The following references need to be added

Microsoft.SqlServer.Smo
Microsoft.SqlServer.SmoEnum
Microsoft.SqlServer.ConnectionInfo

ServerConnection oServerCon = new ServerConnection(txtServer.Text, txtUserName.Text, txtPassword.Text);
Server oServer = new Server(oServerCon);

lvDbOptions.Items.Clear();
String Value = String.Empty;

if (ddlOption.SelectedItem.ToString().ToLower() == "tables")
{                        
     foreach (Table oTable in oServer.Databases[txtDatabase.Text].Tables)
     {
          if (oTable.IsSystemObject == false)
          {
               Value = oTable.ToString().Substring(oTable.ToString().IndexOf(".") + 2, (oTable.ToString().Length - 1) - (oTable.ToString().IndexOf(".") + 2));

               lvDbOptions.Items.Add(new ListViewItem(Value));
          }
     }
}
else if (ddlOption.SelectedItem.ToString().ToLower() == "stored procedures")
{
     foreach (StoredProcedure oProc in oServer.Databases[txtDatabase.Text].StoredProcedures)
     {
          if(oProc.IsSystemObject == false)
          {
               Value = oProc.ToString().Substring(oProc.ToString().IndexOf(".") + 2, (oProc.ToString().Length - 1) - (oProc.ToString().IndexOf(".") + 2));

               lvDbOptions.Items.Add(new ListViewItem(Value));
          }
     }
}
else if (ddlOption.SelectedItem.ToString().ToLower() == "functions")
{
     foreach (UserDefinedFunction oFunc in oServer.Databases[txtDatabase.Text].UserDefinedFunctions)
     {
          if (oFunc.IsSystemObject == false) 
          {
               Value = oFunc.ToString().Substring(oFunc.ToString().IndexOf(".") + 2, (oFunc.ToString().Length - 1) - (oFunc.ToString().IndexOf(".") + 2));

               lvDbOptions.Items.Add(new ListViewItem(Value));
          }
     }
}