一个实现 IFormattable 接口的例子

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;
}
}