C＃：已经有与此命令相关联的打开的DataReader，必须先关闭

Question

 String sql = "SELECT * from mybrknElements; "; 
     String sql2 = ""; 
     String sWord = "" ; 
     String sNum = "" ; 
     int nWords = 0; 
     cnn.Close(); 
     cnn.Open(); 
     SqlCommand command = new SqlCommand(sql, cnn); 
     cnn2.Close(); 
     cnn2.Open(); 

     SqlDataReader drb; 
     drb = command.ExecuteReader(); 
     while (drb.Read()) 
     { 
      sNum = drb["ID"].ToString(); 
      sWord = drb["Element"].ToString(); 
      MessageBox.Show("OUTER loooop sNum = " + sNum + " sWord = " + sWord); 


      sql2 = "SELECT * from mybrknElements2; "; 
      String sWord2 = "" ; 
      String sNum2 = ""; 
      SqlCommand command22 = new SqlCommand(sql2, cnn2); 

      SqlDataReader drcc; 
      drcc = command22.ExecuteReader(); //ERROR comes up after this line 


      while (drcc.Read()) 
      { 
       sNum2 = drcc["ID"].ToString(); 
       sWord2 = drcc["Element"].ToString(); 
       if (Equals(sWord2,sWord2)) 
       { 

        nWords = nWords + 1; 
        MessageBox.Show("sNum2 = " + sNum2 + " sWord2 = " + sWord2); 
       } 
      } 



      //---check occurances-------------- 


     } 

以上是我的代码：我已经使用2个SqlDataReaders，一个内其他 我得到的错误在while循环 结束：已经有一个打开的DataReader与此命令相关联，必须先关闭

有人可以帮忙吗？ 谢谢。

Answer 1

试试这个代码，

 SqlDataReader drb; 
     drb = command.ExecuteReader(); 
     while (drb.Read()) 
     { 
      sNum = drb["ID"].ToString(); 
      sWord = drb["Element"].ToString(); 
      MessageBox.Show("OUTER loooop sNum = " + sNum + " sWord = " + sWord); 


      sql2 = "SELECT * from mybrknElements2; "; 
      String sWord2 = "" ; 
      String sNum2 = ""; 
      SqlCommand command22 = new SqlCommand(sql2, cnn2); 

      SqlDataReader drcc; 
      drcc = command22.ExecuteReader(); //ERROR comes up after this line 

      drb.Close(); 

      while (drcc.Read()) 
      { 
       sNum2 = drcc["ID"].ToString(); 
       sWord2 = drcc["Element"].ToString(); 
       if (Equals(sWord2,sWord2)) 
       { 

        nWords = nWords + 1; 
        MessageBox.Show("sNum2 = " + sNum2 + " sWord2 = " + sWord2); 
       } 
       } 
      drcc.Close(); 
      } 

Answer 2

你应该停止重用你的命令和连接对象，并妥善一旦他们已经用他们的处置。重用对象没有任何好处（.NET会通过连接池重复使用打开的套接字到数据库，即使对于不同的连接对象也是如此），并且这只会导致类似的问题。

所以不重用对象，和那些处置您正确的代码可能最终会是这样的：

string sql = "SELECT ID, Element FROM mybrknElements; "; 
string sql2 = "SELECT ID, Element FROM mybrknElements2;"; 
int nWords = 0; 
string connectionString = "Your Connection String"; 

using (var connection = new SqlConnection(connectionString)) 
using (var command = new SqlCommand(sql1, connection)) 
{ 
    connection.Open(); 
    using (var reader = command.ExecuteReader()) 
    { 
     string sWord = reader.GetString(0); 
     string sNum = reader.GetString(1); 

     using (var connection2 = new SqlConnection(connectionString)) 
     using (var command2 = new SqlCommand(sql2, connection2)) 
     { 
      connection2.Open(); 
      using (var reader2 = command2.ExecuteReader()) 
      { 
       string sWord2 = reader2.GetString(0); 
       string sNum2 = reader2.GetString(1); 

       if (Equals(sWord1,sWord2)) 
       { 
        nWords++; 
       } 
      } 
     }   
    } 
} 

注：在每个数据读取器中，您只使用两列，因此我已将这些列添加到每个SQL语句的选择列表中，这样就不会不必要地检索永远不会使用的数据。

整个循环令我毫无意义和低效虽然，假设整个目的是让nWords了正确的号码，你可以做到这一切在SQL：

SELECT nWords = COUNT(*) 
    FROM mybrknElements AS e 
      INNER JOIN mybrknElements AS e2 
       ON e2.ID = e.ID;