LINQ in .NET framework

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Application Diagram Remote Database
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Figure 711: Sample Application Diagram: Hybrid Version
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objects change, as they are now created from data rows rather than from a data reader, and they store a reference to the row rather than its data For instance, the Order constructor for creating an Order from a data row is as follows:
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public Order(DataRow rowThis) { thisrowThis = rowThis; }
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And the constructor creating a new order, which must create a new data row and then fill it with default values, is as follows:
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public Order() { DataTable myTable = AppDataCurrentdsetEmployeeTables["Orders"]; rowThis = myTableNewRow(); thisOrderID = AppDataCurrentOrdersValues Max(order => orderOrderID) + 1; thisEmployeeID = AppDataCurrentEmployeeID;
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72 The Sample Application
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thisCustomerID = stringEmpty; thisOrderDate = OrderdefaultOrderDate; thisRequiredDate = OrderdefaultRequiredDate; thisShippedDate = OrderdefaultShippedDate; }
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And the OrderDate read-write property, for instance, backed by the matching field in the data row, becomes the following:
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public DateTime OrderDate { get { return (DateTime)rowThis["OrderDate"]; } set { AppDataUtilValidateValueProperty( "OrderDate", value, DateTimeTodayAddYears(-100), DateTimeToday); rowThis["OrderDate"] = value; } }
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The read-only properties of the Order class, which use LINQ to select information from the application s collections, such as OrderNetValue, remain unchanged, as shown here:
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public decimal NetValue { get { return thisDetailsSum(detail => detailNetValue); } }
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All serialization code, and all change tracking/data source update code, is gone, replaced by the capabilities inherent in the data set, data table, and data adapter classes Certainly, we have been able to achieve the best of both worlds: ADONET for retrieving, tracking, and persisting data, and business objects for validation, encapsulation, and use of the user interface with that data All we need now is a nice, flexible way to send, receive, and store this information as XML to, from, and on our device
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LINQ
73 LINQ to XML
Since the earliest planning stages of NET, Microsoft has been authoring classes to provide XML support for developers, introducing new classes, and refining old ones to keep pace with the ever-evolving World Wide Web Consortium specifications With the release of NET 20, developers were presented with an enormous amount of XML support, as Table 73 illustrates This raises the obvious question: Why add even more XML support The answer is this: Microsoft is committed to providing LINQ support for collections of objects, including collections of XML objects; that is, support for XML documents And since an XML document is a collection of XML objects attributes, elements, directives, comments, and so on LINQ to XML became a highly desirable feature, and highly desirable features make it into the released product
TABLE 73: XML Support in NET 20
Feature
Classes
Comments
Readers and writers
XmlReader, XmlWriter
Reads/writes XML to and from a stream one node at a time Tedious to program for Used by other classes to retrieve and store XML Warns of, or rejects, incoming XML that does not comply with the specified schema Holds an entire XML document or fragment in memory Provides XPath querying of the document May provide for modification of document contents Provides for the transformation of an XML document into XML of a different format or into HTML or into any text format Provides for object content to be serialized/deserialized to and from a stream
Schema validation
XmlReader
Document objects
XmlDocument or XPathDocument, plus various
node classes and other supporting classes
XSLT transformations
XslTransform
XML serialization
Generic classes
73 LINQ to XML
Unlike LINQ to Datasets, which did not require the development of new ADONET classes, LINQ to XML did require that new XML classes be written If you think back to our discussion of LINQ to Datasets, you will remember that our LINQ queries were performed against data tables, the same DataTable class that existed in NET 20 The simple rectangular structure of a DataTable combined with its already existing implementation of IEnumerable (actually, its implementation of the even more LINQable IListSource interface) made it a natural for LINQ implementation The same cannot be said for the XmlDocument and XPathDocument classes First, their hierarchical structure is more complex than relational, preventing them from providing a generic enumeration And second, their specific enumerations have already been defined and not by Microsoft These enumerations have been defined by the World Wide Web Consortium, and developers know them as axes, such as Descendant, Child, and Ancestor-or-self Therefore, new classes that paralleled the old XML document and XPath classes and that provided LINQ syntax for making XPath selections were added to NET These classes, referred to as the X classes, duplicate some capabilities that already existed in NET 20, such as loading an XML document from a stream into a document object, saving an XML document from a document object to a stream, and querying/modifying the nodes of a document What the X classes add to NET is LINQ syntax for specifying XPath queries and very flexible constructors That might not seem like much of an addition, but it is To see why, consider that XDocuments and XElements can be constructed from the following:
An incoming XML stream Literal values An object A collection of objects A collection of objects that each contains collections of objects The output from a LINQ to Objects query
It is a somewhat deliberately redundant list, but it all comes down to this: It is easier than it has ever been for a smart-device application to selectively gather and arrange any information that exists anywhere in that application and deliver it to a remote recipient, or to the device store as XML in any format It is a strong claim, so we best support it with some code