How to Use Formatting to Represent Types as Strings and

This article introduces the basic format and parse mechanisms in Microsoft.NET.


The .NET Framework provides a consistent, flexible, and comprehensive means for you to represent any of the numeric, enumeration, and date and time base data types as a string. Formatting is controlled by strings of format specifier characters that indicate how a base type value is to be represented. For example, format specifiers indicate whether a formatted number should be represented in scientific notation, or whether a formatted date should present the month as a number or a name.

You can use format without using parse to display an non-editable result e.g. in a Label or in a report. In this case you are only presenting data to the user; you are not giving the user the ability to edit the data.


More common is the use of format and parse together.  Consider a data entry form that is used to access and modify data in a database.


1.  User searches and selects an existing record in the database.

2.  The application pulls the record into the data entry form.

3.  Instead of showing unformatted numbers from the database, the application formats each number into human readable form.

4.  User modifies a number through a TextBox.  The user enters unformatted numbers.  When the user finishes a TextBox edit the application once again formats the number into human readable form.

5.  User saves the edits.  The application parses the numbers into .Net Framework types before updating the database.


Format Specifiers


Formatting in the .Net Framework is implemented using format specifiers which are more commonly called “format strings”


There are standard and custom format strings.


Standard format strings are used by the ToString methods of the .Net Framework types Int32, Int64, Single, Double, Enumeration, and DateTime.


EXAMPLE:  Date Type Standard Format Strings


The Date type implements standard format strings through its ToString methods.


Dim currentDateTime As Date = Now

‘ Use some of the Date type ToString methods

‘ to format a .Net Framework Date type

‘ into human readable form.



RESULT: 6/20/2003 1:12:40 PM




RESULT: Friday, June 20, 2003




RESULT: 6/20/2003



EXAMPLE:  Numeric Type Standard Format Strings


Numeric types implement standard format strings through their ToString methods.


Dim aDouble As Double

‘ Use some of the numeric standard format strings

‘ to format a .Net Framework Double type

‘ into human readable form.


‘ Format as currency.

aDouble = 19758.1001



RESULT: $19,758.10


‘ Format as percentage.

aDouble = 0.0812



RESULT: 8.12 %


Visit the links below to learn more about standard format strings:


Microsoft.NET 1.x

Standard Numeric Format Strings

Standard DateTime Format Strings

Standard Enumeration Format Strings


Microsoft.NET 2.0

Standard Numeric Format Strings

Standard DateTime Format Strings

Standard Enumeration Format Strings

Custom Format Strings


Custom format strings are created from custom format characters.  Visit the link below for listings of some of the custom format characters you can use to build custom format strings.

Microsoft.NET 1.x

Custom Numeric Format Strings

Custom Datetime Format Strings

Microsoft.NET 2.0

Custom Numeric Format Strings

Custom Enumeration Format Strings


Here is an example that uses Custom Numeric Format Strings to format a Double value to a string representing a telephone number


‘ Create a Double

Dim aDouble As Double = 1234567890

‘ Use Numeric Custom Format Characters to

‘ display the Double as a telephone number.

Dim phoneNumber As String = aDouble.ToString(“(###) ### – ####”)



RESULT:  (123) 456 – 7890


The “#” character is the custom numeric format digit placeholder character. If the value being formatted has a digit in the position where the ‘#’ appears in the format string, then that digit is copied to the output string.


Here is an example that uses Custom Datetime Format Strings to format a Datetime value to provide a unique way to represent a date as a string.


‘ Create a Datetime

Dim aDateTime As DateTime = Now

‘ Use DateTime Custom Format Characters to

‘ display the DateTime in a unique format.

Dim uniqueDateTime As String = aDateTime.ToString(“dddd – d – MMMM”)



RESULT:  Friday – 1 – August


The example combined three custom format strings with two literal dash (-) characters:


dddd   Displays the full name of the day for the specified DateTime.


d   Displays the current day of the month, measured as a number between 1 and 31, inclusive. If the day is a single digit only (1-9), then it is displayed as a single digit. Note that if the’d’ format specifier is used alone, without other custom format strings, it is interpreted as the standard short date pattern format specifier. If the ‘d’ format specifier is passed with other custom format specifiers or the ‘%’ character, it is interpreted as a custom format specifier.


MMMM   Displays the full name of the month for the specified DateTime.


Both examples demonstrate how custom format characters are combined to create a custom format string.  By passing in the custom format string as an argument to a ToString method call, you can create output that is formatted exactly as you want it.




Parsing is the opposite of formatting – it is converting a human readable string representation back into a machine readable .Net Framework base type. For example, convert a date displayed in string format in a Windows Form like this: September 1, 2003; to a .Net Framework DateTime object.


Example:  Parse a string representation of a date to a DateTime object.


‘ Create a string that represents a date.

Dim dateString As String = “September 1, 2003”

‘ Declare a variable of type DateTime named aDate.

Dim aDate As DateTime

‘ Assign a reference to a DateTime object created

‘ by parsing the string representation of a date

‘ to the aDate variable.

aDate = DateTime.Parse(dateString)



Result: 9/1/2003 12:00:00 AM


The parsing method used is determined by the .Net Framework base type object to be created from a string. There are six main parsing methods: Numeric, Date, Time, Enumeration, Char, and Boolean.  The link below leads to details about each.


Parsing Strings .NET 1.x

Parsing Strings .NET 2.0


Some parsing methods have variations which help control how a string is converted into a .Net Framework base type.  For example, when parsing to a number, the Globalization NumberStyles flags are used to parse a string that contains non-numeric characters such as a currency symbol, a decimal point, an exponent, parentheses, and so on.


Example:  Parse a string representation of a number that contains non-numeric characters.


‘ Create a string that represents a decimal number that contains non-numeric characters.

Dim aNumberString As String = “$123,456.56”

‘ Declare a variable of type Decimal named aDecimal.

Dim aDecimal As Decimal

‘ Assign a reference to a Decimal object created

‘ by parsing the string representation of a date,

‘ that includes non-numeric characters,

‘ to the aDecimal variable.

‘ Use Globalization.NumberStyle flags

‘ to allow for thousands group sepearators,

‘ a currency symbol, and a decimal point

‘ that may be included in the the string

‘ representing a decimal number.

aDecimal = Decimal.Parse(aNumberString, _

Globalization.NumberStyles.Number.AllowThousands _

Or Globalization.NumberStyles.Number.AllowCurrencySymbol _

Or Globalization.NumberStyles.AllowDecimalPoint)



Result: 123456.56


~ bởi duriangroup trên Tháng Sáu 1, 2007.

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