首先,你要在你的SQL语句前面将ziduan 赋值。比如 ziduan="sfzh";
然后,将你的sql 改成如下
string sql = "select [xm],[sfzh],[zymc],[byxxmc] from [grinfo] where " + ziduan + "='" + this.txb.Text.Trim() + "'";
把你下面的语句改成下面的即可:
string sql = "select [xm],[sfzh],[zymc],[byxxmc] from [grinfo] where " + ziduan + "= '" + this.txb.Text.Trim() + "'"