Monday, January 14, 2013

URL Encoding in ASP.NET

One potential problem with the query string is that some characters aren’t allowed in a URL. In fact, the list of characters that are allowed in a URL is much shorter than the list of allowed characters in an HTML document. All characters must be alphanumeric or one of a small set of special characters (including $-_.+!*'(),). Some browsers tolerate certain additional special characters (Internet Explorer is notoriously lax), but many do not. Furthermore, some characters have special meaning. For example, the ampersand (&) is used to separate multiple query string parameters, the plus sign (+) is an alternate way to represent a space, and the number sign (#) is used to point to a specific bookmark in a web page. If you try to send query string values that include any of these characters, you’ll lose some of your data. You can test this out with the previous example by adding items with special characters in the list box.

To avoid potential problems, it’s a good idea to perform URL encoding on text values before you place them in the query string. With URL encoding, special characters are replaced by escaped character sequences starting with the percent sign (%), followed by a two-digit hexadecimal representation. For example, the & character becomes %26. The only exception is the space character, which can be represented as the character sequence %20 or the + sign. To performURL encoding, you use the UrlEncode() and UrlDecode() methods of the HttpServerUtility class. As you learned in Chapter 5, an HttpServerUtility object is made available to your code in every web form through the Page.Server property. The following code uses the UrlEncode() method to rewrite the previous example, so it works with product names that contain special characters:

string url = "QueryStringRecipient.aspx?";
url += "Item=" + Server.UrlEncode(lstItems.SelectedItem.Text) + "&";
url += "Mode=" _ chkDetails.Checked.ToString();

Notice that it’s important not to encode everything. In this example, you can’t encode the & character that joins the two query string values, because it truly is a special character. You can use the UrlDecode() method to return a URL-encoded string to its initial value. However, you don’t need to take this step with the query string. That’s because ASP.NET automatically decodes your values when you access them through the Request.QueryString collection. (Many people still make the mistake of decoding the query string values a second time. Usually, decoding already decoded data won’t cause a problem. The only exception is if you have a value that includes the + sign. In this case, using UrlDecode() will convert the + sign to a space, which isn’t what you want.)

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