IFormatProvider Interface

Provides a mechanism for retrieving an object to control formatting.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

[ComVisibleAttribute(true)]
public interface IFormatProvider

The IFormatProvider type exposes the following members.

  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetFormatReturns an object that provides formatting services for the specified type.
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The IFormatProvider interface supplies an object that provides formatting information for formatting and parsing operations. Formatting operations convert the value of a type to the string representation of that value. Typical formatting methods are the ToString methods of a type, as well as Format. Parsing operations convert the string representation of a value to a type with that value. Typical parsing methods are Parse and TryParse.

The IFormatProvider interface consists of a single method, IFormatProvider.GetFormat. GetFormat is a callback method: The parsing or formatting method calls it and passes it a Type object that represents the type of object that the formatting or parsing method expects will provide formatting information. The GetFormat method is responsible for returning an object of that type.

IFormatProvider implementations are often used implicitly by formatting and parsing methods. For example, the DateTime.ToString(String) method implicitly uses an IFormatProvider implementation that represents the system's current culture. IFormatProvider implementations can also be specified explicitly by methods that have a parameter of type IFormatProvider, such as Int32.Parse(String, IFormatProvider) and String.Format(IFormatProvider, String, Object[]).

The .NET Framework includes the following three predefined IFormatProvider implementations to provide culture-specific information that is used in formatting or parsing numeric and date and time values:

The .NET Framework also supports custom formatting. This typically involves the creation of a formatting class that implements both IFormatProvider and ICustomFormatter. An instance of this class is then passed as a parameter to a method that performs a custom formatting operation, such as String.Format(IFormatProvider, String, Object[]) The example provides an illustration of such a custom implementation that formats a number as a 12-digit account number.

The following example illustrates how an IFormatProvider implementation can change the representation of a date and time value. In this case, a single date is displayed by using CultureInfo objects that represent four different cultures.

using System;
using System.Globalization;

public class Example
{
   public static void Main()
   {
      DateTime dateValue = new DateTime(2009, 6, 1, 4, 37, 0);
      CultureInfo[] cultures = { new CultureInfo("en-US"), 
                                 new CultureInfo("fr-FR"),
                                 new CultureInfo("it-IT"),
                                 new CultureInfo("de-DE") };
      foreach (CultureInfo culture in cultures)
         Console.WriteLine("{0}: {1}", culture.Name, dateValue.ToString(culture));
   }
}
// The example displays the following output: 
//       en-US: 6/1/2009 4:37:00 PM 
//       fr-FR: 01/06/2009 16:37:00 
//       it-IT: 01/06/2009 16.37.00 
//       de-DE: 01.06.2009 16:37:00

The following example illustrates the use of a class that implements the IFormatProvider interface and the GetFormat method. The AcctNumberFormat class converts an Int64 value that represents an account number to a formatted 12-digit account number. Its GetFormat method returns a reference to the current AcctNumberFormat instance if the formatType parameter refers to a class that implements ICustomFormatter; otherwise, GetFormat returns null.

public class AcctNumberFormat : IFormatProvider, ICustomFormatter
{
   private const int ACCT_LENGTH = 12;

   public object GetFormat(Type formatType)
   {
      if (formatType == typeof(ICustomFormatter))
         return this;
      else 
         return null;
   }

   public string Format(string fmt, object arg, IFormatProvider formatProvider) 
   {
      // Provide default formatting if arg is not an Int64. 
      if (arg.GetType() != typeof(Int64))
         try {
            return HandleOtherFormats(fmt, arg); 
         }
         catch (FormatException e) {
            throw new FormatException(String.Format("The format of '{0}' is invalid.", fmt), e);
         }

      // Provide default formatting for unsupported format strings. 
      string ufmt = fmt.ToUpper(CultureInfo.InvariantCulture);
      if (! (ufmt == "H" || ufmt == "I")) 
         try {
            return HandleOtherFormats(fmt, arg);
         }
         catch (FormatException e) {
            throw new FormatException(String.Format("The format of '{0}' is invalid.", fmt), e);
         }

      // Convert argument to a string. 
      string result = arg.ToString();

      // If account number is less than 12 characters, pad with leading zeroes. 
      if (result.Length < ACCT_LENGTH)
         result = result.PadLeft(ACCT_LENGTH, '0');
      // If account number is more than 12 characters, truncate to 12 characters. 
      if (result.Length > ACCT_LENGTH)
         result = result.Substring(0, ACCT_LENGTH);   

      if (ufmt == "I")                    // Integer-only format.  
         return result;
      // Add hyphens for H format specifier. 
      else                                          // Hyphenated format. 
         return result.Substring(0, 5) + "-" + result.Substring(5, 3) + "-" + result.Substring(8);
   }

   private string HandleOtherFormats(string format, object arg)
   {
      if (arg is IFormattable) 
         return ((IFormattable)arg).ToString(format, CultureInfo.CurrentCulture);
      else if (arg != null)
         return arg.ToString();
      else 
         return String.Empty;
   }
}

The class that implements IFormatProvider can then be used in a call to a formatting and parsing operation. For example, the following code calls the String.Format(IFormatProvider, String, Object[]) method to generate a string that contains a formatted 12-digit account number.

using System;
using System.Globalization;

public enum DaysOfWeek { Monday=1, Tuesday=2 };

public class TestFormatting
{
   public static void Main()
   {
      long acctNumber;
      double balance; 
      DaysOfWeek wday; 
      string output;

      acctNumber = 104254567890;
      balance = 16.34;
      wday = DaysOfWeek.Monday;

      output = String.Format(new AcctNumberFormat(), 
                             "On {2}, the balance of account {0:H} was {1:C2}.", 
                             acctNumber, balance, wday);
      Console.WriteLine(output);

      wday = DaysOfWeek.Tuesday;
      output = String.Format(new AcctNumberFormat(), 
                             "On {2}, the balance of account {0:I} was {1:C2}.", 
                             acctNumber, balance, wday);
      Console.WriteLine(output);
   }
}
// The example displays the following output: 
//       On Monday, the balance of account 10425-456-7890 was $16.34. 
//       On Tuesday, the balance of account 104254567890 was $16.34.

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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