Checking for Concurrency Violations

The DataAdapter is used to fill the DataTable with both schema and data from table TBL0610 in the database.. The update command is defined for the DataAdapter using the timestamp column

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Recipe 6.10 Checking for Concurrency Violations

Problem

You need to check for concurrency violations

Solution

Use a timestamp column to manage data concurrency violations

The schema of table TBL0610 used in this solution is shown in Table 6-12

Table 6-12 TBL0610 schema

The sample code contains four event handlers and two methods:

Form.Load

Sets up the sample by creating two DataTable objects: A and B For each, a

DataAdapter is created and a handler is attached to handle the RowUpdated event The DataAdapter is used to fill the DataTable with both schema and data from

table TBL0610 in the database The update command is defined for the

DataAdapter using the timestamp column in the WHERE clause to catch

concurrency errors The first row from both tables is displayed on the form

simulating the view of two different simultaneous data users

DataAdapter.RowUpdated

Checks if the row was inserted or updated For those rows, the current value of the timestamp column is retrieved from the TBL0610 in the database and used to

update the row in the DataTable

Update( )

This method copies the specified value for Field1 from the text box into the

DataTable The DataAdapter is used to update DataTable changes to table

TBL0610 in the database If a DBConcurrencyException is encountered, the change to the row is rejected

DisplayRow( )

This method is used to display data from a specified row in DataTable A or B to

the corresponding user display on the form

Update Button.Click

Calls the Update( ) method to update changes made to the row in either DataTable

A or B The DisplayRow( ) method is called to update the corresponding row

displayed on the form

Refresh Button.Click

Gets the latest values for the row in DataTable A or B from table TBL0610 in the

database The DisplayRow( ) method is called to update the corresponding row displayed on the form

The C# code is shown in Example 6-28

Example 6-28 File: RowversionForm.cs

// Namespaces, variables, and constants

using System;

using System.Configuration;

using System.Windows.Forms;

using System.Text;

using System.Data;

using System.Data.SqlClient;

private DataTable dtA, dtB;

private SqlDataAdapter daA, daB;

private const String TABLENAME = "TBL0610";

//

private void RowversionForm_Load(object sender, System.EventArgs e)

{

// Build statements to select only the row for ID = 1

// and to update the data for the DataAdapter

String selectText = "SELECT Id, Field1, rowversion FROM " +

TABLENAME + " WHERE Id = 1";

String updateText = "UPDATE " + TABLENAME + " " +

"SET Field1 = @Field1 " +

"WHERE Id = 1 AND rowversion = @rowversion";

// Create table A and fill it with the schema

dtA = new DataTable("A");

daA = new SqlDataAdapter(selectText,

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

daA.RowUpdated += new SqlRowUpdatedEventHandler(da_RowUpdated);

daA.FillSchema(dtA, SchemaType.Source);

dtA.Columns["rowversion"].ReadOnly = false;

daA.Fill(dtA);

// Create the update command and define the parameters

daA.UpdateCommand = new SqlCommand(updateText,

daA.SelectCommand.Connection);

daA.UpdateCommand.CommandType = CommandType.Text;

daA.UpdateCommand.Parameters.Add("@Id", SqlDbType.Int, 0, "Id");

daA.UpdateCommand.Parameters["@Id"].SourceVersion =

DataRowVersion.Original;

daA.UpdateCommand.Parameters.Add("@Field1", SqlDbType.NVarChar, 50, "Field1");

daA.UpdateCommand.Parameters["@Field1"].SourceVersion =

DataRowVersion.Current;

daA.UpdateCommand.Parameters.Add("@rowversion", SqlDbType.Timestamp, 0, "rowversion");

daA.UpdateCommand.Parameters["@rowversion"].SourceVersion =

DataRowVersion.Original;

// Create table B and fill it with the schema

dtB = new DataTable("B");

daB = new SqlDataAdapter(selectText,

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

daB.RowUpdated += new SqlRowUpdatedEventHandler(da_RowUpdated);

daB.FillSchema(dtB, SchemaType.Source);

dtB.Columns["rowversion"].ReadOnly = false;

daB.Fill(dtB);

