c# listbox add

How to add items in listbox and columns

 public string searchuser()
        {
            try
            {
                DirectoryEntry entry = new DirectoryEntry("LDAP://" + GetSystemDomain());
                DirectorySearcher Dsearch = new DirectorySearcher(entry);

                Dsearch.Filter = "(&((&(objectCategory=Person)(objectClass=User)))(samaccountname=" + txtsearch.Text + "))";

                using (StringWriter sw = new StringWriter())
                {

                    foreach (SearchResult sResultSet in Dsearch.FindAll())
                    {

                        listBox1.Items.Add(sw.WriteLine("{0}", (GetProperty(sResultSet, "Name", "Name"))));

                    }
                    return sw.ToString();
                }
            }
            catch (Exception Ex)
            {
                MessageBox.Show(Ex.Message);
            }

            return null;
        }

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