Applied XML Programming for Microsoft .NET.pdf - Csbdu.in

When the tables are serialized, the Command slot is rendered as follows:ExtendedProperties holds a collection of objects and can accept values of any type, butyou might run into trouble if you store values other than strings there. When the objectis serialized, any extended property is serialized as a string. In particular, the string iswhat the object's ToString method returns. This can pose problems when the DataSetobject is deserialized.Not all types can be successfully and seamlessly rebuilt from a string. For example,consider the Color class. If you call ToString on a Color object (say, Blue), you getsomething like Color [Blue]. However, no constructor on the Color class can rebuild avalid object from such a string. For this reason, pay careful attention to the nonstringtypes you store in the ExtendedProperties collection.Rendering Data RelationsA DataSet object can contain one or more relations gathered under the Relationscollection property. A DataRelation object represents a parent/child relationship setbetween two DataTable objects. The connection takes place on the value of a matchingcolumn and is similar to a primary key/foreign key relationship. In ADO.NET, therelation is entirely implemented in memory and can have any cardinality: one-to-one,one-to-many, and even many-to-one.More often than not, a relation entails table constraints. In ADO.NET, you have twotypes of constraints: foreign-key constraints and unique constraints. A foreign-keyconstraint denotes an action that occurs on the columns involved in the relation when arow is either deleted or updated. A unique constraint denotes a restriction on the parentcolumn whereby duplicate values are not allowed. How are relations rendered in XML?If no schema information is required, relations are simply ignored. When a schema isnot explicitly required, the XML representation of the DataSet object is a plain snapshotof the currently stored data; any ancillary information is ignored. There are two ways toaccurately represent a DataRelation relation within an XML schema: you can use the annotation or specify an element. The WriteXmlprocedure uses the latter solution.The msdata:Relationship AnnotationThe msdata:Relationship annotation is a Microsoft XSD extension that ADO.NET andXML programmers can use to explicitly specify a parent/child relationship between nonnestedtables in a schema. This annotation is ideal for expressing the content of aDataRelation object. In turn, the content of an msdata:Relationship annotation istransformed into a DataRelation object when ReadXml processes the XML file.Let's consider the following relation:DataRelation rel = new DataRelation("Emp2Terr",ds.Tables["Employees"].Columns["employeeid"],ds.Tables["Territories"].Columns["employeeid"]);ds.Relations.Add(rel);The following listing shows how to serialize this relation toXML:335