The past several articles have demonstrated how to use both the XmlTextWriter and XmlTextReader classes to perform basic XML file, or document, tasks. This article takes what you’ve seen thus far to the next logical step—creating a real application that reads and writes data using these classes.
Remember that both the XmlTextWriter and XmlTextReader classes provide for only sequential, forward-only access to an XML file. Therefore, use these classes only in situations where your application design allows for processing every node in the XML file. Examples of such scenarios are XML parsers and applications that maintain single-entity files.
Building the Demo
This article’s demo application is a C++ mixed-mode (native and managed code) example of maintaining a single-entity file, where the information for each movie video is stored in an individual file. If the application were storing the information for all videos in the same file, using the .NET XML DOM (Document Object Model) classes would be best, as they allow direct access to specific nodes without having to process each of the file’s nodes from beginning to end. (Upcoming articles will explore the .NET implementation of the XML DOM.)
The following steps walk you through creating the demo application:
- Create an MFC dialog-based application called VideoInfo.
- From the Project Settings, specify the option Use Managed Extensions.
- Open the stdafx.h file and add the following to the end of the file:
... #using <mscorlib.dll> #using <System.Windows.Forms.dll> #using <System.dll> #using <System.xml.dll> #using <Microsoft.VisualBasic.dll> using namespace System; using namespace System::Windows::Forms; using namespace System::Xml; using namespace System::IO; using namespace Microsoft::VisualBasic; #undef MessageBox
(You’ll see shortly the purpose for the Microsoft.VisualBasic assembly.)
- Open the dialog template resource and add the controls as you see them in Figure 1.
- Hard code two entries into the Format combobox control’s Data property: “DVD;VHS” (without the quotes).
- Set the Actors listbox Sort property to True.
- Set up the DDX variables as listed in Table 1.
Control Variable Type Variable Name Format edit CString m_strFormat Title edit CString m_strTitle Director edit CString m_strDirector Actors listbox CListBox m_lbxActors Studio edit CString m_strStudio Release edit int m_iReleaseYear Runtime edit int m_iRuntime Table 1: DDX Variables to Be Used in Demo
- Add a dialog member variable that will hold the value of the application folder:
class CVideoInfoDlg : public CDialog { ... CString m_strWorkingDir; - Add the following code to set the m_strWorkingDir member just before the end of the dialog’s OnInitDialog function:
TCHAR buff[MAX_PATH]; GetModuleFileName(NULL, buff, MAX_PATH); m_strWorkingDir = Path::GetDirectoryName(buff);
- As the dialog box’s listbox and Add button indicate, the application allows for the entry of multiple actors to each video definition. If you’re a veteran MFC developer, you’re accustomed to having to create a dialog box anytime a value is needed from the user. However, since this is a mixed-mode application (giving access to the full .NET class library) and Microsoft ported the VB 6 InputBox function for backwards compatibility between VB 6 and VB.NET, you also have access to it from C++. Therefore, you can obtain a single value from the user with one line of code.
Add an event handler for the dialog box’s Add button and code it as follows:
void CVideoInfoDlg::OnBnClickedAddActor() { CRect rect; GetWindowRect(&rect); String* actor = Interaction::InputBox(S"Enter the Actor name and click the OK button:", S"New Actor", S"", rect.left + (rect.Width() / 2), rect.top + (rect.Height() / 2)); if (actor->Length) { if (LB_ERR == m_lbxActors.FindStringExact(-1, (CString)actor)) m_lbxActors.AddString((CString)actor); else MessageBox::Show(S"That actor is already listed"); } } - Now add the following function to allow the removal of the currently selected actor in the listbox.
Note:The Add/Remove Actor functions have nothing to do with XML, but they’re small bits of code that help to create a more practical application from which to learn XML.
void CVideoInfoDlg::OnBnClickedRemoveActor() { int iCurrSel; if (LB_ERR == (iCurrSel = m_lbxActors.GetCurSel())) { MessageBox::Show(S"Please select an actor to remove"); } else { m_lbxActors.DeleteString(iCurrSel); } } -
Add an event handler for the dialog box’s Save button. In a real-world application, you would include much more error handling and data validation. However, this example focuses on the XML.
