Getting Values From a Method In A Form Into Another Form
public class Form1 : Form
{
    private string user_code;

  public string UserCode
    {
        get { return user_code; }
    }

    public bool LoginUser()
    {
   user_code = null;


    if(textBox1.Text=="123"){
          user_code="usercode";
             }
         *
         */
    }
}

Form1 form1 = new Form1();
          form1.LoginUser();

        MessageBox.Show(form1.UserName);

objfrm1.loginuser(out U, out v, out w, out a,out b, out c);

public class Form1 : Form
    {
        private string user_pass;
        private string user_name;
        private bool insert_ability;
        private bool update_ability;
        private bool delete_ability;

        public string UserPass
        {
            get { return user_pass; }
        }

        public string UserName
        {
            get { return user_name; }
        }

        public bool InsertAbility
        {
            get { return insert_ability; }
        }

        public bool UpdateAbility
        {
            get { return update_ability; }
        }

        public bool DeleteAbility
        {
            get { return delete_ability; }
        }

        public bool LoginUser()
        {
            /*
              Your code here
              user_pass = "userpass";
              user_name = "username";
              insert_ability = true;

              update_ability = false;
              delete_ability = false;
             *
             */
        }
    }

Form1 form1 = new Form1();
            form1.LoginUser();

            MessageBox.Show(form1.UserName);

public class Form1 : Form
{
    private string user_code;

  public string UserCode
    {
        get { return user_code; }
    }

    private string _testData;                //THIS IS NEEDED
    public string TestData                   //THIS IS NEEDED
    {
        set { _testData = value;}        //THIS IS NEEDED
    }

    public bool LoginUser()
    {
        user_code = null;
        if(textBox1.Text=="123" || TestData=="123")   //THIS IS NEEDED
        {
            user_code="usercode";
        }
    }
}

Form1 form1 = new Form1();
form1.TestData = "123";  //THIS IS NEEDED
form1.LoginUser();

MessageBox.Show(form1.UserName);

class FormLogin : Form
    {
        public string UserName
        {
            get { return textBoxUserName.Text; }
            set { textBoxUserName.Text = value; }
        }

        public string Password
        {
            get { return textBoxPassword.Text; }
            set { textBoxPassword.Text = value; }
        }

        public FormLogin()
        {
            this.Closing += new System.ComponentModel.CancelEventHandler(FormLogin_Closing);
        }

        void FormLogin_Closing(object sender, System.ComponentModel.CancelEventArgs e)
        {
            if (this.DialogResult == DialogResult.Cancel) return;

            if (!LoginUser(UserName, Password))
            {
                MessageBox.Show("Username or password is incorrect");
                e.Cancel = true;
            }

        }

        bool LoginUser(string userName, string password)
        {
            // check login and password in database for example

            return true;
        }
    }

    class Form3 :Form
    {
        void LoginButtonClick()
        {
            FormLogin formLogin = new FormLogin();
            formLogin.UserName = "LastUserName";
            //uncomment if you want hide main form
            //this.Hide();
            if (formLogin.ShowDialog() == DialogResult.OK)
            {
                MessageBox.Show(string.Format("Congratulation! You are logged as {0}", formLogin.UserName));
            }

            //this.Show();
        }
    }