Displaying text and variable contents are a standard aspect
of most development projects. In addition, it is more than likely that the output should be formatted in a particular fashion.

The .NET Framework provides various ways of achieving the
desired output, including the String.Format method and format
. Another way to tackle the problem
involves the IFormattable interface. I’ll examine how you may use this
interface to control text formatting.

What is IFormattable?

The standard ToString method
available with all objects allows you to easily display the contents of a
variable, but it provides no control over how the data appears. The IFormattable interface provides the functionality to format
the value of an object into a string representation. Classes that require more
control over the formatting of strings than the standard ToString
method provides should implement IFormattable.

As a quick overview, an interface is a group of methods and
properties that can be implemented by several classes, regardless of where the
classes are in the class hierarchy. The methods and properties of an interface
are implemented in a class. When methods are implemented, the return data type
and the argument data types must exactly match those of the method described in
the interface. The method name, however, doesn’t need to be the same as that described
in the interface. An example using the IFormattable
interface will make it clearer.

Format providers

Format providers supply information such as the character to
use as the decimal point when formatting numeric strings, or the separation
character to use when formatting a DateTime object.
Format providers define the characters used for formatting by the format specifiers, but do not define the specifiers

Let’s take a look at standard format providers utilized in
the .NET Framework. When formatting numeric types and the DateTime
structure, if no IFormatProvider implementation is
provided, the two standard format provider classes (DateTimeFormatInfo
and NumberFormatInfor) are used automatically to
provide formatting for the related types. A simple example shows how you may be
using these classes without realizing it. The following C# code displays the
number as both currency and a whole number.

inttst = 550000;
System.Console.WriteLine("{0:C}", tst);
System.Console.WriteLine("{0:D}", tst);

Here’s the VB.NET equivalent:

Dim tst As Integer = 550000
System.Console.WriteLine("{0:#0;(#0)}", tst)
System.Console.WriteLine("{0:#0}%", tst)

This is great when working with standard numbers, dates, and
so forth, but problems may arise when working with your own custom data
types/classes. This is when you utilize a custom format provider by overriding
your class’ ToString method.

Custom formatting

A format provider can be passed to the overload of ToString required by the IFormattable
interface, or be predetermined by the method you are using to format text if no
format provider is passed. Generally, classes that implement IFormattable also provide overloads of ToString
that accept only a format specifier or only a format
provider. The default ToString method, which accepts
no parameters, is inherited from the Object class. The IFormattable
interface includes only the one ToString method.

Implementing IFormattable.ToString
is pretty easy, but you should overload ToString to
provide a single parameter for each type. These can simply call the implementation
method, which uses the format specifier and optional
information from the IFormatProvider argument to
format the type as a string.

Listing A contains a C# example class that utilizes this
approach. It contains a property for working the value passed to it, and it
implements the IFormattable interface. Various
flavors of the ToString method are included to handle
various formats.
Listing B features
the VB.NET equivalent of Listing A.

With this class in place, we can utilize it in another
application to demonstrate how it may be used.
Listing C is an ASP.NET example that uses it to display values via
the class’ ToString method. The results are displayed

It is a simple example that accepts user input, and displays
it in a Label control. Now, the text is formatted according to our CustomFormat class. The equivalent ASP.NET using VB.NET is
in Listing D.

While this example is relatively simple, it does showcase
the flexibility afforded by developing your own format classes. Instances always
arise, especially with Web interfaces, when data must be formatted in a certain
manner. Developing a custom class allows you to use the required format as well
as reuse it throughout the application and tweak it in one central location.

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