try
{
string myconn="Integrated Security=SSPI;User Id=sa;database=jindu2;server=localhost";
SqlConnection myconnection=new SqlConnection (myconn);
myconnection.Open();//打开数据库连接
string usname="select * from userinfo where username="+textBox1.Text +" and password="+textBox2.Text +"";
SqlCommand cmdd = new SqlCommand(usname, myconnection);
SqlDataReader myreader=cmdd.ExecuteReader();
if(myreader.Read ())
{
Form1 form1=new Form1 ();
form1.Show ();
this.Dispose ();
}
else
MessageBox.Show ("你输入的密码不正确,请重新输入");
myreader.Close ();
myconnection.Close ();
}

catch(Exception ex)
{
MessageBox.Show (ex.ToString());
}

输入用户名和密码总是提示列名无效,比如用户名li,密码123,则提示列名'li'无效,列名'123'无效