Using a custom control or adding code every time you want to do this can get very cumbersome. The way I handle this is to add an initializer class that can be called from my form at runtime. The behavior of this code happens when the user starts typing, the image disappears. If the text box has no content, the image appears. I am handling the click event for the image window to set focus to the textarea to maintain the illusion that it is part of the control and offsets the left to allow | to show that the textbox has focus and is ready to receive input.

By writing a controller instead of a custom control, I don't have to propagate all events and properties from the textbox through my custom control. This class depends on System.Windows.Forms and can be included directly in your Windows Forms application or added to a control library that can be called from multiple applications.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Drawing;

namespace WindowsFormsApplication1
{
    class TextBoxIcon
    {
        public static TextBoxIcon AddIcon(TextBox textbox, Image icon)
        {
            if (icon != null) {
                return new TextBoxIcon(textbox, icon);
            } else {
                return null;
            }
        }

        private TextBox _TextBox;
        private PictureBox _PictureBox;

        private TextBoxIcon(TextBox textbox, Image icon) {
            this._TextBox = textbox;
            this._PictureBox = new PictureBox();
            this._PictureBox.BackColor = textbox.BackColor;
            this._PictureBox.Image = ScaleImage(icon);
            this._TextBox.Parent.Controls.Add(_PictureBox);
            this._PictureBox.Location = new Point(textbox.Left + 5, textbox.Top + 2);
            this._PictureBox.Size = new Size(textbox.Width - 10, textbox.Height - 4);
            this._PictureBox.Anchor = textbox.Anchor;
            this._PictureBox.Visible = _TextBox.Visible;
            this._PictureBox.BringToFront();
            textbox.Resize += TextBox_Resize;
            textbox.TextChanged += TextBox_TextChanged;
            textbox.Leave += TextBox_Leave;
            _PictureBox.Click +=  PictureBox_Click;
            textbox.VisibleChanged += TextBox_VisibleChanged;
        }

        public static Image ScaleImage(Image img) {
            if (img.Height == 16) {
                return img;
            } else {
                return new Bitmap(img, new Size((int)((img.Height / 16.0) * img.Width), 16));
            }
        }

        private void TextBox_Resize(Object sender, EventArgs e) {
            _PictureBox.Size = new Size(_TextBox.Width - 10, _TextBox.Height - 4);
        }

        private void TextBox_VisibleChanged(Object sender, EventArgs e) {
            _PictureBox.Visible = _TextBox.Visible;
        }

        private void ShowPictureBox() {
            _PictureBox.Visible = String.IsNullOrEmpty(_TextBox.Text);
        }

        private void TextBox_TextChanged(Object sender, EventArgs e) {
            ShowPictureBox();
        }

        private void TextBox_Leave(Object sender, EventArgs e) {
            ShowPictureBox();
        }

        public void PictureBox_Click(object sender, EventArgs e) { 
            _TextBox.Focus();
        }
    }
}

      

        
        
        
      

    

This is how the class will be used from the form:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            TextBoxIcon.AddIcon(txtSearch, Properties.Resources.search);
        }
    }
}

      

        
        
        
      

    

As long as the text box exists and is added to the form, the call can occur at any time.