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Validating ASP.NET Pages

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Validating user input in ASP.NET Web forms is one of the most important tasks for developers. Users are very unpredictable when it comes to supplying information, so you must take extra precautions to make sure they give you the correct data. For example, suppose you ask a user for a credit card number. What happens if he enters non-valid input, such as dgffg934053? Or what if he leaves off one digit of the number? Obviously, this will cause some problems in your application, especially when he tries to place an order.

In ASP.NET, these types of validation are easy to perform. You have a bevy of options for validating user input, including pattern matching and making sure the input is within a given range. The Validation controls provide a number of methods to alert users about errors, and they can do so dynamically without requiring a post to the server. And when one of the built-in options doesn't meet your needs, you can build your own!

Today you're going to look at various ways to validate user input easily with ASP.NET. Specifically, you'll look at the Validation server controls, which make data entry validation easier than ever. You'll also look at examples of creating your own validation routines.

Today's lesson will cover the following:

  • Why and how to validate user input before ASP.NET

  • What ASP.NET Validation controls are and how they work

  • How to implement Validation controls in your pages

  • How to customize error messages displayed to the user

  • How to build custom Validation controls

Validation Scenarios

Imagine going to school to register for classes. The registrar asks you for your name. If you tell him that your name is "56," he's going to wonder if you're telling the truth. Most likely, he'll ask you for your name again. He must either verify that your name is indeed "56" or get your real name.

Any time that you ask people for information, you have to be prepared for the unexpected. You never know what they'll say when presented with a simple question. With computers, this kind of behavior can be disastrous. If an application is expecting one type of data, giving it another type of data may result in an error or even a crash (as you may have already experienced with your ASP.NET pages). Input validation is the process of making sure that the user provides the correct type of data.

Input validation is a must when you allow users to enter information manually into your ASP.NET pages. Most users will do their best to comply and enter the information you ask for in the proper format, but sometimes they make mistakes. Other users may like to try breaking applications by entering invalid information. In either case, getting data that you weren't expecting is a very common situation.

For example, imagine that you've created an e-commerce site that takes orders for office supplies. Eventually, you'll need to take personal information from the user, such as her name, e-mail, billing address, and so on. You must ensure that this information is accurate, or the order won't be processed correctly. For example, if the user enters a number for the shipping state, such as 45, and the application doesn't catch it immediately, the shipment may never be delivered and you'll end up with an irate customer.

So what happens if the user enters a number in place of her name? Or she enters a random string of characters for her e-mail address? Eventually, what you end up with is incorrect data being entered into your system. Detecting invalid input is vital for presenting a solid application.

Input validation also provides benefits for developers—specifically, coherent data. When you're designing your application, you should always be able to count on the data being in a specific format. For example, if you need to add two numbers, you need to make sure they are indeed numbers. Otherwise, the add operation won't work and your application will fail. When the input has been validated beforehand, you know that your add operation will succeed.

Let's examine a typical situation that requires user input validation. Listing 7.1 shows a simple user interface ASP.NET page that allows users to enter some personal information.

Listing 7.1 A Typical Scenario for Validation

1: <html><body>
2:  <form runat="server">
3:   <asp:Label id="lblHeader" runat="server"
4:    Height="25px" Width="100%" BackColor="#ddaa66"
5:    ForeColor="white" Font-Bold="true"
6:    Text="A Validation Example" />
7:   <asp:Label id="lblMessage" runat="server" /><br>
8:   <asp:Panel id="Panel1" runat="server">
9:    <table>
10:    <tr>
11:    <td width="100" valign="top">
12:     First and last name:
13:    </td>
14:    <td width="300" valign="top">
15:     <asp:TextBox id="tbFName" runat="server" />
16:     <asp:TextBox id="tbLName" runat="server" />
17:    </td>
18:    </tr>
19:    <tr>
20:    <td valign="top">Email:</td>
21:    <td valign="top">
22:     <asp:TextBox id="tbEmail"
23:      runat="server" />
24:    </td>
25:    </tr>
26:    <tr>
27:    <td valign="top">Address:</td>
28:    <td valign="top">
29:     <asp:TextBox id="tbAddress"
30:      runat="server" />
31:    </td>
32:    </tr>
33:    <tr>
34:    <td valign="top">City, State, ZIP:</td>
35:    <td valign="top">
36:     <asp:TextBox id="tbCity"
37:      runat="server" />,
38:     <asp:TextBox id="tbState" runat="server"
39:      size=2 />&nbsp;
40:     <asp:TextBox id="tbZIP" runat="server"
41:      size=5 />
42:    </td>
43:    </tr>
44:    <tr>
45:    <td valign="top">Phone:</td>
46:    <td valign="top">
47:     <asp:TextBox id="tbPhone" runat="server"
48:      size=11 /><p>
49:    </td>
50:    </tr>
51:    <tr>
52:    <td colspan="2" valign="top" align="right">
53:     <asp:Button id="btSubmit" runat="server"
54:      text="Add" />
55:    </td>
56:    </tr>
57:    </table>
58:   </asp:Panel>
59:  </form>
60: </body></html>

