c#: Label控件加入AutoHeight属性

此功能在界面布局中颇为实用，录代码以记之：

    public class LabelEx : Label
    {
        private bool autoHeight = true;

        [DefaultValue(true)]
        public bool AutoHeight
        {
            get { return this.autoHeight; }
            set
            {
                this.autoHeight = value;
                RecalcHeight();
            }
        }

        protected override void OnAutoSizeChanged(EventArgs e)
        {
            base.OnAutoSizeChanged(e);
            RecalcHeight();
        }

        protected override void OnFontChanged(EventArgs e)
        {
            base.OnFontChanged(e);
            RecalcHeight();
        }

        protected override void OnSizeChanged(EventArgs e)
        {
            base.OnSizeChanged(e);
            RecalcHeight();
        }

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

        private void RecalcHeight()
        {
            if (this.AutoSize || !this.autoHeight)
                return;

            var size = TextRenderer.MeasureText(this.Text, this.Font);
            int lc = (int)Math.Ceiling(size.Width * 1.0 / this.ClientSize.Width);
            this.Height = lc * size.Height;
        }
    }

顺手，给TextBox加个水印文字(PlaceHolderText)功能：

    public class TextBoxEx : TextBox
    {
        private string placeHolderText;

        [DefaultValue("")]
        [Editor("System.ComponentModel.Design.MultilineStringEditor, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", typeof(UITypeEditor))]
        public string PlaceHolderText
        {
            get { return this.placeHolderText; }
            set
            {
                this.placeHolderText = value;
                PaintPlaceHolderText();
            }
        }

        protected override void WndProc(ref Message m)
        {
            base.WndProc(ref m);

            //若使OnPaint()起作用，须设置this.SetStyle(ControlStyles.UserPaint,true);，但这样控件显示都得重绘，所以直截WM_PAINT消息
            //WM_PAINT消息：0xf  WM_CTLCOLOREDIT：0x133
            if (m.Msg == 0xf || m.Msg == 0x133)
                PaintPlaceHolderText();
        }

        private void PaintPlaceHolderText()
        {
            if (this.Focused || string.IsNullOrEmpty(this.placeHolderText) || !string.IsNullOrEmpty(this.Text))
                return;

            var g = Graphics.FromHwnd(this.Handle);
            var bounds = this.ClientRectangle;
            g.FillRectangle(new SolidBrush(this.BackColor), bounds);
            var sf = new StringFormat();
            switch (this.TextAlign)
            {
                case HorizontalAlignment.Left:
                    sf.Alignment = StringAlignment.Near;
                    break;
                case HorizontalAlignment.Center:
                    sf.Alignment = StringAlignment.Center;
                    break;
                case HorizontalAlignment.Right:
                    sf.Alignment = StringAlignment.Far;
                    break;
            }
            bounds.Offset(-1, 1);
            g.DrawString(this.placeHolderText, this.Font, new SolidBrush(Color.DarkGray), bounds, sf);
        }
    }

参考资料：

Winform-TextBox实现 placeholder - zhishiheng的专栏 - CSDN博客