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SqlConnection cn = new SqlConnection( ConfigurationManager .ConnectionStrings["localPubs"].ConnectionString); SqlCommand cm = new SqlCommand("", cn); foreach (DataRow dr in ds.Tables[0].Rows) { sqlUpdate = ""; sqlWhere = ""; foreach(DataColumn dc in ds.Tables[0].Columns) { if (dr[dc, DataRowVersion.Current] != dr[dc, DataRowVersion.Original]) { sqlUpdate += string.Format("{0} = '{1}', ", dc.ColumnName, dr[dc]); sqlWhere += string.Format(" and {0} = '{1}'", dc.ColumnName, dr[dc, DataRowVersion.Original]); } } if (sqlUpdate.Length > 0) { sqlUpdate = sqlUpdate.Substring (0, sqlUpdate.Length - 2); sql += string.Format(sqlBase, sqlUpdate, dr["au_id", DataRowVersion.Original], sqlWhere); ; } } cm.CommandText = sql; cn.Open(); int updates = cm.ExecuteNonQuery(); cn.Close(); return true; } The next thing to do with this method is to compare the number of rows that were updated to the number of rows that have changed in the DataSet, to make sure no concurrency errors occurred (in the limited definition you ve implemented here). Then the DataSet would need to be refreshed by calling AcceptChanges. if (updates != ds.Tables[0].GetChanges().Rows.Count) return false; else { ds.AcceptChanges();
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return true; } AcceptChanges doesn t move any data to the database (only an adapter can do that); it simply copies current values over original values. The same null checking logic you saw earlier needs to be built into this method as well.
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Timestamp Column
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When you have control of the schema of the database, your best option for managing concurrency is to add a timestamp column to the table, and then build a check of the value into the update query. The database automatically updates the timestamp value whenever a change is made to the data. So as long as the timestamp value that was read still matches it when an update is attempted, you can rest assured that the data has not changed in the interim. A timestamp is binary data, so marshaling the value to and from a web browser requires some transformation along the way. You ll send the timestamp value to the browser in a hidden input. When the user sends changes back, you can retrieve it and use it in the where clause of your update. First, you ll need to modify the authors table and add a timestamp column to it. You ll do this on a copy of the authors table named authors_ts, as shown in Figure 10-8.
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Figure 10-8. The modified version of the authors table Here s the markup from the editing page. The user will pick an author for editing with the DropDownList. You re only creating an interface to edit the first and last name. Of course, usually you would create an interface for editing more of the data. What you re interested in for this demonstration, though, is the user control created just after the HTML table (see Concurrency3.aspx in Web10). <asp:Panel Runat=server ID=pnEdit Visible=false> <table> <tr> <td>First Name</td> <td> <asp:TextBox Runat=server ID=txtFirstName /> </td> </tr>
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<tr> <td>Last Name</td> <td> <asp:TextBox Runat=server ID=txtLastName /> </td> </tr> <tr><td colspan=2 align=center> <asp:Button Runat=server ID=btnSave Text='Save' OnClick="btnSave_Click" /> </td></tr> </table> <uc1:Timestamp ID="tsAuthor" Runat="server" /> </asp:Panel> <br /><br /><asp:Label Runat=server ID=lblOutput /> To facilitate reuse of this logic across several tables, you ll encapsulate the timestamp value in a user control. You can also do this with a custom control, making it usable across different applications. Here s the markup for the user control. (This code is in Timestamp.ascx of the Web10 project.) <%@ Control Language="C#" CodeFile="Timestamp.ascx.cs" Inherits="Timestamp_ascx" %> <asp:TextBox Runat=server ID=txtTimestamp Visible=false /> It gets included on your edit page with the following register directive from Concurrency3.aspx: <%@ Register TagPrefix="uc1" TagName="Timestamp" Src="Timestamp.ascx" %> You can see there s not much to this control. It will have no visible rendering behavior; you re really just using it to store a value in the ViewState. The timestamp will be tracked by the user control with a public property named TimestampValue. It s implemented thusly in Timestamp.ascx. public object TimestampValue { get { byte[] ba = new byte[8]; for(int indx = 0; indx < 8; indx++) { ba[indx] = Convert.ToByte( txtTimestamp.Text.Substring(indx * 3,2),16); } return ba; }
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