.NET Framework Standard Library Annotated Reference, Volume 1: Base Class Library and Extended Numerics Library

Example

using System;
using System.Globalization;

namespace Samples
{
  public class MyFormatProviderSample: IFormatProvider,
                                       ICustomFormatter
  {
    public object GetFormat(Type t)
    {
      if(t == typeof(ICustomFormatter))
          return this;
      return null;
    }
    public string Format(string f, object o,
                         IFormatProvider p)
    {
      if(f == null)
        return String.Format("{0}", o);
      if(!f.StartsWith("-"))
      {
        if(o is IFormattable)
        {
          return ((IFormattable)o).ToString(f, p);
        }
        else if(o != null)
        {
          return o.ToString();
        }
      }
      return "Sample Formatted";
    }
    public static void Main()
    {
      MyFormatProviderSample m =
                            new MyFormatProviderSample();
      Console.WriteLine(String.Format(
                        m,
                        "First (no formatting): {0} \n" +
                        "Second (formatted '-'): {1:-}",
                        new object[] {m, m}));
    }
  }
}

The output is

 First ...