DataTable.Merge Method

Definition

Merge the specified DataTable with the current DataTable.

Overloads

Merge(DataTable, Boolean, MissingSchemaAction)

Merge the specified DataTable with the current DataTable, indicating whether to preserve changes and how to handle missing schema in the current DataTable.

Merge(DataTable, Boolean)

Merge the specified DataTable with the current DataTable, indicating whether to preserve changes in the current DataTable.

Merge(DataTable)

Merge the specified DataTable with the current DataTable.

Examples

The following console application demonstrates the behavior of the missingSchemaAction parameter of the Merge method. This example creates two versions of the same table, modifying the schema for the second version. The code then attempts to merge the second table into the first.

Note

This example shows how to use one of the overloaded versions of Merge. For other examples that might be available, see the individual overload topics.

private static void DemonstrateMergeTable()
{
    DataTable table1 = new DataTable("Items");

    // Add columns
    DataColumn idColumn = new DataColumn("id", typeof(System.Int32));
    DataColumn itemColumn = new DataColumn("item", typeof(System.Int32));
    table1.Columns.Add(idColumn);
    table1.Columns.Add(itemColumn);

    // Set the primary key column.
    table1.PrimaryKey = new DataColumn[] { idColumn };

    // Add RowChanged event handler for the table.
    table1.RowChanged += new 
        System.Data.DataRowChangeEventHandler(Row_Changed);

    // Add ten rows.
    DataRow row;
    for (int i = 0; i <= 9; i++)
    {
        row = table1.NewRow();
        row["id"] = i;
        row["item"] = i;
        table1.Rows.Add(row);
    }

    // Accept changes.
    table1.AcceptChanges();
    PrintValues(table1, "Original values");

    // Create a second DataTable identical to the first.
    DataTable table2 = table1.Clone();

    // Add column to the second column, so that the 
    // schemas no longer match.
    table2.Columns.Add("newColumn", typeof(System.String));

    // Add three rows. Note that the id column can't be the 
    // same as existing rows in the original table.
    row = table2.NewRow();
    row["id"] = 14;
    row["item"] = 774;
    row["newColumn"] = "new column 1";
    table2.Rows.Add(row);

    row = table2.NewRow();
    row["id"] = 12;
    row["item"] = 555;
    row["newColumn"] = "new column 2";
    table2.Rows.Add(row);

    row = table2.NewRow();
    row["id"] = 13;
    row["item"] = 665;
    row["newColumn"] = "new column 3";
    table2.Rows.Add(row);

    // Merge table2 into the table1.
    Console.WriteLine("Merging");
    table1.Merge(table2, false, MissingSchemaAction.Add);
    PrintValues(table1, "Merged With table1, schema added");

}

private static void Row_Changed(object sender, 
    DataRowChangeEventArgs e)
{
    Console.WriteLine("Row changed {0}\t{1}", e.Action, 
        e.Row.ItemArray[0]);
}

private static void PrintValues(DataTable table, string label)
{
    // Display the values in the supplied DataTable:
    Console.WriteLine(label);
    foreach (DataRow row in table.Rows)
    {
        foreach (DataColumn col in table.Columns)
        {
            Console.Write("\t " + row[col].ToString());
        }
        Console.WriteLine();
    }
}
Private Sub DemonstrateMergeTable()
  Dim table1 As New DataTable("Items")

  ' Add columns
  Dim idColumn As New DataColumn("id", GetType(System.Int32))
  Dim itemColumn As New DataColumn("item", GetType(System.Int32))
  table1.Columns.Add(idColumn)
  table1.Columns.Add(itemColumn)

  ' Set the primary key column.
  table1.PrimaryKey = New DataColumn() {idColumn}

  ' Add RowChanged event handler for the table.
  AddHandler table1.RowChanged, AddressOf Row_Changed

  ' Add some rows.
  Dim row As DataRow
  For i As Integer = 0 To 3
    row = table1.NewRow()
    row("id") = i
    row("item") = i
    table1.Rows.Add(row)
  Next i

  ' Accept changes.
  table1.AcceptChanges()
  PrintValues(table1, "Original values")

  ' Create a second DataTable identical to the first.
  Dim table2 As DataTable = table1.Clone()

  ' Add column to the second column, so that the 
  ' schemas no longer match.
  table2.Columns.Add("newColumn", GetType(System.String))

