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CHAPTER 10 MANAGED PROVIDERS OF DATA ACCESS
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pm.SourceVersion = DataRowVersion.Current; pm.SourceColumn = "city"; cm.Parameters.Add(pm); pm = new SqlParameter ("@state", SqlDbType.Char, 2); pm.SourceVersion = DataRowVersion.Current; pm.SourceColumn = "state"; cm.Parameters.Add(pm); pm = new SqlParameter ("@zip", SqlDbType.Char, 5); pm.SourceVersion = DataRowVersion.Current; pm.SourceColumn = "zip"; cm.Parameters.Add(pm); pm = new SqlParameter ("@contract", SqlDbType.Bit); pm.SourceVersion = DataRowVersion.Current; pm.SourceColumn = "contract"; cm.Parameters.Add(pm); pm = new SqlParameter ("@org_au_id", SqlDbType.VarChar, 11); pm.SourceVersion = DataRowVersion.Original; pm.SourceColumn = "au_id"; cm.Parameters.Add(pm); pm = new SqlParameter ("@org_au_lname", SqlDbType.VarChar, 40); pm.SourceVersion = DataRowVersion.Original; pm.SourceColumn = "au_lname"; cm.Parameters.Add(pm); pm = new SqlParameter ("@org_au_fname", SqlDbType.VarChar, 20); pm.SourceVersion = DataRowVersion.Original; pm.SourceColumn = "au_fname"; cm.Parameters.Add(pm); pm = new SqlParameter ("@org_phone", SqlDbType.Char, 12); pm.SourceVersion = DataRowVersion.Original; pm.SourceColumn = "phone"; cm.Parameters.Add(pm); pm = new SqlParameter ("@org_address", SqlDbType.VarChar, 40); pm.SourceVersion = DataRowVersion.Original; pm.SourceColumn = "address"; cm.Parameters.Add(pm); pm = new SqlParameter ("@org_city", SqlDbType.VarChar, 20); pm.SourceVersion = DataRowVersion.Original; pm.SourceColumn = "city";
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cm.Parameters.Add(pm); pm = new SqlParameter ("@org_state", SqlDbType.Char, 2); pm.SourceVersion = DataRowVersion.Original; pm.SourceColumn = "state"; cm.Parameters.Add(pm); pm = new SqlParameter ("@org_zip", SqlDbType.Char, 5); pm.SourceVersion = DataRowVersion.Original; pm.SourceColumn = "zip"; cm.Parameters.Add(pm); pm = new SqlParameter ("@org_contract", SqlDbType.Bit); pm.SourceVersion = DataRowVersion.Original; pm.SourceColumn = "contract"; cm.Parameters.Add(pm); da.UpdateCommand = cm; da.Update(ds); return true; } Notice how you re mapping columns of the DataTable to parameters of the CommandText using the SourceColumn and SourceVersion attributes of the parameter object. For the parameters in the "set" portion of the SQL statement, you re pulling the current values from the DataTable. For columns in the where clause, you re pulling the original values from the DataTable. Realize that "select * from authors" is the SelectCommand of the DataAdapter. What you re building with this code is the UpdateCommand of the adapter, which you set just before calling the Update method. Typically, when using an adapter for retrieving data from the database and later moving it back, you ll do a one-time creation of this adapter and cache it, as it s a lot of work. This code will also only work for updates to the DataSet; you ll also want to create and set an insert and delete command (for the InsertCommand and DeleteCommand properties of the adapter, respectively). When the user gets into a dirty read situation using this approach, the error displayed in Figure 10-7 occurs. In a full-blown application, this is the exception that would be caught and would kick off the rendering of whatever concurrency resolution interface was required by the business rules. This strategy is good when modifications cannot be made to the database in use. Be sure to build only columns that could change into the where clause. The disadvantage to this approach is the amount of code it takes and the amount of work that must be done at runtime, including marshaling of data across the wire and the complexity of the database queries.
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Figure 10-7. A concurrency error raised by the data adapter
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If you have no control over the database schema, but the overhead of the last approach is too much to bear, you can update and query on only those column values that have changed. Unfortunately, this approach requires dynamic generation of SQL statements, so it is generally frowned upon. For tables with high column counts, this approach may be worth the trade-off. There is another business rule that must be present before adopting this approach. When user A moves a change to "address" back to the database, it must be irrelevant if user B has changed "state" in the meantime. Using this approach, "address" will get updated with the new value from user A, but "state" will remain the value that user B set it to. While technically both changes were based on fresh data, the combination of values might be invalid. You could end up with an author from Green Bay, MN. (East coasters: Green Bay is in Wisconsin.) So, if your requirements can overlook all of the glaring deficiencies of this method, here s what it would look like (see Concurrency2.aspx in Web10). private bool UpdateAuthors(DataSet ds) { string sql = ""; string sqlBase = " update authors set {0} where au_id = '{1}'{2}"; string sqlUpdate = ""; string sqlWhere = "";
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