Редактиране

Споделяне чрез

Handling DataAdapter Events

  • Статия

The ADO.NET DataAdapter exposes three events that you can use to respond to changes made to data at the data source. The following table shows the DataAdapter events.

Event Description
RowUpdating An UPDATE, INSERT, or DELETE operation on a row (by a call to one of the Update methods) is about to begin.
RowUpdated An UPDATE, INSERT, or DELETE operation on a row (by a call to one of the Update methods) is complete.
FillError An error has occurred during a Fill operation.

RowUpdating and RowUpdated

RowUpdating is raised before any update to a row from the DataSet has been processed at the data source. RowUpdated is raised after any update to a row from the DataSet has been processed at the data source. As a result, you can use RowUpdating to modify update behavior before it happens, to provide additional handling when an update will occur, to retain a reference to an updated row, to cancel the current update and schedule it for a batch process to be processed later, and so on. RowUpdated is useful for responding to errors and exceptions that occur during the update. You can add error information to the DataSet, as well as retry logic, and so on.

The RowUpdatingEventArgs and RowUpdatedEventArgs arguments passed to the RowUpdating and RowUpdated events include the following: a Command property that references the Command object being used to perform the update; a Row property that references the DataRow object containing the updated information; a StatementType property for what type of update is being performed; the TableMapping, if applicable; and the Status of the operation.

You can use the Status property to determine if an error has occurred during the operation and, if desired, to control the actions against the current and resulting rows. When the event occurs, the Status property equals either Continue or ErrorsOccurred. The following table shows the values to which you can set the Status property in order to control later actions during the update.

Status Description
Continue Continue the update operation.
ErrorsOccurred Abort the update operation and throw an exception.
SkipCurrentRow Ignore the current row and continue the update operation.
SkipAllRemainingRows Abort the update operation but do not throw an exception.

Setting the Status property to ErrorsOccurred causes an exception to be thrown. You can control which exception is thrown by setting the Errors property to the desired exception. Using one of the other values for Status prevents an exception from being thrown.

You can also use the ContinueUpdateOnError property to handle errors for updated rows. If DataAdapter.ContinueUpdateOnError is true, when an update to a row results in an exception being thrown, the text of the exception is placed into the RowError information of the particular row, and processing continues without throwing an exception. This enables you to respond to errors when the Update is complete, in contrast to the RowUpdated event, which enables you to respond to errors when the error is encountered.

The following code sample shows how to both add and remove event handlers. The RowUpdating event handler writes a log of all deleted records with a time stamp. The RowUpdated event handler adds error information to the RowError property of the row in the DataSet, suppresses the exception, and continues processing (mirroring the behavior of ContinueUpdateOnError = true).

' Assumes that connection is a valid SqlConnection object.  
Dim custAdapter As SqlDataAdapter = New SqlDataAdapter( _  
  "SELECT CustomerID, CompanyName FROM Customers", connection)  
  
' Add handlers.  
AddHandler custAdapter.RowUpdating, New SqlRowUpdatingEventHandler( _  
  AddressOf OnRowUpdating)  
AddHandler custAdapter.RowUpdated, New SqlRowUpdatedEventHandler(  
  AddressOf OnRowUpdated)  
  
' Set DataAdapter command properties, fill DataSet, and modify DataSet.  
  
custAdapter.Update(custDS, "Customers")  
  
' Remove handlers.  
RemoveHandler custAdapter.RowUpdating, _  
  New SqlRowUpdatingEventHandler(AddressOf OnRowUpdating)  
RemoveHandler custAdapter.RowUpdated, _  
  New SqlRowUpdatedEventHandler(AddressOf OnRowUpdated)  
  
Private Shared Sub OnRowUpdating(sender As Object, _  
  args As SqlRowUpdatingEventArgs)  
  If args.StatementType = StatementType.Delete Then  
    Dim tw As System.IO.TextWriter = _  
  System.IO.File.AppendText("Deletes.log")  
    tw.WriteLine( _  
      "{0}: Customer {1} Deleted.", DateTime.Now, args.Row(_  
      "CustomerID", DataRowVersion.Original))  
    tw.Close()  
  End If  
End Sub  
  
Private Shared Sub OnRowUpdated( _  
  sender As Object, args As SqlRowUpdatedEventArgs)  
  If args.Status = UpdateStatus.ErrorsOccurred  
    args.Status = UpdateStatus.SkipCurrentRow  
    args.Row.RowError = args.Errors.Message  
  End If  
End Sub  

