一个实现 IFormattable 接口的例子

摘自 《Professional C# 4and .NET 4》

    class Program
    {
        static void Main()
        {
            Vector v1 = new Vector(1, 32, 5);
            Vector v2 = new Vector(845.4, 54.3, -7.8);
            Console.WriteLine("\nIn IJK format,\nv1 is {0,30:IJK}\nv2 is {1,30:IJK}", v1, v2);
            Console.WriteLine("\nIn default format,\nv1 is {0,30}\nv2 is {1,30}", v1, v2);
            Console.WriteLine("\nIn VE format\nv1 is {0,30:VE}\nv2 is {1,30:VE}", v1, v2);
            Console.WriteLine("\nNorms are:\nv1 is {0,20:N}\nv2 is {1,20:N}", v1, v2);
            Console.ReadKey();
        }
    }

    struct Vector : IFormattable
    {
        public double x, y, z;

        public Vector(double x, double y, double z)
        {
            this.x = x;
            this.y = y;
            this.z = z;
        }

        public string ToString(string format, IFormatProvider formatProvider)
        {
            if (format == null)
                return ToString();
            string formatUpper = format.ToUpper();
            switch (formatUpper)
            {
                case "N":
                    return "|| " + Norm() + " ||";
                case "VE":
                    return String.Format("( {0:E}, {1:E}, {2:E} )", x, y, z);
                case "IJK":
                    StringBuilder sb = new StringBuilder(x.ToString(), 30);
                    sb.Append(" i + ");
                    sb.Append(y.ToString());
                    sb.Append(" j + ");
                    sb.Append(z.ToString());
                    sb.Append(" k");
                    return sb.ToString();
                default:
                    return ToString();
            }
        }

        public double Norm()
        {
            return x * x + y * y + z * z;
        }
    }