How to search in object list?

I agree with Wayne, but would add that you should never store passwords in clear anyway. See Password Storage: How to do it.[^]

The logic of your code is wrong: the error condition should be signaled only at the end of the loop.
I'm not a ASP expert but the following code should do the trick:

foreach (Member_Login M in objList)
{
  if (txtUsername.Text == M.Username && txtPassword.Text == M.Password)
  {
    lblresult.Text = M.Member_Id.ToString();
    Session["Memid"] = M.Member_Id.ToString();
    Response.Redirect("Home.aspx");
  }
}
// 'redirect' didn't occur: member not found
MessageBox.Show("You are not found as a registered member", "Error!! Invalid Account", MessageBoxButtons.OK, MessageBoxIcon.Warning);

You are basically comparing all the usernames and passwords against data entered by the user.
If the condition (txtUsername.Text == M.Username && txtPassword.Text == M.Password) fails, the code displays the message box.

This is not correct code. Do not display the messagebox every time the username and password do not match.
Instead use a flag to keep track of the match status. If there is a match, set the flag and break out of the loop.

Check for a condition on the flag after the loop. If the flag is not set, display the message.

Something must have happened when i copied the code coz i only used the Member_Login class. It still gives me the same problem.
Is there another way to structure my foreach so it that it only returns the correct value without returning multiple error messages.

Member_Login objM = new Member_Login();
            List<member_login> objList = new List<member_login>();
            objList = objM.Get_Member_Details();           
 
            foreach (Member_Login M in objList)
            {
                if (txtUsername.Text == M.Username && txtPassword.Text == M.Password)
                {
                    lblresult.Text = M.Member_Id.ToString();
                    Session["Memid"] = M.Member_Id.ToString();
                    Response.Redirect("Home.aspx");
                }
                else
                {
                    MessageBox.Show("You are not found as a registered member", "Error!! Invalid Account", MessageBoxButtons.OK, MessageBoxIcon.Warning);
       
                }
            }
</member_login></member_login>

Edit: Code formatted

lol you guys are the best... its working now.. Thanx guys.