不允许嵌套的 SqlCommand？

Question

我有 2 个 SqlCommand，其中一个是嵌套的。为什么它不允许我发出第二个 SqlCommand（我正在使用单独的 SQLCommand）？它给出了一个错误

There is already an open DataReader associated with this Command which must be closed first.

如果我单独使用SqlConnection，没问题。

 SqlCommand cmd = new SqlCommand(qry, cn);

 SqlDataReader rd = cmd.ExecuteReader();

 while (rd.Read())
 {
      ....    
      try
      {
          SqlCommand cmd2 = new SqlCommand(qry2, cn);
          cmd2.ExecuteNonQuery();
      }
      catch (Exception e)
      {
          // I get this error here
          // System.Data; There is already an open DataReader associated with this Command which must be closed first.
      }        
 }

Answer 1

消息很明显：当 DataReader 仍然打开时，您不能同时对不同的 SqlCommand 实例使用相同的连接。 SqlDataReader实例说明已经说了：

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.

此问题的常见解决方案是在连接字符串上使用 MultipleActiveResultSets=True：

<add name="ConnectionName" connectionString="[connection string];MultipleActiveResultSets=True" ... />

然后，使用DataTable代替直接迭代DataReader：

var dt = new DataTable();
dt.Load(rd);

foreach (DataRow row in dt.Rows)
{
    // other stuff

    try
    {
        SqlCommand cmd2 = new SqlCommand(qry2, cn);
        cmd2.ExecuteNonQuery();
    }
    catch (Exception e)
    {
        // throw exception
    }
}

此外，您可以使用 SqlConnection.State 属性:

简单检查之前的连接是否仍然打开（即服务 DataReader）

// close if connection still open
if (cn.State == ConnectionState.Open)
{
    cn.Close();
}

// open if connection already closed
if (cn.State == ConnectionState.Closed)
{
    cn.Open();
}

上面的简单检查应该放在请求 SqlConnection.

的代码的任何部分

不允许嵌套的 SqlCommand？

Not allow Nested SqlCommand?

c#

sqlcommand

sqldatareader