How to add space before text in combobox

Note: an example of how to add a space character (or any other strings) to the front (prefix) or end (suffix) of the ComboBox Items appears at the end, here.

Please understand that these basic Controls provided by WinForms are essentially "little-black-boxes," an "opaque wrapper" of .NET around old COM based Controls. As a group, they are inconsistent, and I would argue that, with rare exceptions, any time a developer spends "hacking" these Controls using the Win API, is wasting time.

There is already a useful, proven, method ("Composition") to create custom Controls: make a UserControl that contains the "atomic" Controls you want to use as an "entity."

So, add a UserControl to your Project, put a ComboBox, and a Button, into the UserControl. Spend some time tweaking the visual appearance of the UserControl.

The code-behind can look this simple:

using System;
using System.Windows.Forms;

namespace YourNameSpace
{
    public partial class UCComboBox : UserControl
    {
        public UCComboBox()
        {
            InitializeComponent();
        }

        // event handler for new item Button click to be "injected" here
        public Action<ComboBox> GetNewItemAction { set; get; }

        // event handler for ComboBox selection to be "injected" here
        public Action<int, object> ComboSelectionAction { set; get; }

        // one possible way an external "client" can set the content (Items)
        // of the internal ComboBox, with an option to either clear
        // the existing Items, or append the new Items to the existing Items
        public void SetComboItems(bool doClearItemsFirst, params string[] items)
        {
            if(doClearItemsFirst) TheCombo.Items.Clear();

            TheCombo.Items.AddRange(items);
        }

        // the Button is clicked
        private void btnAddNewItem_Click(object sender, EventArgs e)
        {
            if (GetNewItemAction != null) GetNewItemAction(TheCombo);
        }

        // selection in the ComboBox
        private void TheCombo_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (ComboSelectionAction != null) ComboSelectionAction(TheCombo.SelectedIndex, TheCombo.SelectedItem);
        }
    }
}

An example of the use of this UserControl:

a. assuming you've either drag-dropped an instance of the UserControl from the ToolBox, or created one in code: it's named 'UCComboBox1:

// in Form Load EventHandler:
private void Form1_Load(object sender, EventArgs e)
{
    UCComboBox1.SetComboItems(
        true,
        "one", "two", "three", "four",
        "five", "six", "seven", "eight");
    
    UCComboBox1.ComboSelectionAction = ComboSelectionAction;
    
    UCComboBox1.GetNewItemAction = GetNewItemAction;
}

// event that receives notification from UserControl ComboBox
// when a selection is made
private void ComboSelectionAction(int ndx, object item)
{
    Console.WriteLine("selected index: {0} selected item: {1}", ndx, item);
}

// event that receives notification from UserControl Button
// when Button is clicked
private void GetNewItemAction(ComboBox comboBox)
{
    comboBox.Items.Add("nine");
}

But, how do you do add spaces (or anything else) to the Items in the ComboBox ?

Well, you could "automate" that pretty easily:

// requires Linq
// in the UserControl
public void SetComboItems(bool doClearItemsFirst, string prefix, string suffix, params string[] items)
{
    if(doClearItemsFirst) TheCombo.Items.Clear();

    TheCombo.Items.AddRange(AdjustItems(prefix, suffix, items));
}

public string[] AdjustItems(string prefix, string suffix, string[] items)
{
   return items.Select(itm => prefix + itm + suffix).ToArray();
}

To use this:

// in the client
UCComboBox1.SetComboItems(
    true,  // clear items
    "{",   // prefix
    "}",   // suffix
    "one", "two", "three", "four",   // items
    "five", "six", "seven", "eight");

Notes:

0. in case it's not clear to you: 'Action and 'Func are just a convenient way to use the Delegate Type. These facilities were added in .NET 3.5.

1. the choice to pass a reference to the ComboBox itself to the external client/user when the Button is clicked is based on the idea that the client/user may wish to do some editing/adjustment to the ComboBox Items beyond just adding a new Item. if adding a new Item is all that's required, you could use a 'Func that returned some 'object or 'string to add to the ComboBox, rather than an 'Action, and avoid "exposing" the ComboBox outside its host Class.