在ASP.NET的上面，TextBox是表单设计时最常用的控件之一。很多时候为了页面的紧凑和美观，我们需要适当的限制TextBox的显示宽度，但是如果TextBox过于窄了之后，又会给用户的填写带了不便，而且更麻烦的是很多时候我们并不知道用户到底会往那个TextBox里填多少内容。为了解决这些问题，下面给大家推荐一个可自动适应输入内容的宽度的TextBox控件。

    本控件是从TextBox控件继承，设计原理是使用一个agent input(type=text)来做为实际的用户录入的TextBox，在焦点切换的过程中完成background input和agent input的属性同步。 

    下面的代码完成两个input之间的样式和属性同步：
#region function ATB_SwitchToInputAgent(input)
function ATB_SwitchToInputAgent(input)
{
    var spanWrapper = input.parentElement;
    var agentInput = spanWrapper.lastChild;
    var userOffsetTop, userOffsetLeft;
    with(input.style)
    {
        var userMarginTop = parseInt(marginTop);
        var userMarginLeft = parseInt(marginLeft);
        var userBorderTop = parseInt(borderTopWidth);
        var userBorderLeft = parseInt(borderLeftWidth);
        userMarginTop = isNaN(userMarginTop) ? 0 : userMarginTop;
        userMarginLeft = isNaN(userMarginLeft) ? 0 : userMarginLeft;
        userBorderTop = isNaN(userBorderTop) ? 0 : userBorderTop;
        userBorderLeft = isNaN(userBorderLeft) ? 0 : userBorderLeft;
        userOffsetTop = userBorderTop + userMarginTop;
        userOffsetLeft = userBorderLeft + userMarginLeft;
    }
    var retouch = 0;
    agentInput.style.top = userOffsetTop;
    agentInput.style.left = userOffsetLeft-1;
    spanWrapper.style.zIndex = 2;
    agentInput.style.display = 'inline';
    agentInput.value = input.value;
    agentInput.style.borderWdith = input.style.borderWdith;
    agentInput.style.borderColor = input.style.borderColor;
    agentInput.style.backgroundColor = input.style.backgroundColor;
    agentInput.style.color = input.style.color;
    agentInput.style.fontFamily = input.style.fontFamily;
    agentInput.style.fontWegith = input.style.fontWeight;
    agentInput.style.fontSize = input.style.fontSize;
    agentInput.style.height = input.style.height;
    agentInput.style.fontStyle = input.style.fontStyle;
    try { agentInput.style.font = input.style.font } catch(exp){};
    agentInput.style.fontVariant = input.style.fontVariant;
    agentInput.style.zoom = input.style.zoom;
    agentInput.focus();
    agentInput.select();
}
  #endregion 
    注意：不能使用style=style或for( attribute in style)的方式来赋值，否这引起onpropertychange的死循环递归。

    第二个问题是使用onpropertychange属性来同步agent input的宽度和其内容宽度相等，代码如下： 
function ATB_AutoIncreaseWidth(input) 
{ 
    if ( input.style.display == 'none' ) return; 
    var spanWrapper = input.parentElement; 
    var userInput = spanWrapper.firstChild; 
    userInput.value = input.value; 
    var absOffsetWidth = GetAbsoluteOffsetLeft(input); 
    var docClientWidth = window.document.body.clientWidth; 
    if ( input.scrollWidth < userInput.clientWidth ) 
    { 
        if ( absOffsetWidth + styleWidth >= docClientWidth )  
        { 
            input.style.width = docClientWidth - absOffsetWidth; 
        } 
        else 
        { 
            input.style.width = userInput.clientWidth+2; 
        } 
        return; 
    } 
    var styleWidth = parseInt(input.style.width); 
    if ( styleWidth != input.scrollWidth+2 ) 
    { 
        if ( absOffsetWidth + styleWidth >= docClientWidth )  
        { 
            input.style.width = docClientWidth - absOffsetWidth; 
        } 
        else 
        { 
            input.style.width = input.scrollWidth+2; 
        } 
    } 
} 
    这里需要注意的是当增长的agent input的最右端超出了IE的body区域时，需要停止其增长，否则用户看不见输入的东西了。 

