DataSet.Merge Method (DataSet)


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Merges a specified DataSet and its schema into the current DataSet.

Namespace:   System.Data
Assembly:  System.Data (in System.Data.dll)

Public Sub Merge (
	dataSet As DataSet


Type: System.Data.DataSet

The DataSet whose data and schema will be merged.

Exception Condition

One or more constraints cannot be enabled.


The dataSet is null.

The Merge method is used to merge two DataSet objects that have largely similar schemas. A merge is typically used on a client application to incorporate the latest changes from a data source into an existing DataSet. This allows the client application to have a refreshed DataSet with the latest data from the data source.

The Merge method is typically called at the end of a series of procedures that involve validating changes, reconciling errors, updating the data source with the changes, and finally refreshing the existing DataSet.

In a client application, it is common to have a single button that the user can click that gathers the changed data and validates it before sending it back to a middle-tier component. In this scenario, the GetChanges method is first invoked. That method returns a second DataSet optimized for validating and merging. This second DataSet object contains only the DataTable and DataRow objects that were changed, resulting in a subset of the original DataSet. This subset is generally smaller, and thus more efficiently passed back to a middle-tier component. The middle-tier component then updates the original data source with the changes through stored procedures. The middle tier can then send back either a new DataSet that includes original data and the latest data from the data source (by running the original query again), or it can send back the subset with any changes that have been made to it from the data source. (For example, if the data source automatically creates unique primary key values, these values can be propagated back to the client application.) In either case, the returned DataSet can be merged back into the client application's original DataSet with the Merge method.

When the Merge method is called, the schemas of the two DataSet objects are compared because it is possible that the schemas may have been changed. For example, in a business-to-business scenario, new columns may have been added to an XML schema by an automated process. If the source DataSet contains schema elements (added DataColumn objects) that are missing in the target, the schema elements can be added to the target by setting the missingSchemaAction argument to MissingSchemaAction.Add. In that case, the merged DataSet contains the added schema and data.

After merging schemas, the data is merged.

When merging a new source DataSet into the target, any source rows with a DataRowState value of Unchanged, Modified, or Deleted are matched to target rows with the same primary key values. Source rows with a DataRowState value of Added are matched to new target rows with the same primary key values as the new source rows.

During a merge, constraints are disabled. If any constraints cannot be enabled at the end of merge, a ConstraintException is generated and the merged data is retained while the constraints are disabled. In this case, the EnforceConstraints property is set to false, and all rows that are invalid are marked in error. The errors must be resolved before attempting to reset the EnforceConstraints property to true.

The following example uses the GetChanges, Update, and Merge methods on a DataSet.

 Private Sub DemonstrateMerge()
     ' Create a DataSet with one table, two columns, and three rows.
     Dim dataSet As New DataSet("dataSet")
     Dim table As New DataTable("Items")
     Dim idColumn As New DataColumn("id", Type.GetType("System.Int32"))
     idColumn.AutoIncrement = True
     Dim itemColumn As New DataColumn("Item", Type.GetType("System.Int32"))

     ' DataColumn array to set primary key.
     Dim keyColumn(1) As DataColumn
     Dim row As DataRow

     ' Create variable for temporary DataSet. 
     Dim changeDataSet As DataSet

     ' Add columns to table, and table to DataSet.   

     ' Set primary key column.
     keyColumn(0) = idColumn
     table.PrimaryKey = keyColumn

     ' Add ten rows.
     Dim i As Integer
     For i = 0 To 9
         row = table.NewRow()
         row("Item") = i
     Next i

     ' Accept changes.
     PrintValues(dataSet, "Original values")

     ' Change two row values.
     table.Rows(0)("Item") = 50
     table.Rows(1)("Item") = 111

     ' Add one row.
     row = table.NewRow()
     row("Item") = 74

     ' Insert code for error checking. Set one row in error.
     table.Rows(1).RowError = "over 100"
     PrintValues(dataSet, "Modified and New Values")

     ' If the table has changes or errors, create a subset DataSet.
     If dataSet.HasChanges(DataRowState.Modified Or DataRowState.Added) _
        And dataSet.HasErrors Then

         ' Use GetChanges to extract subset.
         changeDataSet = dataSet.GetChanges(DataRowState.Modified _
            Or DataRowState.Added)
         PrintValues(changeDataSet, "Subset values")

         ' Insert code to reconcile errors. In this case, reject changes.
         Dim changeTable As DataTable
         For Each changeTable In  changeDataSet.Tables
             If changeTable.HasErrors Then
                 Dim changeRow As DataRow
                 For Each changeRow In  changeTable.Rows
                     If CInt(changeRow("Item", _
                        DataRowVersion.Current)) > 100 Then
                     End If
                 Next changeRow
             End If
         Next changeTable
         PrintValues(changeDataSet, "Reconciled subset values")

         ' Merge changes back to first DataSet.
         PrintValues(dataSet, "Merged Values")
     End If
 End Sub

Private Sub PrintValues(dataSet As DataSet, label As String)
     Console.WriteLine(ControlChars.Cr & label)
     Dim table As DataTable
     For Each table In  dataSet.Tables
         Console.WriteLine("TableName: " & table.TableName)
         Dim row As DataRow
         For Each row In  table.Rows
             Dim column As DataColumn
             For Each column In  table.Columns
                 Console.Write(ControlChars.Tab & " " _
                    & row(column).ToString())
             Next column
         Next row
     Next table
End Sub

.NET Framework
Available since 1.1
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