Error in Chat Application C#

namespace ServiceClass
{       public class Class1 : MarshalByRefObject
        {
            Dictionary<string, string> Dc = new Dictionary<string, string>();
            public void adduser(string name)
            {
                Dc.Add(name, " ");
            }
            public void setmessage(string from, string to, string message)
            {
                Dc[from] = Dc[from] + from + ":" + message + Environment.NewLine;
                Dc[to] = Dc[to] + from + ":" + message + Environment.NewLine;
            }
            public string getmessage(string u)
            {
                return Dc[u];
            }
            public ICollection<string> users()
            {
                return Dc.Keys;
            }   }}

namespace Trial
{
 public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        public static string UserName;
        private void button1_Click_1(object sender, EventArgs e)
        {
            UserName = textBox1.Text;
            Form2 ob = new Form2();
            ob.Show();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            ServiceClass.Class1 obj = (ServiceClass.Class1)Activator.GetObject(typeof(Class1), "Tcp://localhost:8085/ServiceClass");} } }

namespace Trial
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }
        ServiceClass.Class1 obj = (ServiceClass.Class1)Activator.GetObject(typeof(Class1), "Tcp://localhost:8085/ServiceClass");
       
        private void Form2_Load_1(object sender, EventArgs e)
        {
            
            label1.Text = Form1.UserName;
            obj.adduser(Form1.UserName);
        }
        private void button1_Click_1(object sender, EventArgs e)
        {
            obj.setmessage(label1.Text, comboBox1.SelectedItem.ToString(), textBox1.Text);
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            ICollection<string> ic = obj.users();
            foreach (string s in ic)
            {
                if (comboBox1.Items.Contains(s))
                {
                    comboBox1.Items.Add(s);
                }
                if (comboBox1.SelectedItem != null)
                {
                    textBox1.Text = obj.getmessage(Form1.UserName);
                }
            }
        }

namespace Trial
{
  public   interface Class1
    {
        void adduser(string name);

        void setmessage(string from, string to, string message);

        string getmessage(string u);

        ICollection<string> users();
    }
}