This lesson will cover how to format strings. We already looked at how to concatenate strings, with the “+” and “+=” operators, but this lesson will show you how to handle special formatting conditions, such as the patterns seen in things like phone, social security, or credit card numbers. These all have some form of patterned formatting, or otherwise have some special arrangement of numbers and characters.

Step 1: Create a New Project

We’ll demonstrate this with a very simple loan application form that stores pieces of information like the phone number, social security number, salary and date. It will then display all of this information back to resultLabel with special formatting so that it looks presentable. Begin this lesson by creating a new ASP.NET project called “CS-ASP_019,” and set up a Default.aspx with the following Server Controls, and programmatic IDs:

  1. nameTextBox

  2. phoneTextBox

  3. ssTextBox

  4. loanDateCalendar

  5. salaryTextBox

  6. submitButton

  7. resultLabel



The first thing we will want to do is take the applicant’s name that was input via nameTextBox, and output it within a formal “thank you” statement:


Step 2: String Formatting with string.Format()

This approach uses the string concatenation technique we have become familiar with. This will get the job done, but isn’t as readable owing to the multiple uses of the concatenation operator. A much more readable way of achieving this is by calling the string.Format() method instead:


This method takes in (1) a literal string argument, along with a (2) variable string argument, and then using a (3) special placeholder it transposes the variable string argument into the placeholder position within the literal string:


Note: This placeholder system is zero-based, meaning that the first value passed in will be {0}, then {1}, {2} {3}, etc.

The output when running the application will look like this:


This essentially produces a string “template” with as many placeholders as needed:



The problem now is that the entered number itself should be formatted to look more like a typical Social Security number. We start by converting this value to an integer using int.Parse(), and then storing it into an integer variable:


Step 3: Applying Special Formatting Patterns to the Placeholder

Modify the string.Format() method to take this int value and apply to it a formatting pattern of number sequences delimited by dashes. Here we took the {1} placeholder and appended to it a formatting pattern, with a colon separating the placeholder from its pattern:


The formatting pattern will become much more apparent when running the application:


Tip: You should note that phone numbers are at least 10 digits in length, which reaches the upper-limits allowable for storage into an int variable. If the first digit in the phone number is greater than 1, there is a good chance it will cross the allowable threshold and return an error. To fix this, you can store the number into a long. However, if the application ends up processing thousands of numbers (or more) this could be very inefficient and you would probably want to leave the phone number as a string.

Step 4: Inserting HTML into the Output String

You can also inject HTML straight into the output string, considering that the output is ultimately being delivered through the browser. Here, we injected a simple HTML line break tag “<br />” to put each phrase onto its own line:



Step 5: Breaking Up Code on Separate Lines for Readability

Now, let’s apply the same principles towards formatting the phone number. Add a third placeholder, {2}, which will take the value supplied in yet another argument, phone. We will be applying this process over and over again, to handle the other user inputs, which will lead to a very long statement. To make it more readable, we can use the concatenation operator to break up the statement on separate lines:


The end result will look like this:


Tip: This goes back to the previous lesson on code style. This is very subjective – what looks good to one person, may not look good to another. Here’s a good rule of thumb: keep lines approximately within an 80-column range. You can find the column number for each line at the bottom right of Visual Studio. Also, when separating lines, try to keep a logical separator in mind. Here, the <br /> tag is used as a line separator, as well as the comma after each string argument. This sets a pattern that makes it easier for you to get a sense of what the code is doing, at a glance.

Step 6: Applying Special Date Formatting

Next we will want to format the date returned by the applicant’s Calendar selection. We already saw built-in formatting methods like ToShortDateString(), but sometimes that isn’t flexible enough to handle a particular formatting requirement. Below is a short list of available date formatting options in C#:


This is a partial list that can be found by visiting:

Here we are applying some of these special date formatting rules to the fourth placeholder at {3}, which formats the date given by loanDateCalendar.SelectedDate:


This produces the date formatting shown here:


Step 7: Applying Special Currency Formatting

When working with currency you can place a “C” after the colon, inside the placeholder, to tell the compiler that this is to be formatted as a currency. If you want to allow for cents, you need to first convert the entered value to a double, as well:



For more information on currency formatting options, visit the following page showing you a variety of ways to represent exponential values, fixed point, and other numeric types of values:

Formatting strings is very useful and powerful for you, the developer, when creating forms for users to fill out. Keep these tools in mind, as they are going to be utilized in the next Challenge coming up very soon.

Lessons in this Course