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How to: Define and Use Custom Numeric Format Providers

The .NET Framework gives you extensive control over the string representation of numeric values. It supports the following features for customizing the format of numeric values:

  • Standard numeric format strings, which provide a predefined set of formats for converting numbers to their string representation. You can use them with any numeric formatting method, such as Decimal.ToString(String), that has a format parameter. For details, see Standard Numeric Format Strings.

  • Custom numeric format strings, which provide a set of symbols that can be combined to define custom numeric format specifiers. They can also be used with any numeric formatting method, such as Decimal.ToString(String), that has a format parameter. For details, see Custom Numeric Format Strings.

  • Custom CultureInfo or NumberFormatInfo objects, which define the symbols and format patterns used in displaying the string representations of numeric values. You can use them with any numeric formatting method, such as ToString, that has a provider parameter. Typically, the provider parameter is used to specify culture-specific formatting.

In some cases (such as when an application must display a formatted account number, an identification number, or a postal code) these three techniques are inappropriate. The .NET Framework also enables you to define a formatting object that is neither a CultureInfo nor a NumberFormatInfo object to determine how a numeric value is formatted. This topic provides the step-by-step instructions for implementing such an object, and provides an example that formats telephone numbers.

To define a custom format provider

  1. Define a class that implements the IFormatProvider and ICustomFormatter interfaces.

  2. Implement the IFormatProvider.GetFormat method. GetFormat is a callback method that the formatting method (such as the String.Format(IFormatProvider, String, Object[]) method) invokes to retrieve the object that is actually responsible for performing custom formatting. A typical implementation of GetFormat does the following:

    1. Determines whether the Type object passed as a method parameter represents an ICustomFormatter interface.

    2. If the parameter does represent the ICustomFormatter interface, GetFormat returns an object that implements the ICustomFormatter interface that is responsible for providing custom formatting. Typically, the custom formatting object returns itself.

    3. If the parameter does not represent the ICustomFormatter interface, GetFormat returns null.

  3. Implement the Format method. This method is called by the String.Format(IFormatProvider, String, Object[]) method and is responsible for returning the string representation of a number. Implementing the method typically involves the following:

    1. Optionally, make sure that the method is legitimately intended to provide formatting services by examining the provider parameter. For formatting objects that implement both IFormatProvider and ICustomFormatter, this involves testing the provider parameter for equality with the current formatting object.

    2. Determine whether the formatting object should support custom format specifiers. (For example, an "N" format specifier might indicate that a U.S. telephone number should be output in NANP format, and an "I" might indicate output in ITU-T Recommendation E.123 format.) If format specifiers are used, the method should handle the specific format specifier. It is passed to the method in the format parameter. If no specifier is present, the value of the format parameter is String.Empty.

    3. Retrieve the numeric value passed to the method as the arg parameter. Perform whatever manipulations are required to convert it to its string representation.

    4. Return the string representation of the arg parameter.

To use a custom numeric formatting object

  1. Create a new instance of the custom formatting class.

  2. Call the String.Format(IFormatProvider, String, Object[]) formatting method, passing it the custom formatting object, the formatting specifier (or String.Empty, if one is not used), and the numeric value to be formatted.

The following example defines a custom numeric format provider named TelephoneFormatter that converts a number that represents a U.S. telephone number to its NANP or E.123 format. The method handles two format specifiers, "N" (which outputs the NANP format) and "I" (which outputs the international E.123 format).

using System;
using System.Globalization;

public class TelephoneFormatter : IFormatProvider, ICustomFormatter
{
   public object GetFormat(Type formatType)
   {
      if (formatType == typeof(ICustomFormatter))
         return this;
      else 
         return null;
   }               

   public string Format(string format, object arg, IFormatProvider formatProvider)
   {
      // Check whether this is an appropriate callback              
      if (! this.Equals(formatProvider))
         return null; 

