How to get the category list and set category on a contact

using SuperOffice.CRM.Services;
using SuperOffice;

private void button3_Click(object sender, EventArgs e)
{
  using (SoSession mySession = SoSession.Authenticate("SAL0", ""))
  {
    //Get the MDO agent
    using(MDOAgent mdoAgent = new MDOAgent())
    {
      SelectableMDOListItem[] categoryList =
       mdoAgent.GetSelectableList("category", false  , "", false);

      //Set the datasource of the control
      cmbCategory.DataSource = categoryList;

      //Set the display member
      cmbCategory.DisplayMember = "Name";

      //Set the value member
      cmbCategory.ValueMember = "Id";
    }
  }
}

private void button4_Click(object sender, EventArgs e)
{
  using (SoSession mySession = SoSession.Authenticate("SAL0", ""))
  {
    //Retrieve a contact agent
    using(ContactAgent contactAgent = new ContactAgent())
    {
      //Retrieve the contact entity you want through the contact agent
      ContactEntity myContact = contactAgent.GetContactEntity(4);

      //Set the category ID of the contact using selected value of the combo box control
      myContact.Category.Id = System.Convert.ToInt32(cmbCategory.SelectedValue);

      //Finally save contact entity
      contactAgent.SaveContactEntity(myContact);
    }
  }
}