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DataSet.Merge Method (DataTable)

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

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

'Declaration
Public Sub Merge ( _
	table As DataTable _
)
'Usage
Dim instance As DataSet
Dim table As DataTable

instance.Merge(table)
public void Merge (
	DataTable table
)
public function Merge (
	table : DataTable
)
Not applicable.

Parameters

table

The DataTable whose data and schema will be merged.

Exception typeCondition

ArgumentNullException

The dataSet is a null reference (Nothing in Visual Basic).

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 creates a simple DataSet with one table, two columns, and ten rows. A second DataTable is created that is identical to the first. Two rows are added to the second table, which is then merged into the DataSet.

Private Sub DemonstrateMergeTable()
    ' Create a DataSet with one table, two columns, 
    ' and ten rows.
    Dim dataSet As New DataSet("dataSet")
    Dim table As New DataTable("Items")

    ' Add tables to the DataSet
    dataSet.Tables.Add(table)

    ' Add columns
    Dim c1 As New DataColumn("id", Type.GetType("System.Int32"), "")
    Dim c2 As New DataColumn("Item", Type.GetType("System.Int32"), "")
    table.Columns.Add(c1)
    table.Columns.Add(c2)

    ' DataColumn array to set primary key.
    Dim keyCol(1) As DataColumn

    ' Set primary key column.
    keyCol(0) = c1
    table.PrimaryKey = keyCol

    ' Add RowChanged event handler for the table.
    AddHandler table.RowChanged, AddressOf Row_Changed

    ' Add ten rows.
    Dim i As Integer
    Dim row As DataRow

    For i = 0 To 9
        row = table.NewRow()
        row("id") = i
        row("Item") = i
        table.Rows.Add(row)
    Next i

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

    ' Create a second DataTable identical to the first.
    Dim t2 As DataTable
    t2 = table.Clone()

    ' Add three rows. Note that the id column can't be the 
    ' same as existing rows in the DataSet table.
    Dim newRow As DataRow
    newRow = t2.NewRow()
    newRow("id") = 14
    newRow("Item") = 774
    t2.Rows.Add(newRow)

    newRow = t2.NewRow()
    newRow("id") = 12
    newRow("Item") = 555
    t2.Rows.Add(newRow)

    newRow = t2.NewRow()
    newRow("id") = 13
    newRow("Item") = 665
    t2.Rows.Add(newRow)

    ' Merge the table into the DataSet.
    Console.WriteLine("Merging")
    dataSet.Merge(t2)
    PrintValues(dataSet, "Merged With Table")
 End Sub 
     
 Private Sub Row_Changed( _
    sender As Object, e As DataRowChangeEventArgs)
     Console.WriteLine("Row Changed " & e.Action.ToString() _
        & ControlChars.Tab & e.Row.ItemArray(0).ToString())
 End Sub 'Row_Changed
 
 Private Sub PrintValues(dataSet As DataSet, label As String)
     Console.WriteLine(ControlChars.Cr & label)
     Dim table As DataTable
     Dim row As DataRow
     Dim column As DataColumn
     For Each table In  dataSet.Tables
         Console.WriteLine("TableName: " & table.TableName) 
         For Each row In  table.Rows             
             For Each column In  table.Columns
                 Console.Write(ControlChars.Tab & " " _
                    & row(column).ToString())
             Next column
             Console.WriteLine()
         Next row
     Next table
 End Sub

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0

XNA Framework

Supported in: 1.0

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