Composite Formatting

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The .NET Framework composite formatting feature takes a list of objects and a composite format string as input. A composite format string consists of fixed text intermixed with indexed placeholders, called format items, that correspond to the objects in the list. The formatting operation yields a result string that consists of the original fixed text intermixed with the string representation of the objects in the list.

The composite formatting feature is supported by methods such as Format and AppendFormat. The String.Format method yields a formatted result string, and the AppendFormat method appends a formatted result string to a StringBuilder object.

Composite Format String

A composite format string and object list are used as arguments of methods that support the composite formatting feature. A composite format string consists of zero or more runs of fixed text intermixed with one or more format items. The fixed text is any string that you choose, and each format item corresponds to an object or boxed structure in the list. The composite formatting feature returns a new result string where each format item is replaced by the string representation of the corresponding object in the list.

Consider the following Format code fragment.

Dim myName As String = "Fred"
String.Format("Name = {0}, hours = {1:hh}", myName, DateTime.Now)
string myName = "Fred";
String.Format("Name = {0}, hours = {1:hh}", myName, DateTime.Now);

The fixed text is "Name = " and ", hours = ". The format items are "{0}", whose index is 0, which corresponds to the object myName, and "{1:hh}", whose index is 1, which corresponds to the object DateTime.Now.

Format Item Syntax

Each format item takes the following form and consists of the following components:


The matching braces ("{" and "}") are required.

Index Component

The mandatory index component, also called a parameter specifier, is a number starting from 0 that identifies a corresponding item in the list of objects. That is, the format item whose parameter specifier is 0 formats the first object in the list, the format item whose parameter specifier is 1 formats the second object in the list, and so on.

Multiple format items can refer to the same element in the list of objects by specifying the same parameter specifier. For example, you can format the same numeric value in hexadecimal, scientific, and number format by specifying a composite format string like this: "{0:X} {0:E} {0:N}".

Each format item can refer to any object in the list. For example, if there are three objects, you can format the second, first, and third object by specifying a composite format string like this: "{1} {0} {2}". An object that is not referenced by a format item is ignored. A runtime exception results if a parameter specifier designates an item outside the bounds of the list of objects.

Alignment Component

The optional alignment component is a signed integer indicating the preferred formatted field width. If the value of alignment is less than the length of the formatted string, alignment is ignored and the length of the formatted string is used as the field width. The formatted data in the field is right-aligned if alignment is positive and left-aligned if alignment is negative. If padding is necessary, white space is used. The comma is required if alignment is specified.

Format String Component

The optional formatString component is a format string that is appropriate for the type of object being formatted. Specify a standard numeric format string or a custom numeric format string if the corresponding object is a numeric value, a standard date and time format string or a custom date and time format string if the corresponding object is a DateTime or a DateTimeOffset object, or an enumeration format string if the corrersponding object is an enumeration value. If formatString is not specified, the general ("G") format specifier for a numeric, date and time, or enumeration type is used. The colon is required if formatString is specified.

Escaping Braces

Opening and closing braces are interpreted as starting and ending a format item. Consequently, you must use an escape sequence to display a literal opening brace or closing brace. Specify two opening braces ("{{") in the fixed text to display one opening brace ("{"), or two closing braces ("}}") to display one closing brace ("}"). Braces in a format item are interpreted sequentially in the order they are encountered. Interpreting nested braces is not supported.

The way escaped braces are interpreted can lead to unexpected results. For example, consider the format item "{{{0:D}}}", which is intended to display an opening brace, a numeric value formatted as a decimal number, and a closing brace. However, the format item is actually interpreted in the following manner:

  1. The first two opening braces ("{{") are escaped and yield one opening brace.

  2. The next three characters ("{0:") are interpreted as the start of a format item.

  3. The next character ("D") would be interpreted as the Decimal standard numeric format specifier, but the next two escaped braces ("}}") yield a single brace. Because the resulting string ("D}") is not a standard numeric format specifier, the resulting string is interpreted as a custom format string that means display the literal string "D}".

