尝试多次检索连接字符串时出错

如果我调用一次，我的方法就可以正常工作。如果我两次调用此方法，它将返回以下内容：

“ ConnectionString属性尚未初始化”

这是我的方法：

public void insertWerk(Werk werk)
{
    string conStr = ConfigurationManager.ConnectionStrings["MDSConnectionString"].ConnectionString;
    using (SqlConnection connection = new SqlConnection(conStr))
    {

        if (connection.State == ConnectionState.Closed)
        {
            Console.WriteLine(connection.ConnectionString);
            connection.Open();
        }

        using (SqlCommand cmd = new SqlCommand(@"usr.[gngImportZavody]", connection))
        {
            cmd.CommandType = CommandType.StoredProcedure;

            cmd.Parameters.Add("@Name", SqlDbType.NVarChar).Value = werk.WerksName.Replace("'", "''''");
            cmd.Parameters.Add("@Code", SqlDbType.NVarChar).Value = werk.WerksUniCode;
            cmd.Parameters.Add("@WerksType", SqlDbType.NVarChar).Value = werk.TypeWerks;
            cmd.Parameters.Add("@SapCode", SqlDbType.NVarChar).Value = werk.WerksCodeSAP;
            cmd.Parameters.Add("@BukrsSapCode", SqlDbType.NVarChar).Value = werk.BukrsCodeSAP;
            cmd.Parameters.Add("@Country", SqlDbType.NVarChar).Value = werk.Country;

            cmd.ExecuteNonQuery();
            Console.WriteLine(" GetWerksMDM : " + werk.WerksName);

        }


    }
}