在何处/何时使用connection.close()

您不必显式关闭连接，因为您使用了带有sqlconnection对象的{}块。连接将自动关闭。

第二，如果您想显式关闭连接，那么在所有数据库操作完成后将其关闭。

例如，在适配器填充操作完成时将其关闭。

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using (SqlDataAdapter dAd = new SqlDataAdapter("SELECT * FROM LoanPortfolio where custID like 'OH00002'", conn))
                {
                    DataTable dTable = new DataTable();
                    dAd.Fill(dTable);
                    conn.Close();
                    return dTable;
                }

本节：

conn.Close();
using (SqlDataAdapter dAd = new SqlDataAdapter("SELECT * FROM LoanPortfolio where custID like 'OH00002'", conn))
{
    DataTable dTable = new DataTable();
    dAd.Fill(dTable);
    return dTable;
}

关闭后您正在重新使用同一个连接。请重新打开并重复使用。或者根本不要关闭，因为您仍在同一个using块中。

更新：MSDN：http://msdn.microsoft.com/en-us/library/bh8kx08z（v=vs.100）.aspx

Fill方法隐式打开DataAdapter如果发现连接尚未打开，则使用。如果填充打开连接，当Fill为完成。

因此，在上面的部分"conn.close（）"应该无关紧要，"Fill"将打开并关闭连接。

using块将在完成连接后处理掉连接。因此，不需要显式关闭连接。

Sql dataAdapter本身管理连接。它在填充命令后打开和关闭。然而，使用ExecuteReader或ExecuteNonQuery明确要求打开和关闭连接，您可以在执行nonquery和ExecuteReader命令之前打开连接，如果您的代码遇到错误，并且代码没有到达conn.close（），那么还会发生什么情况命令。

   public class LoanDAL
{
string connString = ConfigurationManager.ConnectionStrings["Oakhorizons"].ToString();
public LoanDAL()
{
    //
    // TODO: Add constructor logic here
    //
}
public DataTable getAllLoanInfoDT()
{
        using (SqlConnection conn = new SqlConnection(connString))
        {
            SqlCommand cmd2 = new SqlCommand();
            cmd2.Connection = conn;
            // cmd.CommandType = CommandType.StoredProcedure;
            cmd2.CommandText = "SELECT DISTINCT loanUpdateDate FROM LoanPortfolio WHERE (custID LIKE 'OH00002') AND (loanType LIKE 'Personal Loan')";
            cmd2.Parameters.AddWithValue("@custID", "OH00002");
            cmd2.Parameters.AddWithValue("@loanType", "Personal Loan");
            DateTime loanUpdateDate = DateTime.Now;
            try
            {
             conn.Open();
            SqlDataReader myReader = cmd2.ExecuteReader();
            while (myReader.Read())
            {
                loanUpdateDate = Convert.ToDateTime(myReader[0]); 
                break; 
            }
               conn.Close();   
            DateTime currDateTime = DateTime.Now;
            }
            Catch(Exception Ex)
            {//Close connection in case of any exception .
              conn.Close();
            }
            int loanToBeAdded = (((currDateTime.Year - loanUpdateDate.Year) * 12) + currDateTime.Month - loanUpdateDate.Month) * 500;
            if (loanToBeAdded > 0)
            {
               try
               {
                String sql = "UPDATE LoanPortfolio SET loanPaid = loanPaid + " + loanToBeAdded.ToString() + ", LastUpdatedLoanPaidDate = " + DateTime.Now.ToString();
                sql += " WHERE (loanType LIKE 'Personal Loan') AND (custID LIKE 'OH00002')";
                cmd2.Connection = conn;
                cmd2.CommandText = sql;
                 conn.Open();
                cmd2.ExecuteNonQuery();
                conn.Close();
                }
                Catch(Exception Ex)
                 {
                 //Close connection in case of exception .
                  Conn.close()
                  throw Ex;
                 }
            }            
            using (SqlDataAdapter dAd = new SqlDataAdapter("SELECT * FROM LoanPortfolio where custID like 'OH00002'", conn))
            {
                DataTable dTable = new DataTable();
                dAd.Fill(dTable);
                return dTable;
            }
        }
}
//Returning a DataSet which contains all the information in the Player Table
public DataSet getAllLoanInfoDS()
{
    using (SqlConnection conn = new SqlConnection(connString))
    {
        using (SqlDataAdapter dAd = new SqlDataAdapter("SELECT * FROM LoanPortfolio where custID like 'OH00002", conn))
        {
            DataSet myDS = new DataSet();
            dAd.Fill(myDS);
            return myDS;
        }

    }
}
}

您的连接字符串需要添加"MultipleActiveResultSets=True"。如果您使用多个命令，就像在一个连接中有两个命令一样，请在连接字符串中启用"MultipleActiveResultSets=True"，或者在与服务器的每次事务后打开并关闭（每个连接一个命令）