A WinForms TextBox that has support for cue banners.

CueTextBox.cs

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

/// <summary>
/// A Windows text box control with a cue banner.
/// </summary>
public class CueTextBox : TextBox
{
    /// <summary>
    /// Contains platform invocation methods used by this class.
    /// </summary>
    private static class NativeMethods
    {
        /// <summary>
        /// The EM_SETCUEBANNER message - sets the textual cue.
        /// </summary>
        internal const uint EM_SETCUEBANNER = 0x1501;

        /// <summary>
        /// Sends the specified message to a window or windows.
        /// </summary>
        /// <param name="hWnd">
        /// A handle to the window whose window procedure will receive the message.
        /// </param>
        /// <param name="Msg">
        /// The message to be sent.
        /// </param>
        /// <param name="wParam">
        /// Additional message-specific information.
        /// </param>
        /// <param name="lParam">
        /// Additional message-specific information.
        /// </param>
        /// <returns>
        /// The result of the message processing.
        /// </returns>
        [DllImport("user32.dll", EntryPoint = "SendMessageW")]
        public static extern IntPtr SendMessage(
            IntPtr hWnd,
            UInt32 Msg,
            IntPtr wParam,
            [MarshalAs(UnmanagedType.LPWStr)] string lParam);
    }

    /// <summary>
    /// The textual cue banner that is displayed by the control to prompt the user for
    /// information.
    /// </summary>
    private string cue;

    /// <summary>
    /// Gets or sets the textual cue banner that is displayed by the control to prompt the
    /// user for information.
    /// </summary>
    public string Cue
    {
        get
        {
            return this.cue;
        }

        set
        {
            this.cue = value;
            this.UpdateCue();
        }
    }

    /// <summary>
    /// Raises the HandleCreated event.
    /// </summary>
    /// <param name="e">An EventArgs that contains no event data.</param>
    protected override void OnHandleCreated(EventArgs e)
    {
        base.OnHandleCreated(e);
        this.UpdateCue();
    }

    /// <summary>
    /// Updates the textual cue.
    /// </summary>
    private void UpdateCue()
    {
        if (this.IsHandleCreated && this.cue != null)
        {
            NativeMethods.SendMessage(
                this.Handle,
                NativeMethods.EM_SETCUEBANNER,
                (IntPtr)1,
                this.cue);
        }
    }
}