IFormattable Interface

Definition

public interface class IFormattable

public interface IFormattable

[System.Runtime.InteropServices.ComVisible(true)]
public interface IFormattable

type IFormattable = interface

[<System.Runtime.InteropServices.ComVisible(true)>]
type IFormattable = interface

Public Interface IFormattable

Derived

System.Byte

System.Char

System.DateOnly

System.DateTime

System.DateTimeOffset

More…

Attributes

ComVisibleAttribute

Examples

The following example defines a Temperature class that implements the IFormattable interface. The class supports four format specifiers: "G" and "C", which indicate that the temperature is to be displayed in Celsius; "F", which indicates that the temperature is to be displayed in Fahrenheit; and "K", which indicates that the temperature is to be displayed in Kelvin. In addition, the IFormattable.ToString implementation also can handle a format string that is null or empty. The other two ToString methods defined by the Temperature class simply wrap a call to the IFormattable.ToString implementation.

using System;
using System.Globalization;

public class Temperature : IFormattable
{
 private decimal temp;

 public Temperature(decimal temperature)
 {
 if (temperature < -273.15m)
 throw new ArgumentOutOfRangeException(String.Format("{0} is less than absolute zero.",
 temperature));
 this.temp = temperature;
 }

 public decimal Celsius
 {
 get { return temp; }
 }

 public decimal Fahrenheit
 {
 get { return temp * 9 / 5 + 32; }
 }

 public decimal Kelvin
 {
 get { return temp + 273.15m; }
 }

 public override string ToString()
 {
 return this.ToString("G", CultureInfo.CurrentCulture);
 }

 public string ToString(string format)
 {
 return this.ToString(format, CultureInfo.CurrentCulture);
 }

 public string ToString(string format, IFormatProvider provider)
 {
 if (String.IsNullOrEmpty(format)) format = "G";
 if (provider == null) provider = CultureInfo.CurrentCulture;

 switch (format.ToUpperInvariant())
 {
 case "G":
 case "C":
 return temp.ToString("F2", provider) + " °C";
 case "F":
 return Fahrenheit.ToString("F2", provider) + " °F";
 case "K":
 return Kelvin.ToString("F2", provider) + " K";
 default:
 throw new FormatException(String.Format("The {0} format string is not supported.", format));
 }
 }
}

open System
open System.Globalization

type Temperature(temperature: decimal) =
 do 
 if temperature < -273.15M then
 raise (ArgumentOutOfRangeException $"{temperature} is less than absolute zero.")

 member _.Celsius =
 temperature

 member _.Fahrenheit =
 temperature * 9M / 5M + 32M

 member _.Kelvin =
 temperature + 273.15m

 override this.ToString() =
 this.ToString("G", CultureInfo.CurrentCulture)

 member this.ToString(format) =
 this.ToString(format, CultureInfo.CurrentCulture)

 member this.ToString(format, provider: IFormatProvider) =
 let format =
 if String.IsNullOrEmpty format then "G"
 else format

 let provider =
 if isNull provider then 
 CultureInfo.CurrentCulture :> IFormatProvider
 else provider

 match format.ToUpperInvariant() with
 | "G" | "C" ->
 temperature.ToString("F2", provider) + " °C"
 | "F" ->
 this.Fahrenheit.ToString("F2", provider) + " °F"
 | "K" ->
 this.Kelvin.ToString("F2", provider) + " K"
 | _ ->
 raise (FormatException $"The {format} format string is not supported.")

 interface IFormattable with
 member this.ToString(format, provider) = this.ToString(format, provider)

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

The following example then calls the IFormattable.ToString implementation either directly or by using a composite format string.

public class Example
{
 public static void Main()
 {
 // Use composite formatting with format string in the format item.
 Temperature temp1 = new Temperature(0);
 Console.WriteLine("{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)\n", temp1);

 // Use composite formatting with a format provider.
 temp1 = new Temperature(-40);
 Console.WriteLine(String.Format(CultureInfo.CurrentCulture, "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)", temp1));
 Console.WriteLine(String.Format(new CultureInfo("fr-FR"), "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)\n", temp1));

 // Call ToString method with format string.
 temp1 = new Temperature(32);
 Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)\n",
 temp1.ToString("C"), temp1.ToString("K"), temp1.ToString("F"));

 // Call ToString with format string and format provider
 temp1 = new Temperature(100) ;
 NumberFormatInfo current = NumberFormatInfo.CurrentInfo;
 CultureInfo nl = new CultureInfo("nl-NL");
 Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)",
 temp1.ToString("C", current), temp1.ToString("K", current), temp1.ToString("F", current));
 Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)",
 temp1.ToString("C", nl), temp1.ToString("K", nl), temp1.ToString("F", nl));
 }
}
// The example displays the following output:
// 0.00 °C (Celsius) = 273.15 K (Kelvin) = 32.00 °F (Fahrenheit)
//
// -40.00 °C (Celsius) = 233.15 K (Kelvin) = -40.00 °F (Fahrenheit)
// -40,00 °C (Celsius) = 233,15 K (Kelvin) = -40,00 °F (Fahrenheit)
//
// 32.00 °C (Celsius) = 305.15 K (Kelvin) = 89.60 °F (Fahrenheit)
//
// 100.00 °C (Celsius) = 373.15 K (Kelvin) = 212.00 °F (Fahrenheit)
// 100,00 °C (Celsius) = 373,15 K (Kelvin) = 212,00 °F (Fahrenheit)

open System
open System.Globalization

[<EntryPoint>]
let main _ =
 // Use composite formatting with format string in the format item.
 let temp1 = Temperature 0
 Console.WriteLine("{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)\n", temp1)

