Esplorazione di oggetti DataTable

  • Articolo

Il tipo DataTableReader presenta il contenuto di uno o più oggetti DataTable sotto forma di uno o più set di risultati forward-only di sola lettura.

Un tipo DataTableReader può contenere più set di risultati se viene creato tramite il metodo CreateDataReader. Quando è presente più di un set di risultati, il metodo NextResult sposta il cursore al set di risultati successivo. Questa proprietà è forward-only. Non è possibile tornare a un set di risultati precedente.

Esempio

Nell'esempio seguente il metodo TestConstructor crea due istanze del tipo DataTable. Per illustrare il funzionamento di questo costruttore per la classe DataTableReader, l'esempio crea un nuovo DataTableReader basato sulla matrice contenente le due DataTable e, mediante una semplice operazione, stampa il contenuto delle prime colonne nella finestra della console.

static void TestConstructor()
{
    // Create two data adapters, one for each of the two
    // DataTables to be filled.
    DataTable customerDataTable = GetCustomers();
    DataTable productDataTable = GetProducts();

    // Create the new DataTableReader.
    using (DataTableReader reader = new(
               new DataTable[] { customerDataTable, productDataTable }))
    {
        // Print the contents of each of the result sets.
        do
        {
            PrintColumns(reader);
        } while (reader.NextResult());
    }

    Console.WriteLine("Press Enter to finish.");
    Console.ReadLine();
}

static DataTable GetCustomers()
{
    // Create sample Customers table, in order
    // to demonstrate the behavior of the DataTableReader.
    DataTable table = new();

    // Create two columns, ID and Name.
    DataColumn idColumn = table.Columns.Add("ID", typeof(int));
    table.Columns.Add("Name", typeof(string));

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

    table.Rows.Add(new object[] { 1, "Mary" });
    table.Rows.Add(new object[] { 2, "Andy" });
    table.Rows.Add(new object[] { 3, "Peter" });
    table.Rows.Add(new object[] { 4, "Russ" });
    return table;
}

static DataTable GetProducts()
{
    // Create sample Products table, in order
    // to demonstrate the behavior of the DataTableReader.
    DataTable table = new();

    // Create two columns, ID and Name.
    DataColumn idColumn = table.Columns.Add("ID", typeof(int));
    table.Columns.Add("Name", typeof(string));

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

    table.Rows.Add(new object[] { 1, "Wireless Network Card" });
    table.Rows.Add(new object[] { 2, "Hard Drive" });
    table.Rows.Add(new object[] { 3, "Monitor" });
    table.Rows.Add(new object[] { 4, "CPU" });
    return table;
}

static void PrintColumns(DataTableReader reader)
{
    // Loop through all the rows in the DataTableReader
    while (reader.Read())
    {
        for (var i = 0; i < reader.FieldCount; i++)
        {
            Console.Write(reader[i] + " ");
        }
        Console.WriteLine();
    }
}

Private Sub TestConstructor()
    ' Create two data adapters, one for each of the two
    ' DataTables to be filled.
    Dim customerDataTable As DataTable = GetCustomers()
    Dim productDataTable As DataTable = GetProducts()

    ' Create the new DataTableReader.
    Using reader As New DataTableReader( _
       New DataTable() {customerDataTable, productDataTable})

        ' Print the contents of each of the result sets.
        Do
            PrintColumns(reader)
        Loop While reader.NextResult()
    End Using

    Console.WriteLine("Press Enter to finish.")
    Console.ReadLine()

End Sub

Private Function GetCustomers() As DataTable
    ' Create sample Customers table, in order
    ' to demonstrate the behavior of the DataTableReader.
    Dim table As New DataTable

    ' Create two columns, ID and Name.
    Dim idColumn As DataColumn = table.Columns.Add("ID", GetType(Integer))
    table.Columns.Add("Name", GetType(String))

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

    table.Rows.Add(New Object() {1, "Mary"})
    table.Rows.Add(New Object() {2, "Andy"})
    table.Rows.Add(New Object() {3, "Peter"})
    table.Rows.Add(New Object() {4, "Russ"})
    Return table
End Function

Private Function GetProducts() As DataTable
    ' Create sample Products table, in order
    ' to demonstrate the behavior of the DataTableReader.
    Dim table As New DataTable

    ' Create two columns, ID and Name.
    Dim idColumn As DataColumn = table.Columns.Add("ID", GetType(Integer))
    table.Columns.Add("Name", GetType(String))

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

    table.Rows.Add(New Object() {1, "Wireless Network Card"})
    table.Rows.Add(New Object() {2, "Hard Drive"})
    table.Rows.Add(New Object() {3, "Monitor"})
    table.Rows.Add(New Object() {4, "CPU"})
    Return table
End Function

Private Sub PrintColumns( _
   ByVal reader As DataTableReader)

    ' Loop through all the rows in the DataTableReader.
    Do While reader.Read()
        For i As Integer = 0 To reader.FieldCount - 1
            Console.Write(reader(i).ToString() & " ")
        Next
        Console.WriteLine()
    Loop
End Sub

Vedi anche