  ' Add three rows. Note that the id column can't be the 
  ' same as existing rows in the original table.
  row = table2.NewRow()
  row("id") = 14
  row("item") = 774
  row("newColumn") = "new column 1"
  table2.Rows.Add(row)

  row = table2.NewRow()
  row("id") = 12
  row("item") = 555
  row("newColumn") = "new column 2"
  table2.Rows.Add(row)

  row = table2.NewRow()
  row("id") = 13
  row("item") = 665
  row("newColumn") = "new column 3"
  table2.Rows.Add(row)

  ' Merge table2 into the table1.
  Console.WriteLine("Merging")
  table1.Merge(table2, False, MissingSchemaAction.Add)
  PrintValues(table1, "Merged With table1, Schema added")
End Sub

Private Sub Row_Changed(ByVal sender As Object, _
      ByVal e As DataRowChangeEventArgs)
  Console.WriteLine("Row changed {0}{1}{2}", _
    e.Action, ControlChars.Tab, e.Row.ItemArray(0))
End Sub

Private Sub PrintValues(ByVal table As DataTable, _
      ByVal label As String)
  ' Display the values in the supplied DataTable:
  Console.WriteLine(label)
  For Each row As DataRow In table.Rows
    For Each col As DataColumn In table.Columns
      Console.Write(ControlChars.Tab + " " + row(col).ToString())
    Next col
    Console.WriteLine()
  Next row
End Sub

Remarks

The Merge method is used to merge two DataTable 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 DataTable. This allows the client application to have a refreshed DataTable with the latest data from the data source.

The merge operation takes into account only the original table, and the table to be merged. Child tables are not affected or included. If a table has one or more child tables, defined as part of a relationship, each child table must be merged individually.

Merge(DataTable, Boolean, MissingSchemaAction)

Merge the specified DataTable with the current DataTable, indicating whether to preserve changes and how to handle missing schema in the current DataTable.

public:
 void Merge(System::Data::DataTable ^ table, bool preserveChanges, System::Data::MissingSchemaAction missingSchemaAction);
public void Merge (System.Data.DataTable table, bool preserveChanges, System.Data.MissingSchemaAction missingSchemaAction);
member this.Merge : System.Data.DataTable * bool * System.Data.MissingSchemaAction -> unit

Parameters

table
DataTable

The DataTable to be merged with the current DataTable.

preserveChanges
Boolean

true, to preserve changes in the current DataTable; otherwise false.

missingSchemaAction
MissingSchemaAction

One of the MissingSchemaAction values.

Examples

The following console application demonstrates the behavior of the missingSchemaAction parameter of the Merge method. This example creates two versions of the same table, modifying the schema for the second version. The code then attempts to merge the second table into the first.

private static void DemonstrateMergeTable()
{
    DataTable itemsTable = new DataTable("Items");

    // Add columns
    DataColumn idColumn = new DataColumn("id", typeof(System.Int32));
    DataColumn itemColumn = new DataColumn("item", typeof(System.Int32));
    itemsTable.Columns.Add(idColumn);
    itemsTable.Columns.Add(itemColumn);

    // Set the primary key column.
    itemsTable.PrimaryKey = new DataColumn[] { idColumn };

    // Add RowChanged event handler for the table.
    itemsTable.RowChanged += 
        new System.Data.DataRowChangeEventHandler(Row_Changed);

    // Add ten rows.
    DataRow row;
    for (int i = 0; i <= 9; i++)
    {
        row = itemsTable.NewRow();
        row["id"] = i;
        row["item"] = i;
        itemsTable.Rows.Add(row);
    }

    // Accept changes.
    itemsTable.AcceptChanges();
    PrintValues(itemsTable, "Original values");

    // Create a second DataTable identical to the first.
    DataTable itemsClone = itemsTable.Clone();

    // Add column to the second column, so that the 
    // schemas no longer match.
    itemsClone.Columns.Add("newColumn", typeof(System.String));

    // Add three rows. Note that the id column can't be the 
    // same as existing rows in the original table.
    row = itemsClone.NewRow();
    row["id"] = 14;
    row["item"] = 774;
    row["newColumn"] = "new column 1";
    itemsClone.Rows.Add(row);

    row = itemsClone.NewRow();
    row["id"] = 12;
    row["item"] = 555;
    row["newColumn"] = "new column 2";
    itemsClone.Rows.Add(row);

    row = itemsClone.NewRow();
    row["id"] = 13;
    row["item"] = 665;
    row["newColumn"] = "new column 3";
    itemsClone.Rows.Add(row);

