Am I wrong assuming that if two identical DataTables are merged the state of each row will be preserved?
Take a look at this simple example. It creates two identical tables and merge the updated table with original table. But the returned table in original.GetChanges() is not Nothing as expected. Also, the state of each row in the original table are changed to Modified.
So what am I missing? Do I really have to create my own merge method to achieve this?
Public Sub Test() Dim original As DataTable = Me.CreateTableWithData() Dim updated As DataTable = Me.CreateTableWithData() Dim preserveChanges As Boolean = True Dim msAction As MissingSchemaAction = MissingSchemaAction.Ignore original.Merge(updated, preserveChanges, msAction) Dim changes As DataTable = original.GetChanges() MessageBox.Show(String.Format("Count={0}", If((changes Is Nothing), 0, changes.Rows.Count)), Me.Text, MessageBoxButtons.OK, MessageBoxIcon.Information) If (Not changes Is Nothing) Then changes.Dispose() : changes = Nothing updated.Dispose() : updated = Nothing original.Dispose() : original = Nothing End Sub Private Function CreateTableWithData() As DataTable Dim table As New DataTable("TEST") table.Columns.Add("ID", GetType(Integer)) table.Columns.Add("VALUE", GetType(String)) table.PrimaryKey = New DataColumn() {table.Columns(0)} table.Rows.Add(1, "Value 1") table.Rows.Add(2, "Value 2") table.AcceptChanges() Return table End Function
Output: Count=2
Edit - The workaround
The following code is a workaround for this strange(?) behavior.
Private Shared Sub Merge(target As DataTable, source As DataTable, preserveChanges As Boolean, msa As MissingSchemaAction) target.Merge(source, preserveChanges, msa) Dim row As DataRow Dim column As DataColumn Dim acceptChanges As Boolean For Each row In target.Rows If ((row.RowState = DataRowState.Modified) AndAlso ((row.HasVersion(DataRowVersion.Original)) AndAlso (row.HasVersion(DataRowVersion.Default)))) Then acceptChanges = True For Each column In target.Columns If (Not Object.Equals(row.Item(column, DataRowVersion.Original), row.Item(column, DataRowVersion.Default))) Then acceptChanges = False Exit For End If Next If (acceptChanges) Then row.AcceptChanges() End If End If Next acceptChanges = Nothing column = Nothing row = Nothing End Sub
After some time of working with DataTable merge I found the best solution to merging data, preserving changes and not setting the RowState to Modified for all of the existing rows.
What I discovered is that all of the rows in the original DataTable would have their RowState set to Modified if you use the DataTable Merge and pass True as the preserve changes property. If you pass false instead, the RowStates remain the same.
Going back to the documentation for the DataTable.Merge(DataTable, Boolean, MissingSchemaAction) Method I found this:
...In this scenario, the GetChanges method is first invoked. That method returns a second DataTable optimized for validating and merging. This second DataTable object contains only the DataTable and DataRow objects objects that were changed, resulting in a subset of the original DataTable...
From there I started to realize that the this merge is not really intended to be used with the original data directly... instead you should merge against the table returned by the GetChanges method (passing true in preserving changes) and then merge the changes table into the original source passing false for the preserving changes parameter.
To demonstrate this I have created the following class:
Class TableManger Implements ComponentModel.INotifyPropertyChanged Private _table1 As System.Data.DataTable Private _table2 As System.Data.DataTable Private _changesDetected As Integer = 0 Public ReadOnly Property Table1 Get Return _table1 End Get End Property Public ReadOnly Property ChangesDetected As Integer Get Return _changesDetected End Get End Property Public Sub New() _table1 = CreateTableWithData() _table1.AcceptChanges() AddHandler _table1.RowChanged, New System.Data.DataRowChangeEventHandler(AddressOf Row_Changed) End Sub Public Sub MergeTables() _table2 = _table1.Clone Dim tableRows As New List(Of System.Data.DataRow) For Each r In _table1.Rows Dim dr2 = _table2.NewRow For Each col As System.Data.DataColumn In _table1.Columns dr2(col.ColumnName) = r(col.ColumnName) Next _table2.Rows.Add(dr2) tableRows.Add(dr2) Next _table2.AcceptChanges() If _table2.Rows.Count > 0 Then _table2.Rows(0)(1) = "TB2 Changed" End If If _table1.Rows.Count > 0 Then '_table1.Rows(0)(1) = "TB1 Change"' _table1.Rows(1)(1) = "TB1 Change" End If _changesDetected = 0 Dim perserveChanges As Boolean = True Dim msAction As System.Data.MissingSchemaAction = System.Data.MissingSchemaAction.Ignore Dim changes As System.Data.DataTable = _table1.GetChanges() If changes IsNot Nothing Then changes.Merge(_table2, perserveChanges, msAction) _table1.Merge(changes, False, msAction) Else _table1.Merge(_table2, False, msAction) End If MessageBox.Show(String.Format("Changes in Change Table: {0} {1}Changes Detected: {2}", If((changes Is Nothing), 0, changes.Rows.Count), System.Environment.NewLine, _changesDetected), "Testing") RaiseEvent PropertyChanged(Me, New ComponentModel.PropertyChangedEventArgs("Table1")) RaiseEvent PropertyChanged(Me, New ComponentModel.PropertyChangedEventArgs("ChangesDetected")) End Sub Private Sub Row_Changed(ByVal sender As Object, ByVal e As System.Data.DataRowChangeEventArgs) Select Case e.Action Case System.Data.DataRowAction.Change If e.Row.RowState <> System.Data.DataRowState.Unchanged Then _changesDetected += 1 End If End Select End Sub Private Function CreateTableWithData() As System.Data.DataTable Dim newTable As New System.Data.DataTable Dim columnID As New System.Data.DataColumn("ID", GetType(Guid)) Dim columnA As New System.Data.DataColumn("ColumnA", GetType(String)) Dim columnB As New System.Data.DataColumn("ColumnB", GetType(String)) newTable.Columns.AddRange({columnID, columnA, columnB}) newTable.PrimaryKey = {newTable.Columns(0)} For i = 0 To 5 Dim dr = newTable.NewRow dr("ID") = Guid.NewGuid dr("ColumnA") = String.Format("Column A Row {0}", i.ToString) dr("ColumnB") = String.Format("Column B Row {0}", i.ToString) newTable.Rows.Add(dr) Next Return newTable End Function Public Event PropertyChanged(sender As Object, e As System.ComponentModel.PropertyChangedEventArgs) Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged End Class
So, in the MergeTables method, I make a change to the first row in _table2 and I make a change in the second row of _table1.
Because I made a change the first row in _table1, the _table1.GetChanges method returns a DataTable with all of the changed rows (just the first row in this case).
I then merge the table containing the changes with _table2 and indicate that I want to preserve changes.
Once that merge is completed I know that the results are preserving the changes that I had made before the merge and that the table will contain the new data as well (so long as there were no conflicts). The result of merging the incoming data into the changes table will resolution of any conflicts in data.
After I have that resolved table I can safely merge into the original _table1 table indicating that preserve change = false. Because passing false as the preserve changes parameter results in no RowState changes for the original data everything works perfectly fine! My changes are preserved And the RowStates are not modified!
Happy Coding!
-Frinny
