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string GetAttribute(int i); string GetAttribute(string name, string namespaceURI); string GetAttribute(string name); bool HasValue
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string LookupNamespace(string prefix); bool MoveToAttribute(string name, string ns); bool MoveToAttribute(string name); bool MoveToElement(); bool MoveToFirstAttribute(); bool MoveToNextAttribute(); XmlNameTable NameTable
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APPEN DIX A CRE ATING CUS TOM XMLRE ADER A ND XMLWRITER C LAS SES
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abstract string NamespaceURI
abstract XmlNodeType NodeType
abstract string Prefix
abstract bool Read(); abstract bool ReadAttributeValue(); abstract ReadState ReadState
abstract void ResolveEntity(); abstract string Value
You can override these properties and methods and write your own data-manipulation logic. If you do not want to override a particular property or method, you still need to have its empty implementation. A better way is to throw an exception in such properties and methods so that the caller knows that these properties and methods are not implemented by you. I will not discuss every property here because you are already familiar with many of them (see 3 for more information).
Creating a TableReader Class
Now that you are familiar with the XmlReader abstract class, let s create our own implementation. To do so, create a new project of type class library by using Visual Studio. Add a class named TableReader. Make sure that references to the System.Xml and System.Data assemblies are added to the project. Import the namespaces as shown in Listing A-2 at the top of the TableReader class and ensure that the TableReader class inherits from the XmlReader class. Listing A-2. Importing Namespaces and Setting Inheritence using System.Xml; using System.Data; using System.Data.OleDb; class TableReader:XmlReader { ...
AP P END I X A CREA TI NG C UST O M X MLREA D ER AN D XM LWRI T ER C LA SS ES
You need to add an implementation of each property and method mentioned. Visual Studio provides a shortcut for adding empty implementations of these members. Right-click on the XmlReader class in the class definition and choose the Implement Abstract Class menu option (Figure A-1).
Figure A-1. Adding empty implementations of properties and methods This will add dummy signatures of all the properties and methods that need to be overridden. Notice how the dummy implementation throws an exception by using the throw keyword. This way, if somebody tries to use unimplemented members, an exception will be thrown indicating that the method or operation is not implemented. Code the TableReader class as shown in Listing A-3. Listing A-3. The TableReader Class public class TableReader:XmlReader { private OleDbConnection cnn; private OleDbCommand cmd; private OleDbDataReader reader; private int intColumnIndex = -1; private string strValue; public TableReader(string connectionString,string tableName) { cnn = new OleDbConnection(connectionString); cmd = new OleDbCommand(); cmd.Connection = cnn; cmd.CommandText = tableName; cmd.CommandType = CommandType.TableDirect; cnn.Open(); reader = cmd.ExecuteReader(); }
APPEN DIX A CRE ATING CUS TOM XMLRE ADER A ND XMLWRITER C LAS SES
public override int AttributeCount { get { return reader.FieldCount; } } public override void Close() { reader.Close(); cnn.Close(); } public override int Depth { get { return reader.Depth; } } public override string GetAttribute(int i) { return reader.GetValue(i).ToString(); } public override string GetAttribute(string name) { return reader.GetValue(reader.GetOrdinal(name)).ToString(); } public override bool MoveToAttribute(string name) { intColumnIndex = reader.GetOrdinal(name); return true; } public override bool MoveToElement() { intColumnIndex = -1; return true; }
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public override bool MoveToFirstAttribute() { intColumnIndex = 0; return true; } public override bool MoveToNextAttribute() { intColumnIndex++; if (intColumnIndex > reader.FieldCount - 1) { return false; } else { return true; } } public override bool Read() { intColumnIndex = -1; strValue = ""; return reader.Read(); } public override bool HasValue { get { return reader.IsDBNull(intColumnIndex); } } public override bool ReadAttributeValue() { if (intColumnIndex < reader.FieldCount) { strValue = reader.GetValue(intColumnIndex).ToString(); return true; } else { return false; } }
APPEN DIX A CRE ATING CUS TOM XMLRE ADER A ND XMLWRITER C LAS SES
public string Name { get { if (intColumnIndex == -1) { return cmd.CommandText; } else { return reader.GetName(intColumnIndex); } } } public override string Value { get { return strValue; } } ... } In the following text, we will dissect the code step by step.
Declaring Class-Level Variables
private private private private private OleDbConnection cnn; OleDbCommand cmd; OleDbDataReader reader; int intColumnIndex = -1; string strValue;
The TableReader class declares private variables of type OleDbConnection, OleDbCommand, and OleDbDataReader classes at the class level: The OleDbConnection class is used to establish a connection with OLEDB-compliant databases such as Access. The OleDbCommand class is used to execute any query, SQL query, or stored procedures against a database. The OleDbDataReader class allows you to iterate through a result set in a cursor-oriented manner. The intColumnIndex integer variable keeps track of the current column index whose value is to be read. Similarly, the strValue string variable stores the value from the column indicated by intColumnIndex.
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