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

Although this solution is clearly not optimal, because unchanged columns are storedtwice, it closely reflects the internal architecture of the DataRow object and, as such,speeds up the restoration of the DataRow object in the destination DataTable object.NoteThe DataRow class maintains its various versions by implementingan array of subobjects—one for the current values, one for theoriginal version, and one for intermediate proposed values. Otherinternal properties indicate at any moment which is the currentversion and what the state of the row is.As a final note, consider that for each column in a DataRow object, only the original andthe current values are tracked, and no intermediate values are buffered. For example,suppose that you perform the following operation on an unchanged row:// 1 is the current value of the fieldrow[0][field] = 2;The row state changes to Modified, the original value (1) is persisted in the Originalcopy of the row, and the new value (2) is registered as the current value. Next thefollowing code runs:// 2 is the current value of the field// 1 is the original value of the fieldrow[0][field] = 3;// 3 is NOW the current value of the field// 1 is the original value of the fieldThe original copy of the row remains intact, but the current version is updated. As aresult, the intermediate value (2) is overwritten and is irreversibly lost.NoteBuilding an automatic mechanism for tracking the entire history of arow is probably unnecessary in most cases. If you need a morepowerful mechanism to track changes, you can build a parallel tableof changes for each row in the table. Each entry in the custom tablewould point to a particular DataRow object and contain a collectionof changes organized as you prefer.Tracking Pending ErrorsThe DataRow class provides a few methods for handling row errors. You can set ageneral error message on the entire row, and you can set a column-specific message.To set a general error message, you use the RowError property. To set a columnspecificmessage, you use the pair of methods SetColumnError and GetColumnError.Other helper methods available are GetColumnsInError and ClearErrors.A column or row with an error is in no way different from a column or row withoutpending errors. In this context, an error is simply a description of contents that the user,or the application, finds erroneous and inconsistent. Nothing prevents you from usingerror properties as general-purpose cargo variables in which to store custominformation and annotations.NoteIf you choose to use error properties as general-purpose cargovariables, keep in mind that some advanced Windows Forms andWeb Forms controls can, in the presence of error flags, refresh theirown user interfaces accordingly. For example, the Windows Forms370