If I recall correctly that when I used yield inside using SqlConnection blocks I got runtime exceptions.

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();
}

这些问题解决了，当我更换收益率通过在那里我添加的项目列表中的每个迭代。

Those problems were solved when I replaced yield by a List where I added items each iteration.

The same problem didn't happen yet to me when inside using StreamReader blocks

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

有没有异常的原因发生在前者的情况下，而不是在后面的任何解释？这是建设建议？

Is there any explanation why Exceptions happened in the former case and not in the latter? Is this construction advisable?

修改要获得我过去做了错误（早处置），你应该叫下面的第一种方法：

EDIT To get the error (early disposal) that I did in the past you should call the first method below:

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;
    }
}

可以延期执行的其他方法，比如可以达到相同的误差 System.Linq.Enumerable.Select（）

推荐答案

See this post for a good explanation of the issues with using and yield. Because you return in enumerator, the using block will already have destroyed the context before anything is accessed. The answers have good solutions, basically, either make the wrapper method an enumerator, or build a list instead.

此外，它通常是更实际有使用周围的读者，而不是连接，并使用 CommandBehavior.CloseConnection 以确保当读者进行资源被释放。虽然它并没有真正的问题在您的情况，如果你曾经从一个方法返回一个数据读取器，这将确保连接正常时，读写器配置被关闭。

Also it's usually more practical to have using around the reader, not the connection, and use CommandBehavior.CloseConnection to ensure resources are released when the reader's done. Though it doesn't really matter in your situation, if you ever return a data reader from a method, this will ensure the connection is closed properly when the reader is disposed.

   using(SqlDataReader reader = 
             command.ExecuteReader(CommandBehavior.CloseConnection)) {
        while (reader.Read())
        {
            yield reader[0];
        }
   }