N ot e in VB.NET

Printing ECC200 in VB.NET N ot e

Opt im ist ic and pessim ist ic concur rency are t wo cont r ast ing ways of m anaging dat a m anipulat ion in a m ult iuser env ironm ent . Wit h pessim ist ic concurr ency , a lock applies t o a row as soon as a user signals t he st art of a dat a m anipulat ion t ask for a r ow. This lock doesn t release unt il t he com plet ion of t he t ask. Wit h opt im ist ic concur rency, no lock s go on a row aft er t he st ar t of a dat a m anipulat ion t ask. Ther efore, anot her user can change dat a before t he current user com m it s a change. I f t he dat a does change before t he com m it m ent of a change by t he curr ent user , t he dat abase server r aises a concurrency v iolat ion w hen t he curr ent user at t em pt s t o com m it t he change. Opt im ist ic concur rency scales bet t er t han pessim ist ic concurr ency. Ther efor e, opt im ist ic concurr ency is m or e suit able for m ult iuser applicat ions par t icularly if t he applicat ions serve m any users.

ADO.NET pr ov ides t wo t echniques for handling except ions result ing fr om opt im ist ic concur r ency v iolat ions. First , y ou can creat e an event procedure for t he RowUpdat ed ev ent . ADO.NET raises t his ev ent aft er each at t em pt t o change a rem ot e dat a source based on a m odified row fr om a local dat a set . Wit h a RowUpdat ed ev ent pr ocedur e, y ou can pr ocess except ions as t hey occur for each row . Second, y ou can set t he Cont inueUpdat eOnEr ror propert y of t he SqlDat aAdapt er obj ect t o Tr ue before inv ok ing t he Updat e m et hod. This causes ADO.NET t o com plet e all v alid updat es and wr it e any err or m essages t o t he local dat a set so t hat you can respond t o t hem aft er t he Updat e m et hod t erm inat es.

The Dat aSet is a m em or y- resident obj ect t hat can cont ain one or m ore t ables and relat ionships bet w een t ables. This m em ory - r esident obj ect and it s child obj ect s m ake up t he disconnect ed dat a source t hat is t he cent erpiece of t he ADO.NET archit ect ure. Figur e 10- 1 present s an overv iew of t he Dat aSet obj ect m odel. The balance of t his sect ion describes select ed com ponent s w it hin t hat m odel. Figu re 1 0 - 1 . Th e D at a Se t ob j e ct m ode l.

ADO.NET r efers t o each indiv idual t able w it hin a dat a set as a Dat aTable obj ect . The collect ion of all t ables wit hin a dat a set is t he Dat aTableCollect ion class. The t ables w it hin a dat a set can r elat e hierarchically t o one anot her . This m akes it possible for y ou t o r epr esent t he schem a for t he t ables in t he Nort hw ind dat abase wit hin a dat a set . The Recordset obj ect from classic ADO doesn t direct ly represent hierarchical r elat ionships. I nst ead, classic ADO flat t ens t he relat ionships bet w een t ables int o a single rowset . Dat aTable obj ect s consist of Dat aColum n and Dat aRow obj ect s. The set of dat a colum ns w it hin a Dat aTable obj ect is t he Dat aColum nCollect ion obj ect . The Dat aColum nCollect ion class defines t he schem a for a Dat aTable obj ect . For exam ple, t he indiv idual Dat aColum n obj ect s specify a dat a t ype t hat each colum n can cont ain. Dat aTable obj ect s can hav e a Prim ary Key pr opert y. You can define t his propert y wit h a Dat aColum n ar ray t hat can cont ain one or m or e Dat aColum n obj ect s. The Dat aRowCollect ion class repr esent s all rows w it hin a Dat aTable. Colum n values for a local t able r eside w it hin t he Dat aRow obj ect s t hat m ake up t he Dat aRow Collect ion obj ect for a Dat aTable obj ect . I nvok e t he New Row m et hod for a Dat aTable t o creat e a new row. You can t hen assign colum n values t o t he

row and add t he new Dat aRow obj ect t o t he Dat aRow Collect ion obj ect for a Dat aTable obj ect . ADO.NET specifies t he r elat ionship bet w een t ables in a dat a set w it h a Dat aRelat ion obj ect . The set of all Dat aRelat ion obj ect s wit hin a dat a set is a Dat aRelat ionCollect ion obj ect . Addit ionally, you can specify const raint s for t ables wit h t he Const raint class. Wit h a Const raint obj ect , you can specify unique or for eign k ey const raint s. A Dat aRelat ion obj ect bet ween t w o t ables denot es a parent - child relat ionship bet w een t he t ables. When y ou creat e a Dat aRelat ion obj ect bet w een t w o t ables, ADO.NET aut om at ically cr eat es a for eign k ey const raint in t he child t able and a unique const r aint on t he pr im ary k ey in t he parent t able. A Dat aRelat ionCollect ion class for a Dat aSet obj ect cont ains each of t he Dat aRelat ion obj ect s in a dat a set . You can access Dat aRelat ion obj ect m em bers t hrough t he ChildRelat ions and Parent Relat ions propert ies of indiv idual Dat aTable obj ect s. Ther e ar e t wo t echniques for elim inat ing a Dat aRow obj ect fr om t he Dat aRow Collect ion of a Dat aTable obj ect . The first of t hese t echniques is t he Delet e m et hod, w hich applies t o indiv idual Dat aRow obj ect s wit hin a Dat aTable. This t echnique assigns Delet ed t o t he Row St at e pr opert y for a Dat aRow, but it doesn t act ually r em ov e t he r ow fr om t he local t able. ( Your applicat ion can rest ore a delet ed r ow wit h t he Rej ect Changes m et hod for a Dat aRow obj ect .) Alt er nat ively, y ou can com m it t he applicat ion of a Delet e m et hod t o a r ow so t hat it isn t r ecov erable by invok ing t he Accept Changes m et hod for t he r ow. The second t echnique for elim inat ing a row from a local t able is t o invok e t he Rem ove m et hod for t he Dat aRow Collect ion obj ect wit hin a Dat aTable obj ect . This m et hod requir es t hat you specify t he index for t he r ow t hat y ou want t o elim inat e. I ndex values st art wit h 0 and progress by 1 for each Dat aRow obj ect wit hin a Dat aTable unt il 1 m inus t he count of rows wit hin t he Dat aTable. This second t echnique doesn t allow you t o rest ore a row.