    // Merge itemsClone into the itemsTable.
    Console.WriteLine("Merging");
    itemsTable.Merge(itemsClone, false, MissingSchemaAction.Add);
    PrintValues(itemsTable, "Merged With itemsTable, schema added");
}

private static void Row_Changed(object sender, 
    DataRowChangeEventArgs e)
{
    Console.WriteLine("Row changed {0}\t{1}", 
        e.Action, e.Row.ItemArray[0]);
}

private static void PrintValues(DataTable table, string label)
{
    // Display the values in the supplied DataTable:
    Console.WriteLine(label);
    foreach (DataRow row in table.Rows)
    {
        foreach (DataColumn col in table.Columns)
        {
            Console.Write("\t " + row[col].ToString());
        }
        Console.WriteLine();
    }
}
Private Sub DemonstrateMergeTable()
  Dim itemsTable As New DataTable("Items")

  ' Add columns
  Dim idColumn As New DataColumn("id", GetType(System.Int32))
  Dim itemColumn As New DataColumn("item", GetType(System.Int32))
  itemsTable.Columns.Add(idColumn)
  itemsTable.Columns.Add(itemColumn)

  ' Set the primary key column.
  itemsTable.PrimaryKey = New DataColumn() {idColumn}

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

  ' Add some rows.
  Dim row As DataRow
  For i As Integer = 0 To 3
    row = itemsTable.NewRow()
    row("id") = i
    row("item") = i
    itemsTable.Rows.Add(row)
  Next i

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

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

  ' Add column to the second column, so that the 
  ' schemas no longer match.
  itemsClone.Columns.Add("newColumn", GetType(System.String))

  ' Add three rows. Note that the id column can't be the 
  ' same as existing rows in the original table.
  row = itemsClone.NewRow()
  row("id") = 14
  row("item") = 774
  row("newColumn") = "new column 1"
  itemsClone.Rows.Add(row)

  row = itemsClone.NewRow()
  row("id") = 12
  row("item") = 555
  row("newColumn") = "new column 2"
  itemsClone.Rows.Add(row)

  row = itemsClone.NewRow()
  row("id") = 13
  row("item") = 665
  row("newColumn") = "new column 3"
  itemsClone.Rows.Add(row)

  ' Merge itemsClone into the itemsTable.
  Console.WriteLine("Merging")
  itemsTable.Merge(itemsClone, False, MissingSchemaAction.Add)
  PrintValues(itemsTable, "Merged With itemsTable, Schema added")
End Sub

Private Sub Row_Changed(ByVal sender As Object, _
  ByVal e As DataRowChangeEventArgs)
  Console.WriteLine("Row changed {0}{1}{2}", _
    e.Action, ControlChars.Tab, e.Row.ItemArray(0))
End Sub

Private Sub PrintValues(ByVal table As DataTable, ByVal label As String)
  ' Display the values in the supplied DataTable:
  Console.WriteLine(label)
  For Each row As DataRow In table.Rows
    For Each col As DataColumn In table.Columns
      Console.Write(ControlChars.Tab + " " + row(col).ToString())
    Next col
    Console.WriteLine()
  Next row
End Sub

Remarks

The Merge method is used to merge two DataTable 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 DataTable. This allows the client application to have a refreshed DataTable with the latest data from the data source.

The merge operation takes into account only the original table, and the table to be merged. Child tables are not affected or included. If a table has one or more child tables, defined as part of a relationship, each child table must be merged individually.

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 DataTable.

When performing a merge, changes made to the existing data before the merge are preserved during the merge operation unless the developer specifies false for the preserveChanges parameter. If the preserveChanges parameter is set to true, incoming values do not overwrite existing values in the Current row version of the existing row. If the preserveChanges parameter is set to false, incoming values do overwrite the existing values in the Current row version of the existing row. For more information about row versions, see Row States and Row Versions.

In a client application, it is usual 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 DataTable optimized for validating and merging. This second DataTable object contains only the DataTable and DataRow objects that were changed, resulting in a subset of the original DataTable. This subset is generally smaller, and thus this subset is 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 DataTable 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 DataTable can be merged back into the client application's original DataTable with the Merge method.

When the Merge method is called, the schemas of the two DataTable 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 DataTable 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 DataTable contains the added schema and data.

