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DataSet.Merge Method (DataTable, Boolean, MissingSchemaAction)

Merges a specified DataTable and its schema into the current DataSet, preserving or discarding changes in the DataSet and handling an incompatible schema according to the given arguments.

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

Public Sub Merge ( _
	table As DataTable, _
	preserveChanges As Boolean, _
	missingSchemaAction As MissingSchemaAction _
Dim instance As DataSet 
Dim table As DataTable 
Dim preserveChanges As Boolean 
Dim missingSchemaAction As MissingSchemaAction

instance.Merge(table, preserveChanges, _


Type: System.Data.DataTable

The DataTable whose data and schema will be merged.

Type: System.Boolean

One of the MissingSchemaAction values.

Type: System.Data.MissingSchemaAction

true to preserve changes in the DataSet; otherwise false.


The dataSet is Nothing.

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.

iOn 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 nearly identical to the first except that a new DataColumn is added to the table. Two rows are added to the second table, which is then merged into the DataSet with the preserveChanges argument set to false, and the missingSchemaAction argument set to MissingSchemaAction.Add.

Private Sub DemonstrateMergeTableAddSchema()
    ' 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

    ' Create and add two columns to the DataTable 
    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 keyCol(1) As DataColumn

    ' Set primary key column.
    keyCol(0) = idColumn
    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("Item") = i
    Next i

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

    ' Create a second DataTable identical to the first 
    ' with one extra column using the Clone method. 
    Dim cloneTable As New DataTable
    cloneTable = table.Clone()

    ' Add column.
    cloneTable.Columns.Add("extra", _

    ' Add two rows. Note that the id column can't be the  
    ' same as existing rows in the DataSet table. 
    Dim newRow As DataRow
    newRow = cloneTable.NewRow()
    newRow("id") = 12
    newRow("Item") = 555
    newRow("extra") = "extra Column 1"

    newRow = cloneTable.NewRow()
    newRow("id") = 13
    newRow("Item") = 665
    newRow("extra") = "extra Column 2"

    ' Merge the table into the DataSet.
    dataSet.Merge(cloneTable, False, MissingSchemaAction.Add)
    PrintValues(dataSet, "Merged With Table, Schema Added")
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 

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
        Next row
    Next table
 End Sub

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0

XNA Framework

Supported in: 3.0, 2.0, 1.0