SqlConnection sqlCnn = new SqlConnection("server=.;database=test;uid=sa");
   sqlCnn.Open();
   SqlCommand sqlCmm = new SqlCommand("Insert into users (username)values('test2');select @@identity",sqlCnn);
   object obj = sqlCmm.ExecuteScalar();
   MessageBox.Show(obj.ToString()); 
//数据库users表结构：
id -- int 标识，自增1
username - varchar 50