The IFormattable.ToString(string, IFormatProvider) method converts a value to a string representation that can be expressed in multiple ways. Its precise format depends on specific symbols or a specified order defined by specific cultures, professions, or industries. You can call the method directly. It is also called automatically by the Convert.ToString(object) and Convert.ToString(object, IFormatProvider) methods, and by methods that use the composite formatting feature in the .NET Framework, such as string.Format(string, Object[]), Console.WriteLine(string, Object[]), and System.Text.StringBuilder.AppendFormat(string, Object[]). (For more information, see Composite Formatting.)

Composite formatting methods call the IFormattable.ToString(string, IFormatProvider) method once for each format item in a format string. The parameters passed to the method depend on the specific formatting method that is called and on the content of the format item, as follows:

• If the format item does not include a format string (for example, if the format item is simply {0}), it is passed null as the value of the string parameter.

• If the format item includes a format string (for example, {0:G}), that format string is passed as the value of the string parameter.

• If the original method call does not include an IFormatProvider parameter, System.Globalization.CultureInfo.CurrentCulture is passed as the value of the IFormatProvider parameter.

• If the original method call includes an IFormatProvider parameter, the provider that is supplied in the method call is passed as the value of the IFormatProvider parameter.

The .NET Framework includes three format providers, all of which implement the IFormatProvider interface:

• System.Globalization.NumberFormatInfo supplies numeric formatting information, such as the characters to use for decimal and group separators, and the spelling and placement of currency symbols in monetary values.

• System.Globalization.DateTimeFormatInfo supplies date-related and time-related formatting information, such as the position of the month, the day, and the year in a date pattern.

• System.Globalization.CultureInfo contains the default formatting information in a specific culture, including the numeric format information, and date-related and time-related formatting information.

In addition, you can define your own custom format provider.

The following example demonstrates using the IFormattable.ToString(string, IFormatProvider) method to display values in a variety of formats. The current system culture is U.S. English, which provides the default values for the formatProvider parameter of IFormattable.ToString(string, IFormatProvider).

using System;
class FormattableExample {
    public static void Main() {
    double d = 123.12345678901234;
    string[] formats = {"C","E","e","F","G","N","P","R"};
    for (int i = 0; i< formats.Length;i++) 
        Console.WriteLine("{0:R} as {1}:  {2}",d,formats[i],d.ToString(formats[i],null));

    string[]intFormats = {"D","x","X"};
    int val = 255;
    for (int i = 0; i< intFormats.Length;i++) 
        Console.WriteLine("{0} as {1}:  {2}",val,intFormats[i],val.ToString(intFormats[i],null));

    }
}

The output is