Populate and Update SharePoint Choice Field

SPFieldChoice and SPFieldMultiChoice are two type of SharePoint Choice field. The first one is single selection and displayed as dropdown menu or the radio buttons; the latter is multiple selections and displayed as a list of check boxes. To populate the choice items in a custom webpart, we can simply go through the Choices collection in the choice field:

    // Populate Dropdown by SharePoint Choices (single selection from dropdown menu or radio buttons)
    private void PopulateSPChoiceDropDown(DropDownList dropdown, SPList list, string fieldName)
    {
        if (dropdown == null || list == null || list.ItemCount == 0)
            return;
            
        dropdown.Items.Clear();
        if (list != null && list.Fields.ContainsField(fieldName))
        {
            SPFieldChoice spChoices = list.Fields[fieldName] as SPFieldChoice;
            if (spChoices != null && spChoices.Choices != null)
            {
                for (int i = 0; i < spChoices.Choices.Count; i++)
                {
                    dropdown.Items.Add(new ListItem(spChoices.Choices[i], spChoices.Choices[i]));
                }
            }
        }
    }

    // Populate CheckBoxList by SharePoint Choices (muliple selction)
    private void PopulateSPChoiceCheckBoxList(CheckBoxList checkboxList, SPList list, string fieldName)
    {
        if (checkboxList == null || list == null || list.ItemCount == 0)
            return;
            
        checkboxList.Items.Clear();
        if (list != null && list.Fields.ContainsField(fieldName))
        {
            SPFieldMultiChoice spChoices = list.Fields[fieldName] as SPFieldMultiChoice;
            if (spChoices != null && spChoices.Choices != null)
            {
                for (int i = 0; i < spChoices.Choices.Count; i++)
                {
                    checkboxList.Items.Add(new ListItem(spChoices.Choices[i], spChoices.Choices[i]));
                }
            }
        }
    }

As a matter of fact, we can merge above two methods into one:

    // Populate List Control such as Dropdown and CheckBoxList by SharePoint Choices
    private void PopulateSPChoiceToListControl(ListControl listControl, SPList list, string fieldName)
    {
        if (listControl == null || list == null || list.ItemCount == 0)
            return;
      
        listControl.Items.Clear();
        if (list != null && list.Fields.ContainsField(fieldName))
        {
            SPFieldMultiChoice spChoices = list.Fields[fieldName] as SPFieldMultiChoice;
            if (spChoices != null && spChoices.Choices != null)
            {
                for (int i = 0; i < spChoices.Choices.Count; i++)
                {
                    listControl.Items.Add(new ListItem(spChoices.Choices[i], spChoices.Choices[i]));
                }
            }
        }
    }

The reason we can merge them together is because SPFieldChoice is inherited from SPFieldMultiChoice, and both DropDownList and CheckBoxList are implementation of abstract ListControl class. To update the SPField:

        SPFieldMultiChoiceValue values = new SPFieldMultiChoiceValue();
        foreach (ListItem listItem in checkList.Items)
        {
            if (listItem.Selected)
            {
                values.Add(listItem.Value);
            }
        }
        item["MultipleChoiceField"] = values;
        item["SingleChoiceField"] = dropdown.SelectedValue;
        item.Update();