// Create the update command and define the parameters

daB.UpdateCommand = new SqlCommand(updateText,

daB.SelectCommand.Connection);

daB.UpdateCommand.CommandType = CommandType.Text;

daB.UpdateCommand.Parameters.Add("@Id", SqlDbType.Int, 0, "Id");

daB.UpdateCommand.Parameters["@Id"].SourceVersion =

DataRowVersion.Original;

daB.UpdateCommand.Parameters.Add("@Field1", SqlDbType.NVarChar, 50, "Field1");

daB.UpdateCommand.Parameters["@Field1"].SourceVersion =

DataRowVersion.Current;

daB.UpdateCommand.Parameters.Add("@rowversion", SqlDbType.Timestamp, 0, "rowversion");

daB.UpdateCommand.Parameters["@rowversion"].SourceVersion =

DataRowVersion.Original;

// Display the first row (ID=1) from both tables on the form

DisplayRow(dtA, 0);

DisplayRow(dtB, 0);

}

private void da_RowUpdated(object sender, SqlRowUpdatedEventArgs e)

{

// Check if an insert or update operation is being performed

if(e.Status == UpdateStatus.Continue &&

(e.StatementType == StatementType.Insert ||

e.StatementType == StatementType.Update))

{

// Build a command object to retrieve the updated timestamp

String sqlGetRowVersion = "SELECT rowversion FROM " +

TABLENAME + " WHERE Id = " + e.Row["Id"];

SqlConnection conn = new SqlConnection(

ConfigurationSettings.AppSettings["Sql_ConnectString"]);

SqlCommand cmd = new SqlCommand(sqlGetRowVersion, conn);

// Set the timestamp to the new value in the data source and

// call accept changes

conn.Open( );

e.Row["rowversion"] = (Byte[])cmd.ExecuteScalar( );

conn.Close( );

e.Row.AcceptChanges( );

}

}

private void Update(SqlDataAdapter da, DataTable dt, TextBox field1TextBox) {

// Move the value for the field named Field1

// from the form to the DataTable for updating

dt.Rows[0]["Field1"] = field1TextBox.Text;

// Update the row table

try

{

da.Update(dt);

}

catch (DBConcurrencyException ex)

{

// Error if timestamp does not match

MessageBox.Show(ex.Message);

dt.RejectChanges( );

}

}

private void DisplayRow(DataTable dt, int index)

{

if (dt.TableName == "A")

{

aIdTextBox.Text = dt.Rows[index]["Id"].ToString( );

aField1TextBox.Text = dt.Rows[index]["Field1"].ToString( );

aRowversionTextBox.Text = Convert.ToBase64String(

(Byte[])dt.Rows[index]["rowversion"]);

}

else

{

bIdTextBox.Text = dt.Rows[index]["Id"].ToString( );

bField1TextBox.Text = dt.Rows[index]["Field1"].ToString( );

bRowversionTextBox.Text = Convert.ToBase64String(

(Byte[])dt.Rows[index]["rowversion"]);

}

}

private void updateButton_Click(object sender, System.EventArgs e)

{

if(((Button)sender).Tag.ToString( ) == "A")

{

Update(daA, dtA, aField1TextBox);

DisplayRow(dtA, 0);

}

else

{

Update(daB, dtB, bField1TextBox);

DisplayRow(dtB, 0);

}

}

private void refreshButton_Click(object sender, System.EventArgs e)

{

if(((Button)sender).Tag.ToString( ) == "A")

{

daA.Fill(dtA);

DisplayRow(dtA, 0);

}

else

{

daB.Fill(dtB);

DisplayRow(dtB, 0);

}

}

Discussion

A timestamp is a data type that automatically generates an 8-byte binary value guaranteed

to be unique within the database

The T-SQL timestamp data type is not the same as the timestamp data type defined in the SQL-92 standard The timestamp data type defined by the SQL-92 standard is equivalent

to the T-SQL datetime data type

SQL Server 2000 introduced the data type synonym rowversion for the timestamp data

type and recommends its use wherever possible in DDL statements One difference

between the timestamp data type and its data type synonym rowversion is that the

timestamp data type does not require a column name while the rowversion data type does

A table can have only one timestamp column The value of the timestamp field is updated with the current database timestamp value from the @@DBTS function each time a row having a timestamp column is inserted or updated This makes it a poor choice as part of

a key, especially a primary key, since the index would be rebuilt with each row inserted

or updated and the performance would suffer

The timestamp value is not related to the date and time that data is inserted or updated

To record the time of record inserts and updates, define a datetime column in the table and create update and insert triggers to set its value

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