IFormattable.ToString Method

The ToString 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 StringBuilder.AppendFormat(String, Object()). (For more information, see Composite Formatting.)

Composite formatting methods call the ToString 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 Nothing as the value of the System.String parameter.

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

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

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

Note
An object's ToString implementation is called by composite formatting methods only if they are not passed an ICustomFormatter format provider, or if the Format method of the custom format provider returns Nothing.

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

• 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.

• 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.

• 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 ToString method must support the "G" (general) format specifier. Besides the "G" specifier, the class can define the list of format specifiers that it supports. In addition, the class must be prepared to handle a format specifier that is Nothing. For more information about formatting and formatting codes, see Formatting Types.

The following example demonstrates a Temperature class that implements the ToString method. This code example is part of a larger example provided for the IFormattable class.

Imports System.Globalization

Public Class Temperature : Implements IFormattable
   Private temp As Decimal 

   Public Sub New(temperature As Decimal)
      If temperature < -273.15 Then _ 
        Throw New ArgumentOutOfRangeException(String.Format("{0} is less than absolute zero.", _
                                              temperature))
      Me.temp = temperature
   End Sub 

   Public ReadOnly Property Celsius As Decimal 
      Get 
         Return temp
      End Get 
   End Property 

   Public ReadOnly Property Fahrenheit As Decimal 
      Get 
         Return temp * 9 / 5 + 32
      End Get 
   End Property 

   Public ReadOnly Property Kelvin As Decimal 
      Get 
         Return temp + 273.15d
      End Get 
   End Property 

   Public Overrides Function ToString() As String 
      Return Me.ToString("G", CultureInfo.CurrentCulture)
   End Function 

   Public Overloads Function ToString(fmt As String) As String 
      Return Me.ToString(fmt, CultureInfo.CurrentCulture)
   End Function 

   Public Overloads Function ToString(fmt As String, provider As IFormatProvider) _
                   As String _
                   Implements IFormattable.ToString
      If String.IsNullOrEmpty(fmt) Then fmt = "G" 
      If provider Is Nothing Then provider = CultureInfo.CurrentCulture

      Select Case fmt.ToUpperInvariant()
         Case "G", "C" 
            Return temp.ToString("F2", provider) + " °C"  
         Case "F" 
            Return Fahrenheit.ToString("F2", provider) + " °F" 
         Case "K" 
            Return Kelvin.ToString("F2", provider) + " K" 
         Case Else 
            Throw New FormatException(String.Format("The {0} format string is not supported.", fmt))
      End Select 
   End Function 
End Class

.NET Framework

.NET Framework Client Profile

Portable Class Library

.NET for Windows Store apps

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.

Reference