IMPLEMENTING LDIM interface, or a Mapfunction (a function

IMPLEMENTING LDIM interface, or a Mapfunction (a function that takes a UDT instance and returns an orderable scalar value). SQL Server UDTs only sup port ordering by state values. Note that this is done, as explained previously, only if the SqlUserDefinedAttributeproperty ByteOrderedis truefor the class that implements the user- defined type. XML Serialization The FOR XML clause of a SELECT statement that includes a user-defined type and DataSetsthat include user-defined types depend on those types being XML serializable. Though a user-defined type is not required to be XML serializable, if it is not, it may produce errors or corrupt data when used with FORXMLand DataSets. It is best practice to ensure that the class you implement is XML serializable. An explanation of the requirements for XML serializability is beyond the scope of this book. Typically, it is accomplished by having the class that implements the user-defined type also implement the IXmlSerializable interface. Refer to the XmlSerializer class in MSDN for details of XML serialization. Maintaining User-Defined Type Definitions There are two ways to replace a user-defined type. One is to use DROP TYPE followed by CREATE TYPE. The other is to use the ALTER ASSEMBLY command to replace the assembly that contains the implementation of the user-defined type. Changing a user-defined type is a fundamental change to a database. Think about how you would feel if Microsoft said they were going to change how the SQLDECIMALdata type worked, even if they said it would just improve the accuracy of decimal. The fact that calculations you had done in the past might produce different, albeit more accurate, results could easily have an overwhelming impact on your database because things that compared as equal in the past might not do so after the change. You must keep this level of impact in mind whenever you make a change to a user-defined type. The safest way to change user-defined type is to export tables that refer to the type to a text file using SQL Server Data Transformation Services (DTS), drop and re-create the type and everything that references it, and then reload the tables. You will not miss anything that refers to the user- defined type, because you cannot drop a type if any table, stored procedure,

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