This documentation is archived and is not being maintained.

StringBuilder.AppendFormat Method (IFormatProvider, String, Object())

Appends the string returned by processing a composite format string, which contains zero or more format items, to this instance. Each format item is replaced by the string representation of a corresponding argument in a parameter array using a specified format provider.

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

Public Function AppendFormat ( _
	provider As IFormatProvider, _
	format As String, _
	ParamArray args As Object() _
) As StringBuilder


Type: System.IFormatProvider
An object that supplies culture-specific formatting information.
Type: System.String
A composite format string (see Remarks).
Type: System.Object()
An array of objects to format.

Return Value

Type: System.Text.StringBuilder
A reference to this instance after the append operation has completed. After the append operation, this instance contains any data that existed before the operation, suffixed by a copy of format where any format specification is replaced by the string representation of the corresponding object argument.


format is Nothing.


format is invalid.


The index of a format item is less than 0 (zero), or greater than or equal to the length of the args array.


The length of the expanded string would exceed MaxCapacity.

This method uses the composite formatting feature of the .NET Framework to convert the value of an object to its text representation and embed that representation in the current StringBuilder object.

The format parameter consists of zero or more runs of text intermixed with zero or more indexed placeholders, called format items, that correspond to objects in the parameter list of this method. The formatting process replaces each format item with the string representation of the corresponding object.

The syntax of a format item is as follows:


Elements in square brackets are optional. The following table describes each element.




The zero-based position in the parameter list of the object to be formatted. If the object specified by index is Nothing, the format item is replaced by String.Empty. If there is no parameter in the index position, a FormatException is thrown.


The minimum number of characters in the string representation of the parameter. If positive, the parameter is right-aligned; if negative, it is left-aligned.


A standard or custom format string that is supported by the parameter.


For the standard and custom format strings used with date and time values, see Standard Date and Time Format Strings and Custom Date and Time Format Strings. For the standard and custom format strings used with numeric values, see Standard Numeric Format Strings and Custom Numeric Format Strings. For the standard format strings used with enumerations, see Enumeration Format Strings.

The provider parameter specifies an IFormatProvider implementation that can provide formatting information for the objects in args. provider can be any of the following:

  • A CultureInfo object that provides culture-specific formatting information.

  • A NumberFormatInfo object that provides culture-specific formatting information for numeric values in args.

  • A DateTimeFormatInfo object that provides culture-specific formatting information for date and time values in args.

  • A custom IFormatProvider implementation that provides formatting information for one or more of the objects in args. Typically, such an implementation also implements the ICustomFormatter interface. The second example in the next section illustrates an StringBuilder.AppendFormat(IFormatProvider, String, Object()) method call with a custom IFormatProvider implementation.

If the provider parameter is Nothing, format provider information is obtained from the current culture.

args represents the objects to be formatted. Each format item in format is replaced with the string representation of the corresponding object in args. If the format item includes formatString and the corresponding object in args implements the IFormattable interface, then args[index].Format(formatString, provider) defines the formatting. Otherwise, args[index].ToString(provider) defines the formatting.

Notes to Callers

In the .NET Framework 4, when you instantiate the StringBuilder object by calling the StringBuilder constructor, both the length and the capacity of the StringBuilder instance can grow beyond the value of its MaxCapacity property. This can occur particularly when you call the Append and AppendFormat methods to append small strings.

The following example demonstrates the AppendFormat method.

Imports System
Imports System.Text
Imports System.Globalization

Class Sample
   Private Shared sb As New StringBuilder()

   Public Shared Sub Main()
      Dim var1 As Integer = 111
      Dim var2 As Single = 2.22F
      Dim var3 As String = "abcd"
      Dim var4 As Object() =  {3, 4.4, "X"c}

      Console.WriteLine("StringBuilder.AppendFormat method:")
      sb.AppendFormat("1) {0}", var1)
      sb.AppendFormat("2) {0}, {1}", var1, var2)
      sb.AppendFormat("3) {0}, {1}, {2}", var1, var2, var3)
      sb.AppendFormat("4) {0}, {1}, {2}", var4)
      Dim ci As New CultureInfo("es-ES", True)
      sb.AppendFormat(ci, "5) {0}", var2)
   End Sub 'Main

   Public Shared Sub Show(sbs As StringBuilder)
      sb.Length = 0
   End Sub 'Show
End Class 'Sample
'This example produces the following results:
'StringBuilder.AppendFormat method:
'1) 111
'2) 111, 2.22
'3) 111, 2.22, abcd
'4) 3, 4.4, X
'5) 2,22

The following example defines a custom IFormatProvider implementation named CustomerFormatter that formats a 10-digit customer number with hyphens after the fourth and seventh digits. It is passed to the StringBuilder.AppendFormat(IFormatProvider, String, Object()) method to create a string that includes the formatted customer number and customer name.

Imports System.Text

Public Class Customer
   Private custName As String
   Private custNumber As Integer

   Public Sub New(name As String, number As Integer)
      custName = name
      custNumber = number
   End Sub

   Public ReadOnly Property Name As String
         Return Me.custName
      End Get
   End Property

   Public ReadOnly Property CustomerNumber As Integer
         Return Me.custNumber
      End Get
   End Property
End Class

Public Class CustomerNumberFormatter 
   Implements IFormatProvider, ICustomFormatter

   Public Function GetFormat(formatType As Type) As Object _
                   Implements IFormatProvider.GetFormat
      If formatType Is GetType(ICustomFormatter) Then
         Return Me
      End If
      Return Nothing
   End Function

   Public Function Format(fmt As String, arg As Object, provider As IFormatProvider) As String _
                   Implements ICustomFormatter.Format
      If typeof arg Is Int32 Then
         Dim custNumber As String = CInt(arg).ToString("D10")
         Return custNumber.Substring(0, 4) + "-" + custNumber.SubString(4, 3) + _
                "-" + custNumber.Substring(7, 3)
         Return Nothing
      End If
   End Function                   
End Class

Module Example
   Public Sub Main()
      Dim customer As New Customer("A Plus Software", 903654)
      Dim sb As New StringBuilder()
      sb.AppendFormat(New CustomerNumberFormatter, "{0}: {1}", _
                      customer.CustomerNumber, customer.Name)
   End Sub
End Module
' The example displays the following output:
'      0000-903-654: A Plus Software

.NET Framework

Supported in: 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

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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