Home > Articles > Programming > C#

  • Print
  • + Share This
Like this article? We recommend

Like this article? We recommend

Change 1: URL-->RawHTML

The first information change we need to code is as follows:

string:URL-->string:RawHTML-->ArrayList:ImageList-->Disk:Files

We start by coding the method declaration. To help you and others understand and maintain your code, one convention you ought to adopt is naming your methods after the changes they perform. In our case this would be URLtoRawHTML. This method takes a URL as a string type and returns the HTML source code (raw HTML) that it finds at that URL as a string. You would implement the declaration for this method as shown in Listing 16:

Listing 16—Method Declaration: URLtoRawHTML

public string URLtoRawHTML(string URL)
{
}

The hard part is figuring out how to get the raw HTML from a URL. Unless you want to write low-level networking code, which would also entail handling the necessary protocols for interacting with web servers, it's best to search the documentation for Microsoft's .NET Framework Class Library to find out whether there are existing objects that you can use that do (most of) the work for you. It turns out that the System.Net and System.IO namespaces contain descriptions of objects that allow us to code this change using two motifs. Remember, you should really try to understand these motifs because they're general building blocks that you can use in other applications.

Motif 1: URL to Stream

The first motif you'll learn is the URL to Stream motif. This motif is useful whenever you're in a programming situation where you have a web address and you want to do something with the contents at this address. For our vampire bot example, we have the address of a web page and we want to extract all the images from this page. But before you can do anything with the content of a web page you need to first convert it to a Stream object. The motif that does so looks like Listing 17.

Listing 17—Motif 1: URL to Stream

using System.Net;
using System.IO;

WebRequest req;
WebResponse res;
Stream   str;

req = WebRequest.Create(URL);
res = req.GetResponse();
str = res.GetResponseStream();

This motif takes a web address as a string (URL) and returns a Stream object (str). As a matter of notational convention, when describing motifs we'll depict invariant code in bold and variables in italics. Moreover, we'll represent any entry variables in red italics (URL) and exit variables in blue italics (str).

Detailed Explanation

You first create a WebRequest object (req) by passing a URL as a string to the Create() method of that object (see Listing 18):

Listing 18—Explanation Motif 1: WebRequest.Create()

using System.Net;
using System.IO;

WebRequest req;
WebResponse res;
Stream   str;

req = WebRequest.Create(URL);
res = req.GetResponse();
str = res.GetResponseStream();

The WebRequest (req) object doesn't do much yet except remember the web page that you want to access. To read from the web page you need to first create a WebResponse object (res) by calling req's GetResponse() method (see Listing 19).

Listing 19—Explanation Motif 1: GetResponse()

using System.Net;
using System.IO;

WebRequest req;
WebResponse res;
Stream   str;

req = WebRequest.Create(URL);
res = req.GetResponse();
str = res.GetResponseStream();

But res also isn't in a form that you can do much with. You need to instruct res to return a Stream object, which you can work with (see Listing 20).

Listing 20—Explanation Motif 1: GetResponseStream()

using System.Net;
using System.IO;

WebRequest req;
WebResponse res;
Stream   str;

req = WebRequest.Create(URL);
res = req.GetResponse();
str = res.GetResponseStream();

The Stream object (str) gives you a lot of flexibility. From this object you can create files, strings, and various other representations of the streamed information. In our case, we're going to create a string that holds the contents of the web address (URL).

Motif 2: Building a String from a Stream (Stream to String)

Given a Stream object, it's quite common to read all the information from that Stream and store it locally in either an array or a file. For our vampire bot, where the Stream consists of HTML source code, we want to store the information into a string; strings contain built-in methods for searching and extracting substrings, which is what our vampire bot needs to do (for example, search and extract all the GIF filenames from the HTML source code). One basic motif for building a string from a stream is shown in Listing 21.

Listing 21—Motif 2: Stream to String

using System.IO;

string   RawHTML;
int     ch;

RawHTML = "";
while ((ch=str.ReadByte())!=-1)
 RawHTML=RawHTML+Convert.ToChar(ch);

This motif assumes that you already have a Stream object (str in our example; also see motif 1). The code loops through, reading one character (ch) at a time from the Stream (str), and appending that character to the destination string (RawHTML). Of course, we should point out that there are faster motifs for converting Streams to strings. For instance, instead of reading one byte at a time, you can read and append a block of characters each time through the loop. However, for instructional purposes this is a straightforward motif.

Detailed Explanation

First define a string variable (RawHTML) and initialize its value to empty (see Listing 22).

Listing 22—Explanation Motif 2: String Setup

using System.IO;

string   RawHTML;
int     ch;

RawHTML = "";
while ((ch=str.ReadByte())!=-1)
 RawHTML=RawHTML+Convert.ToChar(ch);

Read one byte (ch) from the Stream (str) using the Stream's built-in ReadByte() method (see Listing 23).

Listing 23—Explanation Motif 2: ReadByte()

using System.IO;

string   RawHTML;
int     ch;

RawHTML = "";
while ((ch=str.ReadByte())!=-1)
 RawHTML=RawHTML+Convert.ToChar(ch);

Use the + operator to append this character (ch) to the destination string (RawHTML), as shown in Listing 24.

Listing 24—Explanation Motif 2: Concatenation and Convert.ToChar()

using System.IO;

string   RawHTML;
int     ch;

RawHTML = "";
while ((ch=str.ReadByte())!=-1)
 RawHTML=RawHTML+Convert.ToChar(ch);

Note that ch is actually an integer because the ReadByte() method returns an integer. Thus, you must convert ch to a character via the Convert.ToChar() method prior to appending it.

To complete the motif, simply loop through until there are no more characters in the stream, which is denoted by ReadByte() returning a value of -1. One way to implement this loop is by putting the code that reads the character from the stream (ch=str.ReadByte()) in the condition portion of a while loop (see Listing 25).

Listing 25—Explanation Motif 2: Looping to Read All Bytes

using System.IO;

string   RawHTML;
int     ch;

RawHTML = "";
while ((ch=str.ReadByte())!=-1)
 RawHTML=RawHTML+Convert.ToChar(ch);

Completing the Method: Combining Motifs

To combine these motifs, move the motifs' variable declarations near the top of the method, and put the code blocks one after another. In Listing 26, we've highlighted motifs 1 and 2 in green and aqua, respectively, so you can better see how we've combined them.

Listing 26—URLtoRawHTML, Completed Method: Motifs 1 and 2 Combined

public string URLtoRawHTML(string URL)
{
WebRequest req;
WebResponse res;
Stream   str;

string   RawHTML;
int     ch;

req = WebRequest.Create(URL);
res = req.GetResponse();
str = res.GetResponseStream();

RawHTML = "";
while ((ch=str.ReadByte())!=-1)
 RawHTML=RawHTML+Convert.ToChar(ch);

str.Close();
res.Close();

return RawHTML;
}

Note that prior to returning RawHTML (return RawHTML), you need to close both the Stream object (str) and the WebResponse object (res), using each object's .Close() method; in other words, str.Close() and res.Close(), respectively.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020