After merging schemas, the data is merged.

When merging a new source DataTable 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.

See also

Merge(DataTable, Boolean)

Merge the specified DataTable with the current DataTable, indicating whether to preserve changes in the current DataTable.

public:
 void Merge(System::Data::DataTable ^ table, bool preserveChanges);
public void Merge (System.Data.DataTable table, bool preserveChanges);
member this.Merge : System.Data.DataTable * bool -> unit
Public Sub Merge (table As DataTable, preserveChanges As Boolean)

Parameters

table
DataTable

The DataTable to be merged with the current DataTable.

preserveChanges
Boolean

true, to preserve changes in the current DataTable; otherwise false.

Examples

The following console application creates a DataTable containing rows, modifies some of the data in those rows, and attempts to merge data from a different DataTable. The example demonstrates the different behaviors for the preserveChanges parameter.


private static void DemonstrateMergeTable()
{
    // Demonstrate merging, within and without
    // preserving changes.

    // In this example, take these actions:
    // 1. Create a DataTable (table1) and fill the table with data.
    // 2. Create a copy of table1, and modify its data (modifiedTable).
    // 3. Modify data in table1.
    // 4. Make a copy of table1 (table1Copy).
    // 5. Merge the data from modifiedTable into table1 and table1Copy, 
    //    showing the difference between setting the preserveChanges 
    //    parameter to true and false.

    // Create a new DataTable.
    DataTable table1 = new DataTable("Items");

    // Add two columns to the table:
    DataColumn column = new DataColumn("id", typeof(System.Int32));
    column.AutoIncrement = true;
    table1.Columns.Add(column);

    column = new DataColumn("item", typeof(System.String));
    table1.Columns.Add(column);

    // Set primary key column.
    table1.PrimaryKey = new DataColumn[] { table1.Columns[0] };

    // Add some rows.
    DataRow row;
    for (int i = 0; i <= 3; i++)
    {
        row = table1.NewRow();
        row["item"] = "Item " + i;
        table1.Rows.Add(row);
    }

    // Accept changes.
    table1.AcceptChanges();
    PrintValues(table1, "Original values");

    // Using the same schema as the original table, 
    // modify the data for later merge.
    DataTable modifiedTable = table1.Copy();
    foreach (DataRow rowModified in modifiedTable.Rows)
    {
        rowModified["item"] = rowModified["item"].ToString() 
            + " modified";
    }
    modifiedTable.AcceptChanges();

    // Change row values, and add a new row:
    table1.Rows[0]["item"] = "new Item 0";
    table1.Rows[1]["item"] = "new Item 1";

    row = table1.NewRow();
    row["id"] = 4;
    row["item"] = "Item 4";
    table1.Rows.Add(row);

    // Get a copy of the modified data:
    DataTable table1Copy = table1.Copy();
    PrintValues(table1, "Modified and new Values");
    PrintValues(modifiedTable, "Data to be merged into table1");

    // Merge new data into the modified data.
    table1.Merge(modifiedTable, true);
    PrintValues(table1, "Merged data (preserve changes)");

    table1Copy.Merge(modifiedTable, false);
    PrintValues(modifiedTable, "Merged data (don't preserve changes)");
}


private static void PrintValues(DataTable table, string label)
{
    // Display the values in the supplied DataTable:
    Console.WriteLine(label);
    foreach (DataRow row in table.Rows)
    {
        foreach (DataColumn column in table.Columns)
        {
            Console.Write("\t{0}", row[column, DataRowVersion.Current]);
        }
        Console.WriteLine();
    }
}
Private Sub DemonstrateMergeTable()
  ' Demonstrate merging, within and without
  ' preserving changes.

  ' In this example, take these actions:
  ' 1. Create a DataTable (table1) and fill the table with data.
  ' 2. Create a copy of table1, and modify its data (modifiedTable).
  ' 3. Modify data in table1.
  ' 4. Make a copy of table1 (table1Copy).
  ' 5. Merge the data from modifiedTable into table1 and table1Copy, 
  '    showing the difference between setting the preserveChanges 
  '    parameter to true and false.

