Class1.cs

using System;
using System.ComponentModel;
using System.Drawing;
using System.Windows.Forms;

namespace Demo
{
    /// <summary>
    /// Represents a Windows text box control with placeholder.
    /// </summary>
    public class PlaceholderTextBox : TextBox
    {
        #region Properties

        string _placeholderText = DEFAULT_PLACEHOLDER;
        bool _isPlaceholderActive;


        /// <summary>
        /// Gets a value indicating whether the Placeholder is active.
        /// </summary>
        [Browsable(false)]
        public bool IsPlaceholderActive
        {
            get
            {
                return _isPlaceholderActive;
            }
            private set
            {
                if (value != _isPlaceholderActive)
                {
                    _isPlaceholderActive = value;

                    OnPlaceholderActiveChanged(value);
                }
            }
        }


        /// <summary>
        /// Gets or sets the placeholder in the PlaceholderTextBox.
        /// </summary>
        [Description("The placeholder associated with the control."), Category("Placeholder"), DefaultValue(DEFAULT_PLACEHOLDER)]
        public string PlaceholderText
        {
            get { return _placeholderText; }
            set
            {
                _placeholderText = value;

                // Only use the new value if the placeholder is active.
                if (this.IsPlaceholderActive)
                    this.Text = value;
            }
        }


        /// <summary>
        /// Gets or sets the current text in the TextBox.
        /// </summary>
        [Browsable(false)]
        public override string Text
        {
            get
            {
                // Check 'IsPlaceholderActive' to avoid this if-clause when the text is the same as the placeholder but actually it's not the placeholder.
                // Check 'base.Text == this.Placeholder' because in some cases IsPlaceholderActive changes too late although it isn't the placeholder anymore.
                // If you want to get the Text Property and it still contains the placeholder, an empty string will return.
                if (this.IsPlaceholderActive && base.Text == this.PlaceholderText)
                    return String.Empty;

                return base.Text;
            }
            set { base.Text = value; }
        }

        /// <summary>
        /// Gets or sets the foreground color of the control.
        /// </summary>
        public override Color ForeColor
        {
            get
            {
                // We have to differentiate whether the system is asking for the ForeColor to draw it
                // or the developer is asking for the color.
                if (this.IsPlaceholderActive && Environment.StackTrace.Contains("System.Windows.Forms.Control.InitializeDCForWmCtlColor(IntPtr dc, Int32 msg)"))
                    return Color.LightGray;

                return base.ForeColor;
            }
            set
            {
                base.ForeColor = value;
            }
        }


        /// <summary>
        /// Occurs when the value of the IsPlaceholderActive property has changed.
        /// </summary>
        [Description("Occurs when the value of the IsPlaceholderInside property has changed.")]
        public event EventHandler<PlaceholderActiveChangedEventArgs> PlaceholderActiveChanged;

        #endregion


        #region Global Variables

        /// <summary>
        /// Specifies the default placeholder text.
        /// </summary>
        const string DEFAULT_PLACEHOLDER = "<Input>";

        /// <summary>
        /// Flag to avoid the TextChanged Event. Don't access directly, use Method 'ActionWithoutTextChanged(Action act)' instead.
        /// </summary>
        bool avoidTextChanged;

        #endregion


        #region Constructor

        /// <summary>
        /// Initializes a new instance of the PlaceholderTextBox class.
        /// </summary>
        public PlaceholderTextBox()
        {
            // Through this line the default placeholder gets displayed in designer
            base.Text = this.PlaceholderText;

            SubscribeEvents();

            // Set Default
            this.IsPlaceholderActive = true;
        }

        #endregion


        #region Functions

        /// <summary>
        /// Inserts placeholder, assigns placeholder style and sets cursor to first position.
        /// </summary>
        public void Reset()
        {
            this.IsPlaceholderActive = true;

            ActionWithoutTextChanged(() => this.Text = this.PlaceholderText);
            this.Select(0, 0);
        }

        /// <summary>
        /// Run an action with avoiding the TextChanged event.
        /// </summary>
        /// <param name="act">Specifies the action to run.</param>
        private void ActionWithoutTextChanged(Action act)
        {
            avoidTextChanged = true;

            act.Invoke();

            avoidTextChanged = false;
        }

        /// <summary>
        /// Subscribe necessary Events.
        /// </summary>
        private void SubscribeEvents()
        {
            this.TextChanged += PlaceholderTextBox_TextChanged;
        }

        #endregion


        #region Events

        private void PlaceholderTextBox_TextChanged(object sender, EventArgs e)
        {
            // Check flag
            if (avoidTextChanged) return;

            // Run code with avoiding recursive call
            ActionWithoutTextChanged(delegate
            {
                // If the Text is empty, insert placeholder and set cursor to to first position
                if (String.IsNullOrEmpty(this.Text))
                {
                    Reset();
                    return;
                }

                // If the placeholder is active, revert state to a usual TextBox
                if (this.IsPlaceholderActive)
                {
                    this.IsPlaceholderActive = false;

                    // Throw away the placeholder but leave the new typed char
                    this.Text = this.Text.Replace(this.PlaceholderText, String.Empty);

                    // Set Selection to last position
                    this.Select(this.TextLength, 0);
                }
            });

            this.Font = this.Font;
        }

        protected override void OnGotFocus(EventArgs e)
        {
            // Without this line it would highlight the placeholder when getting focus
            this.Select(0, 0);
            base.OnGotFocus(e);
        }

        protected override void OnMouseDown(MouseEventArgs e)
        {
            // When you click on the placerholderTextBox and the placerholder is active, jump to first position
            if (this.IsPlaceholderActive)
                Reset();

            base.OnMouseDown(e);
        }

        protected override void OnKeyDown(KeyEventArgs e)
        {
            // Prevents that the user can go through the placeholder with arrow keys
            if (IsPlaceholderActive && (e.KeyCode == Keys.Left || e.KeyCode == Keys.Right || e.KeyCode == Keys.Up || e.KeyCode == Keys.Down))
                e.Handled = true;

            base.OnKeyDown(e);
        }

        protected virtual void OnPlaceholderActiveChanged(bool newValue)
        {
            if (PlaceholderActiveChanged != null)
                PlaceholderActiveChanged(this, new PlaceholderActiveChangedEventArgs(newValue));
        }

        #endregion
    }

    /// <summary>
    /// Provides data for the PlaceholderActiveChanged event.
    /// </summary>
    public class PlaceholderActiveChangedEventArgs : EventArgs
    {
        /// <summary>
        /// Initializes a new instance of the PlaceholderInsideChangedEventArgs class.
        /// </summary>
        /// <param name="newValue">The new value of the IsPlaceholderInside Property.</param>
        public PlaceholderActiveChangedEventArgs(bool newValue)
        {
            this.NewValue = newValue;
        }

        /// <summary>
        /// Gets the new value of the IsPlaceholderActive property.
        /// </summary>
        public bool NewValue { get; private set; }
    }
}