Writing unit tests
In this section, we will look at how you can write unit tests for your C# code. To do so, we will consider the following implementation of a rectangle:
public struct Rectangle
{
    public readonly int Left;
    public readonly int Top;
    public readonly int Right;
    public readonly int Bottom;
    public int Width => Right - Left;
    public int Height => Bottom - Top;
    public int Area => Width * Height;
    public Rectangle(int left, int top, int right, int bottom)
    {
        Left = left;
        Top = top;
        Right = right;
        Bottom = bottom;
    }
    public...