  ' Create a new DataTable.
  Dim table1 As New DataTable("Items")

  ' Add two columns to the table:
  Dim column As New DataColumn("id", GetType(System.Int32))
  column.AutoIncrement = True
  table1.Columns.Add(column)

  column = New DataColumn("item", GetType(System.String))
  table1.Columns.Add(column)

  ' Set primary key column.
  table1.PrimaryKey = New DataColumn() {table1.Columns(0)}

  ' Add some rows.
  Dim row As DataRow
  For i As Integer = 0 To 3
    row = table1.NewRow()
    row("item") = "Item " & i
    table1.Rows.Add(row)
  Next i

  ' Accept changes.
  table1.AcceptChanges()
  PrintValues(table1, "Original values")

  ' Using the same schema as the original table, 
  ' modify the data for later merge.
  Dim modifiedTable As DataTable = table1.Copy()
  For Each row In modifiedTable.Rows
    row("item") = row("item").ToString() & " modified"
  Next
  modifiedTable.AcceptChanges()

  ' Change row values, and add a new row:
  table1.Rows(0)("item") = "New Item 0"
  table1.Rows(1)("item") = "New Item 1"

  row = table1.NewRow()
  row("id") = 4
  row("item") = "Item 4"
  table1.Rows.Add(row)

  ' Get a copy of the modified data:
  Dim table1Copy As DataTable = table1.Copy()
  PrintValues(table1, "Modified and New Values")
  PrintValues(modifiedTable, "Data to be merged into table1")


  ' Merge new data into the modified data.
  table1.Merge(modifiedTable, True)
  PrintValues(table1, "Merged data (preserve changes)")

  table1Copy.Merge(modifiedTable, False)
  PrintValues(modifiedTable, "Merged data (don't preserve changes)")

End Sub

Private Sub PrintValues(ByVal table As DataTable, _
  ByVal label As String)

  ' Display the values in the supplied DataTable:
  Console.WriteLine(label)
  For Each row As DataRow In table.Rows
    For Each column As DataColumn In table.Columns
      Console.Write("{0}{1}", ControlChars.Tab, row(column, _
          DataRowVersion.Current))
    Next column
    Console.WriteLine()
  Next row
End Sub

Remarks

The Merge method is used to merge two DataTable 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 DataTable. This allows the client application to have a refreshed DataTable with the latest data from the data source.

The merge operation takes into account only the original table, and the table to be merged. Child tables are not affected or included. If a table has one or more child tables, defined as part of a relationship, each child table must be merged individually.

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 DataTable.

When performing a merge, changes made to the existing data before the merge are preserved during the merge operation unless the developer specifies false for the preserveChanges parameter. If the preserveChanges parameter is set to true, incoming values do not overwrite existing values in the Current row version of the existing row. If the preserveChanges parameter is set to false, incoming values do overwrite the existing values in the Current row version of the existing row. For more information about row versions, see Row States and Row Versions.

In a client application, it is usual 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 DataTable optimized for validating and merging. This second DataTable object contains only the DataTable and DataRow objects that were changed, resulting in a subset of the original DataTable. This subset is generally smaller, and thus this subset is 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 DataTable 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 DataTable can be merged back into the client application's original DataTable with the Merge method.

When merging a new source DataTable 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.

See also

Merge(DataTable)

Merge the specified DataTable with the current DataTable.

public:
 void Merge(System::Data::DataTable ^ table);
public void Merge (System.Data.DataTable table);
member this.Merge : System.Data.DataTable -> unit
Public Sub Merge (table As DataTable)

Parameters

table
DataTable

The DataTable to be merged with the current DataTable.

Examples

The following console application creates a simple DataTable and adds data to the table. The example then creates a copy of the table, adding rows to the copy. Finally, the example calls the Merge method to merge the data in the second table with the data in the first table.

private static void DemonstrateMergeTable()
{
    DataTable table1 = new DataTable("Items");

    // Add columns
    DataColumn column1 = new DataColumn("id", typeof(System.Int32));
    DataColumn column2 = new DataColumn("item", typeof(System.Int32));
    table1.Columns.Add(column1);
    table1.Columns.Add(column2);

    // Set the primary key column.
    table1.PrimaryKey = new DataColumn[] { column1 };

    // Add RowChanged event handler for the table.
    table1.RowChanged += 
        new System.Data.DataRowChangeEventHandler(Row_Changed);

    // Add some rows.
    DataRow row;
    for (int i = 0; i <= 3; i++)
    {
        row = table1.NewRow();
        row["id"] = i;
        row["item"] = i;
        table1.Rows.Add(row);
    }

