AppendFormat Method (IFormatProvider, String, Object[])

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 StringBuilder AppendFormat(
	IFormatProvider provider,
	string format,
	params object[] args


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.

Exception Condition

format is null.


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 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 null, 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 null, 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].ToString(formatString, provider) defines the formatting. Otherwise, args[index].ToString() defines the formatting.

Notes to Callers:

In the.NET Framework 4 and the .NET Framework 4.5, when you instantiate the StringBuilder object by calling the StringBuilder(Int32, Int32) 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.

using System;
using System.Text;
using System.Globalization;

class Sample 
    static StringBuilder sb = new StringBuilder();

    public static void Main() 
    int    var1   = 111;
    float  var2   = 2.22F;
    string var3   = "abcd";
    object[] var4 = {3, 4.4, 'X'};

    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);
    CultureInfo ci = new CultureInfo("es-ES", true);
    sb.AppendFormat(ci, "5) {0}", var2);

    public static void Show(StringBuilder sbs)
    sb.Length = 0;
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.

using System;
using System.Text;

public class Customer
   private string custName;
   private int custNumber;

   public Customer(string name, int number)
      this.custName = name;
      this.custNumber = number;

   public string Name
      get { return this.custName; }

   public int CustomerNumber
      get { return this.custNumber; }

public class CustomerNumberFormatter : IFormatProvider, ICustomFormatter
   public object GetFormat(Type formatType)
      if (formatType == typeof(ICustomFormatter))
         return this;
      return null;

   public string Format(string format, object arg, IFormatProvider provider)
      if (arg is Int32)
         string custNumber = ((int) arg).ToString("D10");
         return custNumber.Substring(0, 4) + "-" + custNumber.Substring(4, 3) + 
                "-" + custNumber.Substring(7, 3);
         return null;

public class Example
   public static void Main()
      Customer customer = new Customer("A Plus Software", 903654);
      StringBuilder sb = new StringBuilder();
      sb.AppendFormat(new CustomerNumberFormatter(), "{0}: {1}", 
                      customer.CustomerNumber, customer.Name);
// The example displays the following output:
//      0000-903-654: A Plus Software

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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