 // Use composite formatting with a format provider.
 let temp1 = Temperature -40
 String.Format(CultureInfo.CurrentCulture, "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)", temp1)
 |> printfn "%s"
 String.Format(CultureInfo "fr-FR", "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)\n", temp1)
 |> printfn "%s"

 // Call ToString method with format string.
 let temp1 = Temperature 32
 Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)\n",
 temp1.ToString "C", temp1.ToString "K", temp1.ToString "F")

 // Call ToString with format string and format provider
 let temp1 = Temperature 100 
 let current = NumberFormatInfo.CurrentInfo
 let nl = CultureInfo "nl-NL"
 Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)",
 temp1.ToString("C", current), temp1.ToString("K", current), temp1.ToString("F", current))
 Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)",
 temp1.ToString("C", nl), temp1.ToString("K", nl), temp1.ToString("F", nl))
 0

// The example displays the following output:
// 0.00 °C (Celsius) = 273.15 K (Kelvin) = 32.00 °F (Fahrenheit)
//
// -40.00 °C (Celsius) = 233.15 K (Kelvin) = -40.00 °F (Fahrenheit)
// -40,00 °C (Celsius) = 233,15 K (Kelvin) = -40,00 °F (Fahrenheit)
//
// 32.00 °C (Celsius) = 305.15 K (Kelvin) = 89.60 °F (Fahrenheit)
//
// 100.00 °C (Celsius) = 373.15 K (Kelvin) = 212.00 °F (Fahrenheit)
// 100,00 °C (Celsius) = 373,15 K (Kelvin) = 212,00 °F (Fahrenheit)

Module Example
 Public Sub Main()
 ' Use composite formatting with format string in the format item.
 Dim temp1 As New Temperature(0)
 Console.WriteLine("{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)", temp1)
 Console.WriteLine()
 
 ' Use composite formatting with a format provider.
 temp1 = New Temperature(-40)
 Console.WriteLine(String.Format(CultureInfo.CurrentCulture, "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)", temp1))
 Console.WriteLine(String.Format(New CultureInfo("fr-FR"), "{0:C} (Celsius) = {0:K} (Kelvin) = {0:F} (Fahrenheit)", temp1))
 Console.WriteLine()
 
 ' Call ToString method with format string.
 temp1 = New Temperature(32)
 Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)", _
 temp1.ToString("C"), temp1.ToString("K"), temp1.ToString("F"))
 Console.WriteLine()

 ' Call ToString with format string and format provider
 temp1 = New Temperature(100) 
 Dim current As NumberFormatInfo = NumberFormatInfo.CurrentInfo
 Dim nl As New CultureInfo("nl-NL") 
 Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)", _
 temp1.ToString("C", current), temp1.ToString("K", current), temp1.ToString("F", current))
 Console.WriteLine("{0} (Celsius) = {1} (Kelvin) = {2} (Fahrenheit)", _
 temp1.ToString("C", nl), temp1.ToString("K", nl), temp1.ToString("F", nl))
 End Sub
End Module
' The example displays the following output:
' 0.00 °C (Celsius) = 273.15 K (Kelvin) = 32.00 °F (Fahrenheit)
' 
' -40.00 °C (Celsius) = 233.15 K (Kelvin) = -40.00 °F (Fahrenheit)
' -40,00 °C (Celsius) = 233,15 K (Kelvin) = -40,00 °F (Fahrenheit)
' 
' 32.00 °C (Celsius) = 305.15 K (Kelvin) = 89.60 °F (Fahrenheit)
' 
' 100.00 °C (Celsius) = 373.15 K (Kelvin) = 212.00 °F (Fahrenheit)
' 100,00 °C (Celsius) = 373,15 K (Kelvin) = 212,00 °F (Fahrenheit)

Remarks

The IFormattable interface converts an object to its string representation based on a format string and a format provider.

A format string typically defines the general appearance of an object. For example, the .NET Framework supports the following:

• Standard format strings for formatting enumeration values (see Enumeration Format Strings).

• Standard and custom format strings for formatting numeric values (see Standard Numeric Format Strings and Custom Numeric Format Strings).

• Standard and custom format strings for formatting date and time values (see Standard Date and Time Format Strings and Custom Date and Time Format Strings).

• Standard and custom format strings for formatting time intervals (see Standard TimeSpan Format Strings and Custom TimeSpan Format Strings).

You can also define your own format strings to support formatting of your application-defined types.

A format provider returns a formatting object that typically defines the symbols used in converting an object to its string representation. For example, when you convert a number to a currency value, a format provider defines the currency symbol that appears in the result string. The .NET Framework defines three format providers:

• The System.Globalization.CultureInfo class, which returns either a NumberFormatInfo object for formatting numeric values, or a DateTimeFormatInfo object for formatting date and time values.

• The System.Globalization.NumberFormatInfo class, which returns an instance of itself for formatting numeric values.

• The System.Globalization.DateTimeFormatInfo class, which returns an instance of itself for formatting date and time values.

In addition, you can define your own custom format providers to supply culture-specific, profession-specific, or industry-specific information used in formatting. For more information about implementing custom formatting by using a custom format provider, see ICustomFormatter.

The IFormattable interface defines a single method, ToString, that supplies formatting services for the implementing type. The ToString method can be called directly. In addition, it is 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 methods include Console.WriteLine(String, Object), String.Format, and StringBuilder.AppendFormat(String, Object), among others. The ToString method is called for each format item in the method's format string.

The IFormattable interface is implemented by the base data types.

Notes to Implementers

Classes that require more control over the formatting of strings than ToString() provides should implement IFormattable.

A class that implements IFormattable 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 null. For more information about formatting and formatting codes, see Formatting Types

See also

• IFormatProvider
• ToString()
• CurrentCulture
• Formatting Types in .NET