    演示效果如下：
    
*

#region 附 AdjustableTextBox 控件源码
using System;
using System.IO;
using System.Drawing;
using System.Text;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.ComponentModel;

namespace WebExcel.UI.WebControls
{
    /// <summary>
    /// Summary description for AdjustableTextBox.
    /// </summary>
    [DefaultProperty("Text")]
    [ToolboxData("<{0}:AdjustableTextBox runat=server></{0}:AdjustableTextBox>")]
    public class AdjustableTextBox : TextBox
    {
        public Color AgentBorderColor
        {
            get
            {
                object obj = ViewState["AgentBorderColor"];
                return obj == null ? Color.Gray : (Color)obj;
            }
            set
            {
                ViewState["AgentBorderColor"] = value;
            }
        }

        public bool AutoIncrease
        {
            get
            {
                object obj = ViewState["AutoIncrease"];
                return obj == null ? true : (bool)obj;
            }
            set
            {
                ViewState["AutoIncrease"] = value;
            }
        }

        // the property is always TextBoxMode.SingleLine.
        public override TextBoxMode TextMode
        {
            get
            {
                return base.TextMode;
            }
            set
            {
                base.TextMode = value;
                if ( value != TextBoxMode.SingleLine )
                {
                    this.AutoIncrease = false;
                }
                else
                {
                    this.AutoIncrease = true;
                }
            }
        }

        public new Unit BorderWidth
        {
            get
            {
                if ( base.BorderWidth == Unit.Empty )
                {
                    base.BorderWidth = 1;
                }
                return base.BorderWidth;
            }
            set
            {
                if ( value != Unit.Empty )
                {
                    base.BorderWidth = value;
                }
            }
        }

        /// <summary> 
        /// Render this control to the output parameter specified.
        /// </summary>
        /// <param name="output"> The HTML writer to write out to </param>
        protected override void Render(HtmlTextWriter output)
        {
            if ( this.AutoIncrease )
            {
                this.RegisterClientScript();
                base.Attributes["onfocus"] = "ATB_SwitchToInputAgent(this)";
                base.Attributes.CssStyle["position"] = "relative";
                if ( base.BorderWidth == Unit.Empty )
                {
                    base.BorderWidth = 1;
                }
                string spanWrapper = @"<span style='z-index: 1; position: relative; border: solid 0px black;'>{0}<input type='text' onblur='ATB_GetAgentValue(this)' onpropertychange='ATB_AutoIncreaseWidth(this)' style='border: solid 1px gray; position: absolute; display:none;' /></span>";
                StringBuilder sb = new StringBuilder();
                StringWriter sw = new StringWriter(sb);
                HtmlTextWriter htw = new HtmlTextWriter(sw);
                base.Render(htw);
                output.Write(string.Format(spanWrapper, sb.ToString(), ColorTranslator.ToHtml(this.AgentBorderColor)));
            }
            else
            {
                // base.Attributes["onfocus"] = "this.height='100%'";
                base.Render(output);
            }
        }

        private void RegisterClientScript()
        {
            const string COMMON_SCRIPT_KEY = "__CommonJavaScript__";
            string strCommon = @"<script language='javascript'>
            function GetAbsoluteOffsetLeft(element)
            {
                if ( element == null || arguments.length != 1 )
                {
                    return;
                }
                var offsetLeft = element.offsetLeft;
                while( element = element.offsetParent )
                {
                    offsetLeft += element.offsetLeft;
                }
                return offsetLeft;
            }
            </script>";
            if ( !this.Page.IsStartupScriptRegistered(COMMON_SCRIPT_KEY) )
            {
                this.Page.RegisterStartupScript(COMMON_SCRIPT_KEY, strCommon);
            }