    // Accept changes.
    table1.AcceptChanges();
    PrintValues(table1, "Original values");

    // Create a second DataTable identical to the first.
    DataTable table2 = table1.Clone();

    // Add three rows. Note that the id column can't be the 
    // same as existing rows in the original table.
    row = table2.NewRow();
    row["id"] = 14;
    row["item"] = 774;
    table2.Rows.Add(row);

    row = table2.NewRow();
    row["id"] = 12;
    row["item"] = 555;
    table2.Rows.Add(row);

    row = table2.NewRow();
    row["id"] = 13;
    row["item"] = 665;
    table2.Rows.Add(row);

    // Merge table2 into the table1.
    Console.WriteLine("Merging");
    table1.Merge(table2);
    PrintValues(table1, "Merged With table1");

}

private static void Row_Changed(object sender, 
    DataRowChangeEventArgs e)
{
    Console.WriteLine("Row changed {0}\t{1}", 
        e.Action, e.Row.ItemArray[0]);
}

private static void PrintValues(DataTable table, string label)
{
    // Display the values in the supplied DataTable:
    Console.WriteLine(label);
    foreach (DataRow row in table.Rows)
    {
        foreach (DataColumn col in table.Columns)
        {
            Console.Write("\t " + row[col].ToString());
        }
        Console.WriteLine();
    }
}
Private Sub DemonstrateMergeTable()
  Dim table1 As New DataTable("Items")

  ' Add columns
  Dim column1 As New DataColumn("id", GetType(System.Int32))
  Dim column2 As New DataColumn("item", GetType(System.Int32))
  table1.Columns.Add(column1)
  table1.Columns.Add(column2)

  ' Set the primary key column.
  table1.PrimaryKey = New DataColumn() {column1}

  ' Add RowChanged event handler for the table.
  AddHandler table1.RowChanged, AddressOf Row_Changed

  ' Add some rows.
  Dim row As DataRow
  For i As Integer = 0 To 3
    row = table1.NewRow()
    row("id") = i
    row("item") = i
    table1.Rows.Add(row)
  Next i

  ' Accept changes.
  table1.AcceptChanges()
  PrintValues(table1, "Original values")

  ' Create a second DataTable identical to the first.
  Dim table2 As DataTable = table1.Clone()

  ' Add three rows. Note that the id column can't be the 
  ' same as existing rows in the original table.
  row = table2.NewRow()
  row("id") = 14
  row("item") = 774
  table2.Rows.Add(row)

  row = table2.NewRow()
  row("id") = 12
  row("item") = 555
  table2.Rows.Add(row)

  row = table2.NewRow()
  row("id") = 13
  row("item") = 665
  table2.Rows.Add(row)

  ' Merge table2 into the table1.
  Console.WriteLine("Merging")
  table1.Merge(table2)
  PrintValues(table1, "Merged With table1")

End Sub

Private Sub Row_Changed(ByVal sender As Object, _
  ByVal e As DataRowChangeEventArgs)
  Console.WriteLine("Row changed {0}{1}{2}", _
    e.Action, ControlChars.Tab, e.Row.ItemArray(0))
End Sub

Private Sub PrintValues(ByVal table As DataTable, _
  ByVal label As String)
  ' Display the values in the supplied DataTable:
  Console.WriteLine(label)
  For Each row As DataRow In table.Rows
    For Each col As DataColumn In table.Columns
      Console.Write(ControlChars.Tab + " " + row(col).ToString())
    Next col
    Console.WriteLine()
  Next row
End Sub

Remarks

The Merge method is used to merge two DataTable 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 DataTable. This allows the client application to have a refreshed DataTable with the latest data from the data source.

The merge operation takes into account only the original table, and the table to be merged. Child tables are not affected or included. If a table has one or more child tables, defined as part of a relationship, each child table must be merged individually.

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 DataTable.

When performing a merge, changes made to the existing data before the merge are preserved by default during the merge operation. Developers can modify this behavior by calling one of the other two overloads for this method, and specifying a false value for the preserveChanges parameter.

In a client application, it is usual 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 DataTable optimized for validating and merging. This second DataTable object contains only the DataRow objects that were changed, resulting in a subset of the original DataTable. 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 DataTable 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 DataTable can be merged back into the client application's original DataTable with the Merge method.

When merging a new source DataTable into the target, any source rows with a DataRowState value of Unchanged, Modified, or Deleted, is 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.

See also

Applies to