try
                    {
                        conn.Open();
                        trans = conn.BeginTransaction();
                        cmd.Transaction = trans;  //命令的Transaction属性初始化
                        int i=cmd.ExecuteNonQuery();
                         = "update [table] set accountamount=accountamount+100 where accountid='B'";
                        cmd.ExecuteNonQuery();
                        ();
                    }
                    catch (SqlException ex)
                    {
                        trans.Rollback();//显示错误，trans未赋值，是什么原因啊？？？
                       throw ex;
                    }
                    finally
                    {
                        conn.Close();
                    }

[ 本帖最后由 niaomingjian 于 2012-2-1 15:13 编辑 ]

trans申请在try的外面

谢谢！这样就可以了
conn.Open();
trans = conn.BeginTransaction();  
try
{
  cmd.Transaction = trans;
  ......

为什么这样做就可以啊，可以解释一下吗？谢谢

在try块中申明或者赋值的变量，其作用域只在try块中。当程序遇到异常时，此时定义的变量已经失效。

原来这样啊，与作用域有关系啊，谢谢！