spectre.console/src/Spectre.Console/Rendering/Borders/Boxes/SquareBoxBorder.cs

using System;

namespace Spectre.Console.Rendering
{
    /// <summary>
    /// Represents a square border.
    /// </summary>
    public sealed class SquareBoxBorder : BoxBorder
    {
        /// <inheritdoc/>
        protected override string GetBorderPart(BoxBorderPart part)
        {
            return part switch
            {
                BoxBorderPart.TopLeft => "┌",
                BoxBorderPart.Top => "─",
                BoxBorderPart.TopRight => "┐",
                BoxBorderPart.Left => "│",
                BoxBorderPart.Right => "│",
                BoxBorderPart.BottomLeft => "└",
                BoxBorderPart.Bottom => "─",
                BoxBorderPart.BottomRight => "┘",
                _ => throw new InvalidOperationException("Unknown border part."),
            };
        }
    }
}