c# code 39 checksum Possible Values of ValidationType in C#.NET

Maker Code 39 in C#.NET Possible Values of ValidationType

Table 5-3. Possible Values of ValidationType
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None Auto DTD Schema XDR
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No validation will be performed. This is the default. Automatically decides whether to validate against a DTD or schema by observing the XML document. Validation will be performed against a DTD. Validation will be performed against an XSD schema. Validation will be performed against an XDR schema.
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To trap the validation errors, the XmlReaderSettings class raises a ValidationEventHandler event. This event is raised only when the ValidationType property is other than None. The signature of the event-handler function (OnValidationError()in our example) must match the one shown here: void OnValidationError(object sender, ValidationEventArgs e) { MessageBox.Show(e.Message); } The event handler receives a ValidationEventArgs object as an event argument, which allows you to examine the underlying exception. You can get the descriptive error message by using the Message property as we do in our example. In this case, we simply display a message box with the validation error message. The code from Listing 5-15 then creates an instance of the XmlReader class by calling its Create() static method. The URL of the XML document and the instance of XmlReaderSettings are the arguments. A while loop then reads the XML document. If any validation error is detected during this reading operation, the ValidationEventHandler event is raised. We could have placed code to read the element and attribute values inside the while loop if required (refer to 3 for information about reading XML documents by using the XmlReader class). Finally, the reader is closed.
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To test the preceding code, you need to run the application and supply the full path and filenames of an XML document and a DTD or schema. You can use the same Employees.xml file that we have used throughout this chapter. We also created a DTD and an XSD schema for Employees.xml previously. After you click the Validate button, the XmlReader will attempt to validate the XML document and notify you of any validation errors. Figure 5-17 shows a message box generated after deliberately removing the required attribute employeeid.
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Figure 5-17. Detecting a validation error
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Using XmlDocument to Validate XML Documents Being Loaded
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You are not limited to the XmlReader approach to validate your XML documents; you can also use XmlDocument to validate them. This is useful when you are modifying documents and want to ensure that the new data is consistent with the underlying schema or DTD. The XmlDocument class allows you to validate XML documents in two ways: You can validate the document while it is being loaded by the XmlDocument class. You can validate the document explicitly when you perform any modification on it such as adding or removing nodes. In the following example, you will learn how both of the preceding approaches can be used. We will modify the same example that we developed in the Modifying XML Documents section of 2. Figure 5-18 shows the user interface of the application.
Figure 5-18. Application for validating XML documents by using XmlDocument
CHAPTER 5 VA LIDATIN G XML DOCUMEN TS
Because we have already dissected the complete code in 2, I will not discuss it again here. I will discuss only the modifications that are necessary to validate XML documents. Previously in this section, it was mentioned that XmlDocument allows you to validate XML documents when they are being loaded. This is accomplished by passing a validating reader to the Load() method of the XmlDocument class. Listing 5-16 shows the modified version of the Form_Load event handler. Listing 5-16. Validating an XML Document When It Is Being Loaded private void Form1_Load(object sender, EventArgs e) { XmlReaderSettings settings = new XmlReaderSettings(); settings.ValidationType = ValidationType.Schema; settings.Schemas.Add("", Application.StartupPath + @"\employees.xsd"); settings.ValidationEventHandler += new ValidationEventHandler(OnValidationError); XmlReader reader = XmlReader.Create(Application.StartupPath + @"\employees.xml", settings); doc.Load(reader); reader.Close(); foreach (XmlNode node in doc.DocumentElement.ChildNodes) { comboBox1.Items.Add(node.Attributes["employeeid"].Value); } FillControls(); } Notice the code marked in bold. This code should be familiar to you, because we discussed it in the earlier sections of this chapter: it essentially creates an XmlReaderSettings object and configures it to validate Employees.xml against Employees.xsd. The ValidationEventHandler event is handled by the OnValidationError() method. The XmlReaderSettings object is then passed to the Create() method of the XmlReader class to get an XmlReader object. The Load() method of XmlDocument accepts the newly created XmlReader object as a parameter, internally iterates through the XmlReader, and validation takes place. If there are any validation errors, the OnValidationError() method gets called. Now comes the tricky part. The XmlDocument class allows you to modify the document. Thus a document can be valid when loaded but can become invalid after modification. For example, as per our schema, the telephone number cannot be greater than 20 characters. The user of the form can, however, ignore this restriction, and the loaded document can now have invalid data. This makes it necessary to revalidate the changes made to the document. Fortunately, the XmlDocument class provides a method called Validate() that does the job. Listing 5-17 shows the use of Validate() during the update operation.
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