            const string SCRIPT_KEY = "__AdjustableTextBoxKey__";
            string strScript = @"
                <script language='javascript'>
                function ATB_SwitchToInputAgent(input)
                {
                    if ( input.disabled ) return;
                    var spanWrapper = input.parentElement;
                    var agentInput = spanWrapper.lastChild;
                    var userOffsetTop, userOffsetLeft;
                    with(input.style)
                    {
                        var userMarginTop = parseInt(marginTop);
                        var userMarginLeft = parseInt(marginLeft);
                        var userBorderTop = parseInt(borderTopWidth);
                        var userBorderLeft = parseInt(borderLeftWidth);
                        userMarginTop = isNaN(userMarginTop) ? 0 : userMarginTop;
                        userMarginLeft = isNaN(userMarginLeft) ? 0 : userMarginLeft;
                        userBorderTop = isNaN(userBorderTop) ? 0 : userBorderTop;
                        userBorderLeft = isNaN(userBorderLeft) ? 0 : userBorderLeft;
                        userOffsetTop = userBorderTop + userMarginTop;
                        userOffsetLeft = userBorderLeft + userMarginLeft;
                    }
                    var retouch = 0;
                    agentInput.value = input.value; // must be mdified at first
                    agentInput.style.top = userOffsetTop;
                    agentInput.style.left = userOffsetLeft-1;
                    spanWrapper.style.zIndex = 10;
                    agentInput.style.display = 'inline';
                    agentInput.style.borderWdith = input.style.borderWdith;
                    agentInput.style.borderColor = input.style.borderColor;
                    agentInput.style.backgroundColor = input.style.backgroundColor;
                    agentInput.style.color = input.style.color;
                    agentInput.style.fontFamily = input.style.fontFamily;
                    agentInput.style.fontWegith = input.style.fontWeight;
                    agentInput.style.fontSize = input.style.fontSize;
                    agentInput.style.height = input.style.height;
                    var absOffsetWidth = GetAbsoluteOffsetLeft(input);
                    var docClientWidth = window.document.body.clientWidth;
                    var styleWidth = parseInt(agentInput.style.width);
                    if ( absOffsetWidth + styleWidth >= docClientWidth ) 
                    {
                        agentInput.style.width = docClientWidth - absOffsetWidth;
                    }
                    else
                    {
                        agentInput.style.width = input.clientWidth+2;
                    }
                    agentInput.style.fontStyle = input.style.fontStyle;
                    try { agentInput.style.font = input.style.font } catch(exp){};
                    agentInput.style.fontVariant = input.style.fontVariant;
                    agentInput.style.zoom = input.style.zoom;
                    agentInput.readOnly = input.readOnly;
                    //agentInput.focus(); must remove!!!
                    if ( !agentInput.readOnly )
                    {        
                        agentInput.select();
                    }
                }

                function ATB_GetAgentValue(input)
                {
                    var spanWrapper = input.parentElement;
                    var userInput = spanWrapper.firstChild;
                    input.style.width = userInput.clientWidth+2;
                    spanWrapper.style.zIndex = 1;
                    userInput.value = input.value;
                    input.style.display = 'none';
                }
                
                function ATB_AutoIncreaseWidth(input)
                {
                    if ( input.style.display == 'none' ) return;
                    var spanWrapper = input.parentElement;
                    var userInput = spanWrapper.firstChild;
                    userInput.value = input.value;
                    var absOffsetWidth = GetAbsoluteOffsetLeft(input);
                    var docClientWidth = window.document.body.clientWidth;
                    if ( input.scrollWidth < userInput.clientWidth )
                    {
                        if ( absOffsetWidth + styleWidth >= docClientWidth ) 
                        {
                            input.style.width = docClientWidth - absOffsetWidth;
                        }
                        else
                        {
                            input.style.width = userInput.clientWidth+2;
                        }
                        return;
                    }
                    var styleWidth = parseInt(input.style.width);
                    if ( styleWidth != input.scrollWidth+2 )
                    {
                        if ( absOffsetWidth + styleWidth >= docClientWidth ) 
                        {
                            input.style.width = docClientWidth - absOffsetWidth;
                        }
                        else
                        {
                            input.style.width = input.scrollWidth+2;
                        }
                    }
                }
                </script>";
            if ( !this.Page.IsStartupScriptRegistered(SCRIPT_KEY) )
            {
                this.Page.RegisterStartupScript(SCRIPT_KEY, strScript);
            }
        }
    }
}
#endregion