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

using System;

namespace Spectre.Console.Rendering
{
    /// <summary>
    /// Represents a double border.
    /// </summary>
    public sealed class DoubleBorder : Border
    {
        /// <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."),
            };
        }
    }
}