
        {
            return part switch
            {
                BorderPart.HeaderTopLeft => "╭",
                BorderPart.HeaderTop => "─",
                BorderPart.HeaderTopSeparator => "┬",
                BorderPart.HeaderTopRight => "╮",
                BorderPart.HeaderLeft => "│",
                BorderPart.HeaderSeparator => "│",
                BorderPart.HeaderRight => "│",
                BorderPart.HeaderBottomLeft => "├",
                BorderPart.HeaderBottom => "─",
                BorderPart.HeaderBottomSeparator => "┼",
                BorderPart.HeaderBottomRight => "┤",
                BorderPart.CellLeft => "│",
                BorderPart.CellSeparator => "│",
                BorderPart.CellRight => "│",
                BorderPart.FooterBottomLeft => "╰",
                BorderPart.FooterBottom => "─",
                BorderPart.FooterBottomSeparator => "┴",
                BorderPart.FooterBottomRight => "╯",
                _ => throw new InvalidOperationException("Unknown box part."),
            };
        }
    }
}