  4. The last brace ("}") is interpreted as the end of the format item.

  5. The final result that is displayed is the literal string, "{D}". The numeric value that was to be formatted is not displayed.

One way to write your code to avoid misinterpreting escaped braces and format items is to format the braces and format item separately. That is, in the first format operation display a literal opening brace, in the next operation display the result of the format item, then in the final operation display a literal closing brace. The following example illustrates this approach.

Dim value As Integer = 6324
Dim output As String = String.Format("{0}{1:D}{2}", _
                                     "{", value, "}")
outputBlock.Text += output & vbCrLf
' The example displays the following output:
'       {6324}
int value = 6324;
string output = string.Format("{0}{1:D}{2}",
                             "{", value, "}");
outputBlock.Text += output + "\n";
// The example displays the following output:
//       {6324}                            

Processing Order

Each value in the parameter list that corresponds to a format item is converted to a string by performing the steps in the following list. If any condition in the first three steps is true, the string representation of the value is returned in that step, and subsequent steps are not executed.

  1. If the value to be formatted is null, an empty string ("") is returned.

  2. If the composite formatting method includes a parameter of type IFormatProvider that also implements the ICustomFormatter interface, the value is passed to the ICustomFormatter.Format method.

  3. If the value implements the IFormattable interface, its IFormattable.ToString method is called.

  4. The type's ToString method, which is either overridden or inherited from the Object class, is called.

Alignment is applied after the preceding steps have been performed.

Code Examples

The following example shows one string created using composite formatting and another created using an object's ToString method. Both types of formatting produce equivalent results.

Dim FormatString1 As String = String.Format("{0:dddd MMMM}", DateTime.Now)
Dim FormatString2 As String = DateTime.Now.ToString("dddd MMMM") 
string FormatString1 = String.Format("{0:dddd MMMM}", DateTime.Now);
string FormatString2 = DateTime.Now.ToString("dddd MMMM");

Assuming that the current day is a Thursday in May, the value of both strings in the preceding example is Thursday May in the U.S. English culture.

The following example demonstrates formatting multiple objects, including formatting one object two different ways.

Dim myName As String = "Fred"
String.Format("Name = {0}, hours = {1:hh}, minutes = {1:mm}",
      myName, DateTime.Now)
string myName = "Fred";
String.Format("Name = {0}, hours = {1:hh}, minutes = {1:mm}",
      myName, DateTime.Now);

The string value returned by the preceding code is "Name = Fred, hours = 07, minutes = 23", where the current time reflects these numbers.

The following examples demonstrate the use of alignment in formatting. The arguments that are formatted are placed between vertical bar characters (|) to highlight the resulting alignment.

Dim myFName As String = "Fred"
Dim myLName As String = "Opals"
Dim myInt As Integer = 100
Dim FormatFName As String = String.Format("First Name = |{0,10}|", myFName)
Dim FormatLName As String = String.Format("Last Name = |{0,10}|", myLName)
Dim FormatPrice As String = String.Format("Price = |{0,10:C }|", myInt)

FormatFName = String.Format("First Name = |{0,-10}|", myFName)
FormatLName = String.Format("Last Name = |{0,-10}|", myLName)
FormatPrice = String.Format("Price = |{0,-10:C }|", myInt)
string myFName = "Fred";
string myLName = "Opals";
int myInt = 100;
string FormatFName = String.Format("First Name = |{0,10}|", myFName);
string FormatLName = String.Format("Last Name = |{0,10}|", myLName);
string FormatPrice = String.Format("Price = |{0,10:C}|", myInt); 

FormatFName = String.Format("First Name = |{0,-10}|", myFName);
FormatLName = String.Format("Last Name = |{0,-10}|", myLName);
FormatPrice = String.Format("Price = |{0,-10:C}|", myInt);

The preceding code returns the following string values in the U.S. English culture. Different cultures display different currency symbols and separators.

First Name = |          Fred|
Last Name = |         Opals|
Price = |           $100.00|
First Name = |Fred      |
Last Name = |Opals     |
Price = |$100.00   |