这是代码 为什么结果直接显示输入有误    为什么if里面的没有执行  
string txt1 = "Data Source=pc2011091520fha;DataBase=学生;Integrated Security=SSPI;User Instance=False";
            SqlConnection conn = new SqlConnection(txt1);
            string sql = "select * from [zhuce]";
            SqlCommand cmd = new SqlCommand(sql, conn);
            conn.Open();
            SqlDataReader reader = cmd.ExecuteReader();
            while (reader.Read())
            {
                bool s=reader["Z_no"].ToString() == textBox1.Text.Trim() && (reader["passw"].ToString() == textBox2.Text.Trim());
                if (s ==true)
                {
                    MessageBox.Show("登录成功！");
                    
                    conn.Close();
                    reader.Close();
                    frmain fmain = new frmain();
                    fmain.Show();
                    
                    textBox1.Text = "";
                    textBox2.Text = "";
                    
                }
                else
                {
                    MessageBox.Show("输入有误!"); break;

你应该试用优化这样：

string sql = string.Format("select * from [zhuce] where Z_no='{0}' and passw='{1}'", textBox1.Text.Trim(), textBox2.Text.Trim());
SqlCommand cmd = new SqlCommand(sql, conn);
conn.Open();
SqlDataReader reader = cmd.ExecuteReader();
if (reader.Read())
{
   ...
}

bool s=reader["Z_no"].ToString() == textBox1.Text.Trim() && (reader["passw"].ToString() == textBox2.Text.Trim());
前面的==判断也要加括号，&&的优先级大于==