After calling UpdateData, the function builds the file name using the video title and format. An XMLTextWriter object is then instantiated and several of its methods are used to perform such tasks as starting the document and writing elements and attributes. Note how in some cases the WriteElementString is used and in other situations where more information is needed—such as when writing nested elements or elements with attributes—the WriteStartElement is first called, the addition information is written, and then the WriteEndElement function is called to terminate that element:
void CVideoInfoDlg::OnBnClickedSave() { #pragma push_macro("new") #undef new if (UpdateData()) { XmlTextWriter* xmlwriter; try { String* fileName = String::Format(S"{0}-{1}.xml", (String*)m_strTitle, (String*)m_strFormat); fileName = Path::Combine(m_strWorkingDir, fileName); xmlwriter = new XmlTextWriter(fileName, Text::Encoding::ASCII); xmlwriter->WriteStartDocument(true); xmlwriter->WriteStartElement(S"VideoInfo"); xmlwriter->WriteAttributeString(S"Format", m_strFormat); xmlwriter->WriteElementString(S"Title", m_strTitle); xmlwriter->WriteElementString(S"Director", m_strDirector); xmlwriter->WriteStartElement(S"Actors"); for (int i = 0; i < m_lbxActors.GetCount(); i++) { CString strActor; m_lbxActors.GetText(i, strActor); xmlwriter->WriteElementString(S"Actor", strActor); } xmlwriter->WriteEndElement(); // end of Actors xmlwriter->WriteElementString(S"Studio", m_strStudio); xmlwriter->WriteElementString(S"ReleaseYear", m_iReleaseYear.ToString()); xmlwriter->WriteElementString(S"Runtime", m_iRuntime.ToString()); xmlwriter->WriteEndElement(); // end of VideoInfo xmlwriter->WriteEndDocument(); MessageBox::Show(S"Video information successfully saved"); } catch(Exception* e) { MessageBox::Show(e->Message); } __finally { xmlwriter->Flush(); xmlwriter->Close(); } } #pragma pop_macro("new") } -
Now you add the function to open an existing video-information file. Add an event handler for the Open button and code it as follows.
The function begins by displaying a dialog box in order for the user to specify a file to open. An XMLTextReader object is instantiated to read that file’s nodes. A basic while loop is used, where the XMLTextReader::Read method is called until it returns false (indicating end of file).
Within the loop, the XmlTextReader::NodeType property is inspected to ensure that only elements nodes (XmlNodeType::Element) are processed. If the node type is an element, the function then determines if the current element is one whose value needs to be retrieved for display on the dialog box. If it is, the value is retrieved using the XmlTextReader::ReadString method. Since the Format value is stored as an attribute of the VideoInfo element, the XmlTextReader::GetAttribute method is used to obtain that value if the current element’s Name value is equal to “VideoInfo”:
void CVideoInfoDlg::OnBnClickedOpen() { #pragma push_macro("new") #undef new CFileDialog dlg(TRUE); dlg.m_pOFN->lpstrInitialDir = m_strWorkingDir; dlg.m_pOFN->lpstrTitle = _T("Open a VideoInfo XML File"); dlg.m_pOFN->lpstrFilter = _T("XML Files (*.xml) *.xml "); if (IDOK == dlg.DoModal()) { m_lbxActors.ResetContent(); XmlTextReader* xmlreader; try { xmlreader = new XmlTextReader(dlg.GetPathName()); while (xmlreader->Read()) { if (XmlNodeType::Element == xmlreader->NodeType) { CString name = (CString)xmlreader->Name; if (0 == name.CompareNoCase(_T("VideoInfo"))) m_strFormat = (CString)xmlreader->GetAttribute(S"Format"); else if (0 == name.CompareNoCase(_T("Title"))) m_strTitle = (CString)xmlreader->ReadString(); else if (0 == name.CompareNoCase(_T("Director"))) m_strDirector = (CString)xmlreader->ReadString(); else if (0 == name.CompareNoCase(_T("Actor"))) m_lbxActors.AddString((CString)xmlreader->ReadString()); else if (0 == name.CompareNoCase(_T("Studio"))) m_strStudio = (CString)xmlreader->ReadString(); else if (0 == name.CompareNoCase(_T("ReleaseYear"))) m_iReleaseYear = Convert::ToInt32(xmlreader->ReadString()); else if (0 == name.CompareNoCase(_T("Runtime"))) m_iRuntime = Convert::ToInt32(xmlreader->ReadString()); } } UpdateData(FALSE); } catch(Exception* e) { MessageBox::Show(e->Message); } __finally { xmlreader->Close(); } } #pragma pop_macro("new") } - Finally, build and test the application. You should have a fully functional application that uses the .NET XML classes to store and retrieve data. Figure 2 shows an example of running this application with information from the famous Reservoir Dogs movie displayed.
Figure 2: Example Running of the Demo Application.Figure 3 shows the same video file’s information as displayed in Internet Explorer.
Figure 3: You Can Open the File Using Any XML Parser (including Internet Explorer)
Looking Ahead
As previously mentioned, the XmlTextWriter/XmlTextReader classes are used in situations where your application’s design allows for the writing or reading of an XML file’s nodes in sequential order. While this works fine for certain scenarios, there are many situations where your application design or data schema require that you be able to randomly read or update a specific node. An obvious example is if your data is being stored in a large file where writing or reading every node would be impractical and inefficient. For those scenarios, there is the DOM. The .NET class library implements the DOM via the XmlDocument class that upcoming articles will cover.
Download the Code
To download the accompanying source code for the demo, click here.
About the Author
Tom Archer owns his own training company, Archer Consulting Group, which specializes in educating and mentoring .NET programmers and providing project management consulting. If you would like to find out how the Archer Consulting Group can help you reduce development costs, get your software to market faster, and increase product revenue, contact Tom through his Web site.
