C#: Connection between IFormattable, IFormatProvider and ICustomFormatter, and when to use what

Question

What are the difference and connection between IFormattable, IFormatProvider and ICustomFormatter and when would they be used? A simple implementation example would be very nice too.

And I don't really mean when it is used in the .net framework, but when I would implement these myself and in that case what classes would typically implement what interface and how to do it properly.

Answer 1

IFormattable is an object that supports different (named/custom) formats - for example, numbers, etc. By using an interface, multiple blocks of code can use the value and a format string, and this is common (for example) in data-binding and string.Format.

An IFormatProvider fills in some gaps dealing with formatting - particularly i18n. Most commonly, a CultureInfo is used as the provider, either giving a specific local format, or the invariant culture.

As far as I know, ICustomFormatter is unrelated, and ties more into serialization (BinaryFormatter). I could be wrong...

An example of an IFormattable object:

IFormattable d = 123.45M;
string s1 = d.ToString("c", CultureInfo.CurrentCulture), // local currency
       s2 = d.ToString("c", CultureInfo.InvariantCulture); // invariant currency

Answer 2

• IFormattable is an object which supports formats in string.Format, i.e. the xxx in {0:xxx}. string.Format will delegate to an object's IFormattable.ToString method if the object supports the interface.

• IFormatProvider is a source of configuration information that formatters use for things like culture-specific date and currency layout.

• However, for situations like e.g. DateTime, where the instance you want to format already implements IFormattable yet you don't control the implementation (DateTime is supplied in the BCL, you can't replace it easily), there is a mechanism to prevent string.Format from simply using IFormattable.ToString. Instead, you implement IFormatProvider, and when asked for an ICustomFormatter implementation, return one. string.Format checks the provider for an ICustomFormatter before it delegates to the object's IFormattable.Format, which would in turn likely ask the IFormatProvider for culture-specific data like CultureInfo.

Here is a program which shows what string.Format asks the IFormatProvider for, and how the flow of control goes:

using System;
using System.Globalization;

class MyCustomObject : IFormattable
{
    public string ToString(string format, IFormatProvider provider)
    {
        Console.WriteLine("ToString(\"{0}\", provider) called", format);
        return "arbitrary value";
    }
}

class MyFormatProvider : IFormatProvider
{
    public object GetFormat(Type formatType)
    {
        Console.WriteLine("Asked for {0}", formatType);
        return CultureInfo.CurrentCulture.GetFormat(formatType);
    }
}

class App
{
    static void Main()
    {
        Console.WriteLine(
            string.Format(new MyFormatProvider(), "{0:foobar}", 
                new MyCustomObject()));
    }
}

It prints this:

Asked for System.ICustomFormatter
ToString("foobar", provider) called
arbitrary value

If the format provider is changed to return a custom formatter, it takes over:

class MyFormatProvider : IFormatProvider
{
    public object GetFormat(Type formatType)
    {
        Console.WriteLine("Asked for {0}", formatType);
        if (formatType == typeof(ICustomFormatter))
            return new MyCustomFormatter();
        return CultureInfo.CurrentCulture.GetFormat(formatType);
    }
}

class MyCustomFormatter : ICustomFormatter
{
    public string Format(string format, object arg, IFormatProvider provider)
    {
        return string.Format("(format was \"{0}\")", format);
    }
}

When run:

Asked for System.ICustomFormatter
(format was "foobar")