      // Set default format specifier              
      if (string.IsNullOrEmpty(format)) 
         format = "N";

      string numericString = arg.ToString();

      if (format == "N")
      {
         if (numericString.Length <= 4)
            return numericString;
         else if (numericString.Length == 7)
            return numericString.Substring(0, 3) + "-" + numericString.Substring(3, 4); 
         else if (numericString.Length == 10)
               return "(" + numericString.Substring(0, 3) + ") " +
                      numericString.Substring(3, 3) + "-" + numericString.Substring(6);   
         else 
            throw new FormatException( 
                      string.Format("'{0}' cannot be used to format {1}.", 
                                    format, arg.ToString()));
      }
      else if (format == "I")
      {
         if (numericString.Length < 10)
            throw new FormatException(string.Format("{0} does not have 10 digits.", arg.ToString()));
         else
            numericString = "+1 " + numericString.Substring(0, 3) + " " + numericString.Substring(3, 3) + " " + numericString.Substring(6);
      }
      else
      {
         throw new FormatException(string.Format("The {0} format specifier is invalid.", format));
      } 
      return numericString;  
   }
}

public class TestTelephoneFormatter
{
   public static void Main()
   {
      Console.WriteLine(String.Format(new TelephoneFormatter(), "{0}", 0));
      Console.WriteLine(String.Format(new TelephoneFormatter(), "{0}", 911));
      Console.WriteLine(String.Format(new TelephoneFormatter(), "{0}", 8490216));
      Console.WriteLine(String.Format(new TelephoneFormatter(), "{0}", 4257884748));

      Console.WriteLine(String.Format(new TelephoneFormatter(), "{0:N}", 0));
      Console.WriteLine(String.Format(new TelephoneFormatter(), "{0:N}", 911));
      Console.WriteLine(String.Format(new TelephoneFormatter(), "{0:N}", 8490216));
      Console.WriteLine(String.Format(new TelephoneFormatter(), "{0:N}", 4257884748));

      Console.WriteLine(String.Format(new TelephoneFormatter(), "{0:I}", 4257884748));
   }
}

The custom numeric format provider can be used only with the String.Format(IFormatProvider, String, Object[]) method. The other overloads of numeric formatting methods (such as ToString) that have a parameter of type IFormatProvider all pass the IFormatProvider.GetFormat implementation a Type object that represents the NumberFormatInfo type. In return, they expect the method to return a NumberFormatInfo object. If it does not, the custom numeric format provider is ignored, and the NumberFormatInfo object for the current culture is used in its place. In the example, the TelephoneFormatter.GetFormat method handles the possibility that it may be inappropriately passed to a numeric formatting method by examining the method parameter and returning null if it represents a type other than ICustomFormatter.

If a custom numeric format provider supports a set of format specifiers, make sure you provide a default behavior if no format specifier is supplied in the format item used in the String.Format(IFormatProvider, String, Object[]) method call. In the example, "N" is the default format specifier. This allows for a number to be converted to a formatted telephone number by providing an explicit format specifier. The following example illustrates such a method call.

Console.WriteLine(String.Format(new TelephoneFormatter(), "{0:N}", 4257884748));

But it also allows the conversion to occur if no format specifier is present. The following example illustrates such a method call.

Console.WriteLine(String.Format(new TelephoneFormatter(), "{0}", 4257884748));

If no default format specifier is defined, your implementation of the ICustomFormatter.Format method should include code such as the following so that the .NET Framework can provide formatting that your code does not support.

if (arg is IFormattable) 
   s = ((IFormattable)arg).ToString(format, formatProvider);
else if (arg != null)    
   s = arg.ToString();

In the case of this example, the method that implements ICustomFormatter.Format is intended to serve as a callback method for the String.Format(IFormatProvider, String, Object[]) method. Therefore, it examines the formatProvider parameter to determine whether it contains a reference to the current TelephoneFormatter object. However, the method can also be called directly from code. In that case, you can use the formatProvider parameter to provide a CultureInfo or NumberFormatInfo object that supplies culture-specific formatting information.

Compile the code at the command line using csc.exe or vb.exe. To compile the code in Visual Studio, put it in a console application project template.

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