// Assumes that connection is a valid SqlConnection object.  
SqlDataAdapter custAdapter = new SqlDataAdapter(  
  "SELECT CustomerID, CompanyName FROM Customers", connection);  
  
// Add handlers.  
custAdapter.RowUpdating += new SqlRowUpdatingEventHandler(OnRowUpdating);  
custAdapter.RowUpdated += new SqlRowUpdatedEventHandler(OnRowUpdated);  
  
// Set DataAdapter command properties, fill DataSet, modify DataSet.  
  
custAdapter.Update(custDS, "Customers");  
  
// Remove handlers.  
custAdapter.RowUpdating -= new SqlRowUpdatingEventHandler(OnRowUpdating);  
custAdapter.RowUpdated -= new SqlRowUpdatedEventHandler(OnRowUpdated);  
  
protected static void OnRowUpdating(  
  object sender, SqlRowUpdatingEventArgs args)  
{  
  if (args.StatementType == StatementType.Delete)  
  {  
    System.IO.TextWriter tw = System.IO.File.AppendText("Deletes.log");  
    tw.WriteLine(  
      "{0}: Customer {1} Deleted.", DateTime.Now,
       args.Row["CustomerID", DataRowVersion.Original]);  
    tw.Close();  
  }  
}  
  
protected static void OnRowUpdated(  
  object sender, SqlRowUpdatedEventArgs args)  
{  
  if (args.Status == UpdateStatus.ErrorsOccurred)  
  {  
    args.Row.RowError = args.Errors.Message;  
    args.Status = UpdateStatus.SkipCurrentRow;  
  }  
}

FillError

The DataAdapter issues the FillError event when an error occurs during a Fill operation. This type of error commonly occurs when the data in the row being added could not be converted to a .NET Framework type without some loss of precision.

If an error occurs during a Fill operation, the current row is not added to the DataTable. The FillError event enables you to resolve the error and add the row, or to ignore the excluded row and continue the Fill operation.

The FillErrorEventArgs passed to the FillError event can contain several properties that enable you to respond to and resolve errors. The following table shows the properties of the FillErrorEventArgs object.

Property Description
Errors The Exception that occurred.
DataTable The DataTable object being filled when the error occurred.
Values An array of objects that contains the values of the row being added when the error occurred. The ordinal references of the Values array correspond to the ordinal references of the columns of the row being added. For example, Values[0] is the value that was being added as the first column of the row.
Continue Allows you to choose whether or not to throw an exception. Setting the Continue property to false will halt the current Fill operation, and an exception will be thrown. Setting Continue to true continues the Fill operation despite the error.

The following code example adds an event handler for the FillError event of the DataAdapter. In the FillError event code, the example determines if there is the potential for precision loss, providing the opportunity to respond to the exception.

AddHandler adapter.FillError, New FillErrorEventHandler( _  
  AddressOf FillError)  
  
Dim dataSet As DataSet = New DataSet  
adapter.Fill(dataSet, "ThisTable")  
  
Private Shared Sub FillError(sender As Object, _  
  args As FillErrorEventArgs)  
  If args.Errors.GetType() Is Type.GetType("System.OverflowException") Then  
    ' Code to handle precision loss.  
    ' Add a row to table using the values from the first two columns.  
    DataRow myRow = args.DataTable.Rows.Add(New Object() _  
      {args.Values(0), args.Values(1), DBNull.Value})  
    ' Set the RowError containing the value for the third column.  
    myRow.RowError = _  
      "OverflowException encountered. Value from data source: " & _  
      args.Values(2)  
    args.Continue = True  
  End If  
End Sub  

adapter.FillError += new FillErrorEventHandler(FillError);  
  
DataSet dataSet = new DataSet();  
adapter.Fill(dataSet, "ThisTable");  
  
protected static void FillError(object sender, FillErrorEventArgs args)  
{  
  if (args.Errors.GetType() == typeof(System.OverflowException))  
  {  
    // Code to handle precision loss.  
    //Add a row to table using the values from the first two columns.  
    DataRow myRow = args.DataTable.Rows.Add(new object[]  
       {args.Values[0], args.Values[1], DBNull.Value});  
    //Set the RowError containing the value for the third column.  
    myRow.RowError =
       "OverflowException Encountered. Value from data source: " +  
       args.Values[2];  
    args.Continue = true;  
  }  
}

See also