AutoHeightTextBox.cs

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

namespace DigitalPlatform.Forms
{
    // http://stackoverflow.com/questions/10574998/multiline-textbox-auto-adjust-its-height-according-to-the-amount-of-text
    /// <summary>
    /// 能根据文本内容自动调整高度的订制 TextBox 控件
    /// </summary>
    public class AutoHeightTextBox : TextBox
    {
        int _lineCount = 1;

        public event EventHandler HeightChanged = null;

        // 因为文本变化调整高度引起的高度变化
        public virtual void OnHeightChanged()
        {
            if (this.HeightChanged != null)
                this.HeightChanged(this, new EventArgs());
        }

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

            if (this.Visible == true)
                _lineCount = 0; // 迫使后面重新初始化 Height
        }

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

            if (this.Visible == true)
                SetHeight();
        }

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

            SetHeight();
        }

        public void SetHeight()
        {
            var numberOfLines = API.SendMessage(this.Handle, API.EM_GETLINECOUNT, 0, 0).ToInt32();
            numberOfLines = Math.Max(1, numberOfLines);

            if (numberOfLines != _lineCount)
            {
                int nBorderWidth = 0;
                if (this.BorderStyle == System.Windows.Forms.BorderStyle.Fixed3D)
                    nBorderWidth = SystemInformation.Border3DSize.Height * 2;
                else if (this.BorderStyle == System.Windows.Forms.BorderStyle.FixedSingle)
                    nBorderWidth = SystemInformation.BorderSize.Height * 2;

                /*
                 * Set TextBox Height -
http://www.codeproject.com/Articles/29140/Set-TextBox-Height
Textboxes have a 3-pixel lower and 4-pixel upper white space around
the font height. Therefore, the calculation increases the height by 7
pixels.
                 * */
                int nNewHeight = (this.Font.Height + 1) * numberOfLines + nBorderWidth + 7;
                if (this.Height != nNewHeight)
                {
                    this.Height = nNewHeight;
                    this.OnHeightChanged();
                }

                _lineCount = numberOfLines;
            }
        }
    }

}