To create a SqlDataReader, you must call the ExecuteReader method of the SqlCommand object, instead of directly using a constructor.

While the SqlDataReader is being used, the associated SqlConnection is busy serving the SqlDataReader, and no other operations can be performed on the SqlConnection other than closing it. This is the case until the Close method of the SqlDataReader is called. For example, you cannot retrieve output parameters until after you call Close.

Changes made to a result set by another process or thread while data is being read may be visible to the user of the SqlDataReader. However, the precise behavior is timing dependent.

IsClosed and RecordsAffected are the only properties that you can call after the SqlDataReader is closed. Although the RecordsAffected property may be accessed while the SqlDataReader exists, always call Close before returning the value of RecordsAffected to guarantee an accurate return value.

Note
For optimal performance, SqlDataReader avoids creating unnecessary objects or making unnecessary copies of data. Therefore, multiple calls to methods such as GetValue return a reference to the same object. Use caution if you are modifying the underlying value of the objects returned by methods such as GetValue.

The following example creates a SqlConnection, a SqlCommand, and a SqlDataReader. The example reads through the data, writing it out to the console window. The code then closes the SqlDataReader. The SqlConnection is closed automatically at the end of the using code block.

Private Sub ReadOrderData(ByVal connectionString As String)
    Dim queryString As String = _
        "SELECT OrderID, CustomerID FROM dbo.Orders;"

    Using connection As New SqlConnection(connectionString)
        Dim command As New SqlCommand(queryString, connection)
        connection.Open()

        Dim reader As SqlDataReader = command.ExecuteReader()

        ' Call Read before accessing data.
        While reader.Read()
            Console.WriteLine(String.Format("{0}, {1}", _
                reader(0), reader(1)))
        End While

        ' Call Close when done reading.
        reader.Close()
    End Using
End Sub

private static void ReadOrderData(string connectionString)
{
    string queryString =
        "SELECT OrderID, CustomerID FROM dbo.Orders;";

    using (SqlConnection connection =
               new SqlConnection(connectionString))
    {
        SqlCommand command =
            new SqlCommand(queryString, connection);
        connection.Open();

        SqlDataReader reader = command.ExecuteReader();

        // Call Read before accessing data.
        while (reader.Read())
        {
            Console.WriteLine(String.Format("{0}, {1}",
                reader[0], reader[1]));
        }

        // Call Close when done reading.
        reader.Close();
    }
}

.NET Framework

.NET Framework Client Profile

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role not supported), Windows Server 2003 SP2

Reference

Other Resources

Para crear un SqlDataReader, se debe llamar al método ExecuteReader del objeto SqlCommand, en lugar de utilizar directamente un constructor.

Mientras se está utilizando SqlDataReader, el objeto SqlConnection asociado está ocupado dando servicio a SqlDataReader, y en SqlConnection no se puede realizar ninguna otra operación, excepto la de cerrar. Esto es así hasta que se llama al método Close de SqlDataReader. Por ejemplo, no se pueden recuperar parámetros de salida hasta después de llamar a Close.

Los cambios que realiza otro proceso o subproceso en un conjunto de resultados mientras se leen los datos, pueden ser visibles para el usuario de SqlDataReader. Sin embargo, el comportamiento exacto depende de los intervalos de tiempo.

IsClosed y RecordsAffected son las únicas propiedades que se pueden llamar después de cerrar SqlDataReader. Si bien se puede obtener acceso a la propiedad RecordsAffected mientras exista SqlDataReader, se debe llamar siempre al método Close antes de devolver el valor de RecordsAffected para garantizar que el valor devuelto sea exacto.

Nota
Para obtener un rendimiento óptimo, SqlDataReader evita crear objetos innecesarios o hacer copias de objetos innecesarias. Por tanto, varias llamadas a los métodos como GetValue devuelven una referencia al mismo objeto. Conviene tener mucho cuidado si se modifica el valor subyacente de los objetos devueltos por métodos como GetValue.

En el siguiente ejemplo, se crean una conexión SqlConnection, un objeto SqlCommand y un SqlDataReader. En el ejemplo se leen los datos y se escriben en la ventana de la consola. Seguidamente, el código cierra SqlDataReader. SqlConnection se cierra automáticamente al final del bloque de código using.

Private Sub ReadOrderData(ByVal connectionString As String)
    Dim queryString As String = _
        "SELECT OrderID, CustomerID FROM dbo.Orders;"

    Using connection As New SqlConnection(connectionString)
        Dim command As New SqlCommand(queryString, connection)
        connection.Open()

        Dim reader As SqlDataReader = command.ExecuteReader()

        ' Call Read before accessing data.
        While reader.Read()
            Console.WriteLine(String.Format("{0}, {1}", _
                reader(0), reader(1)))
        End While

        ' Call Close when done reading.
        reader.Close()
    End Using
End Sub

private static void ReadOrderData(string connectionString)
{
    string queryString =
        "SELECT OrderID, CustomerID FROM dbo.Orders;";

    using (SqlConnection connection =
               new SqlConnection(connectionString))
    {
        SqlCommand command =
            new SqlCommand(queryString, connection);
        connection.Open();

        SqlDataReader reader = command.ExecuteReader();

        // Call Read before accessing data.
        while (reader.Read())
        {
            Console.WriteLine(String.Format("{0}, {1}",
                reader[0], reader[1]));
        }

        // Call Close when done reading.
        reader.Close();
    }
}

.NET Framework

.NET Framework Client Profile

Windows 7, Windows Vista SP1 o posterior, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (no se admite Server Core), Windows Server 2008 R2 (se admite Server Core con SP1 o posterior), Windows Server 2003 SP2

Referencia

Otros recursos