如何使用当用户进入文本框时，文本消失？ C＃

Question

Visual C#.NET:

    private void tbAddress_GotFocus()
    {
        tbAddress.Text = "";
    }

    private void tbAddress_LostFocus()
    {
        if (tbAddress.Text == "") { tbAddress.Text = "Email Address"; }
    }

So, I'm trying to use that code to make it so there is text in a (Windows Forms) textbox, and then when the textbox gets focus (when the user clicks inside of it), the text disappears. That way it looks like a form with the label inside the textbox.

So, why doesn't that code work, or is there a better way to do it?

Answer 1

Here's a tutorial on how to do watermarks in textboxes.

http://vidmar.net/weblog/archive/2008/11/05/watermarked-textbox-in-windows-forms-on-.net.aspx

Answer 2

Use the textBox1.GotFocus event

textBox1.GotFocus += textBox1_GotFocus;//at the designer, constructor or form load...

private void textBox1_GotFocus(object sender, EventArgs e)
{
    textBox1.Clear();//clear the text.
}

//Update:

If you already did that, then the problem must be in somewhere else, this code should be running without any problem.

Answer 3

+1 to @RexM for the pointer to the simplest, and probably best answer.

There's a more complex approach to this which is loosely based on the idea from this answer . It allows for customizing the color of the displayed text, but it probably has some other problems.

I'd really recommend using RexM's answer. Anyhow, for reference, here's the code for the other approach:

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

namespace MyControls
{
    public class HintedTextBox : TextBox
    {
        public HintedTextBox() : base() { ResetHintColor(); }

        [Description("The color of the hint text to display"),
        Category("Appearance")]
        public Color HintColor { get; set; }
        // Default value handling for HintColor
        private Color DefaultHintColor { get { return Color.LightGray; } }
        public void ResetHintColor() { HintColor = Color.LightGray; }
        public bool ShouldSerializeHintColor() { return !HintColor.Equals(DefaultHintColor); }

        [Description("The textual hint to display in the textbox"),
        Category("Behavior"),
        Localizable(true)]
        public string HintText
        {
            get { return m_hintText; }
            set
            {
                if (m_hintText != value)
                {
                    m_hintText = value;
                    UpdateHintTextState(true);
                }
            }
        }
        private string m_hintText = "";

        protected override void OnGotFocus(EventArgs e)
        {
            base.OnGotFocus(e);
            HasFocus = true;
            UpdateHintTextState();
        }

        protected override void OnLostFocus(EventArgs e)
        {
            base.OnLostFocus(e);
            HasFocus = false;
            UpdateHintTextState();
        }

        protected override void OnTextChanged(EventArgs e)
        {
            base.OnTextChanged(e);
            UpdateHintTextState();
        }

        protected override void WndProc(ref Message m)
        {
            base.WndProc(ref m);
            if (m.Msg == 15) // WM_PAINT
            {
                PaintHintText();
            }
        }

        private bool DisplayHintText { get; set; }

        private bool HasFocus { get; set; }

        private void PaintHintText()
        {
            if (DisplayHintText)
            {
                using (Graphics g = Graphics.FromHwnd(this.Handle))
                using (SolidBrush b = new SolidBrush(HintColor))
                {
                    StringFormat sf = new StringFormat();
                    switch (this.TextAlign)
                    {
                        case HorizontalAlignment.Center:
                            sf.Alignment = StringAlignment.Center;
                            break;
                        case HorizontalAlignment.Right:
                            sf.Alignment = StringAlignment.Far;
                            break;
                        default:
                            sf.Alignment = StringAlignment.Near;
                            break;
                    }
                    g.DrawString(HintText, Font, b, ClientRectangle, sf);
                }
            }
        }

        private void UpdateHintTextState() { UpdateHintTextState(false); }
        private void UpdateHintTextState(bool forceInvalidate)
        {
            bool prevState = DisplayHintText;

            if (HintText.Length == 0)
                DisplayHintText = false;
            else if (Text.Length != 0)
                DisplayHintText = false;
            else
                DisplayHintText = !HasFocus;

            if (DisplayHintText != prevState || forceInvalidate)
                Invalidate();
        }
    }
}

Answer 4

It doesn't work because by default the textbox does not trigger a postback, and without a postback, server side events do not process.

Your best option here is to use client side javascript to handle this functionality - because round tripping to to the server on each textbox event is wasteful and annoying for the user.