收益率内部使用率

using SqlConnection

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

    SqlDataReader reader = command.ExecuteReader();

    // Call Read before accessing data.
    while (reader.Read())
    {
        yield reader[0];
    }

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

yield

using StreamReader

using (var streamReader = new StreamReader(fileName))
{
    string line;
    while ((line = streamReader.ReadLine()) != null)
    {
        yield return line;
    }
}

IEnumerable<string> Read(string fileName)
{
    using (var streamReader = new StreamReader(fileName))
    {
        return Read(streamReader);
    } // Dispose will be executed before ReadLine() because of deffered execution
}

IEnumerable<string> Read(StreamReader streamReader)
{
    string line;
    while ((line = streamReader.ReadLine()) != null)
    {
        yield return line;
    }
}