This listing uses a few Web controls to display a user interface. The user interface prompts the user for first and last name, e-mail address, street address, city, state, ZIP code, and phone number. The result of Listing 7.1 is shown in Figure 7.1.

Once the form is submitted, you have to verify that each field contains a valid entry before entering the data in a database. A common way to do this is through a series of if statements, as shown in Listing 7.2. This series of if statements validates all of the server controls in Listing 7.1.

Listing 7.2 Validating User Input from Listing 7.1 Through if Statements

1: <script runat="server">
2:  sub Submit(obj as Object, e as EventArgs)
3:   if tbFName.Text <> "" and not IsNumeric(tbFName.Text) then
4:    if tbLName.Text <> "" and not IsNumeric(tbLName.Text) then
5:    if tbAddress.Text <> "" then
6:     if tbCity.Text <> "" and not IsNumeric(tbCity.Text) then
7:      if tbState.Text <> "" and not IsNumeric(tbState.Text) then
8:       if tbZIP.Text <> "" and IsNumeric(tbZIP.Text) then
9:       if tbPhone.Text <> "" then
10:        lblMessage.Text = "Success!"
11:       else
12:        lblMessage.Text = "Phone is incorrect!"
13:       end if
14:       else
15:       lblMessage.Text = "ZIP is incorrect!"
16:       end if
17:      else
18:       lblMessage.Text = "State is incorrect!"
19:      end if
20:     else
21:      lblMessage.Text = "City is incorrect!"
22:     end if
23:    else
24:     lblMessage.Text = "Address is incorrect!"
25:    end if
26:    else
27:    lblMessage.Text = "Last name is incorrect!"
28:    end if
29:   else
30:    lblMessage.Text = "First name is incorrect!"
31:   end if
32:  end sub
33: </script>

As you can see, validating all of the controls in this manner is rather long and tedious. The if statement on line 3 verfies that the user has filled in the first name field, and also that the supplied data is not a number. The second if statement on line 4 does the same for the last name field. You proceed this way until you've checked every control that contains user input. Don't forget the else statements, which display a message telling the user what the error is.

After checking all of the user inputs in this fashion, you can take one of two courses. If all of the information entered by the user is in the correct format, you can insert the data in the data source. If any of the information is formatted improperly, you need to alert the user to the errors in the input (done with the else statements) and allow her to fix them before inserting the results into the data store (by simply redisplaying the page).

Figure 7.1 A typical user-entry page.

However, this can get quite complex because you often have to check for multiple cases on each input. For example, for the first name, you have to check that the field isn't blank, doesn't contain any numbers, doesn't contain spaces, and so on. This makes for a lot of if statements. As tedious as this may be, it's a typical method of validating user input. The process boils down to the following steps:

  1. Display the form to the user and allow her to enter data.

  2. Upon form submission, use if statements to check every user input. This includes checking for blank entries, the proper format and data type, valid ranges of information (for dates), and so on.

  3. If all if statements pass, perform your functionality.

  4. If it fails, redisplay the form, preferably with the fields prefilled with the previously entered user input. This allows the user to correct only the incorrect fields and leave the correct ones. Start over at step 2.

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