TextBox的TextBoxMode设置为MultiLine，maxlength属性失效原因小探

项目会遇到标题所示的原因，今天趁空闲时间大概找了一下原因，借此抛砖引玉，希望大家提出更好的方法。

Asp.net TextBox控件TextBoxMode设置为MultiLine，maxlength属性失效是正常的，当TextBoxMode设置为 MultiLine时，生成的前台控件是textArea，而它是没有maxlength的属性的，而<input type="text">则有此属性。从TextBox的源码我们也可以看出一斑。

protected override void AddAttributesToRender(HtmlTextWriter writer)



{



int maxLength;



if (this.Page != null)



{



this.Page.VerifyRenderingInServerForm(this);



}



writer.AddAttribute(HtmlTextWriterAttribute.Name, this.UniqueID);



switch (this.TextMode)



{



case TextBoxMode.SingleLine:



{



writer.AddAttribute(HtmlTextWriterAttribute.Type, "text");



string text = this.Text;



if (text.Length > 0)



{



writer.AddAttribute(HtmlTextWriterAttribute.Value, text);



}



break;



}



case TextBoxMode.Password:



writer.AddAttribute(HtmlTextWriterAttribute.Type, "password");



break;







 case TextBoxMode.MultiLine:



maxLength = this.Rows;



if (maxLength > 0)



{



writer.AddAttribute(HtmlTextWriterAttribute.Rows, maxLength.ToString(NumberFormatInfo.InvariantInfo));



}



maxLength = this.Columns;



if (maxLength > 0)



{



writer.AddAttribute(HtmlTextWriterAttribute.Cols, maxLength.ToString(NumberFormatInfo.InvariantInfo));



}



if (!this.Wrap)



{



writer.AddAttribute(HtmlTextWriterAttribute.Wrap, "off");



}



goto Label_00FF;



}



maxLength = this.MaxLength;



if (maxLength > 0)



{



writer.AddAttribute(HtmlTextWriterAttribute.Maxlength, maxLength.ToString(NumberFormatInfo.InvariantInfo));



}



maxLength = this.Columns;



if (maxLength > 0)



{



writer.AddAttribute(HtmlTextWriterAttribute.Size, maxLength.ToString(NumberFormatInfo.InvariantInfo));



}



Label_00FF:



if (this.ReadOnly)



{



writer.AddAttribute(HtmlTextWriterAttribute.ReadOnly, "readonly");



}



if (this.AutoPostBack && (this.Page != null))



{



string postBackClientEvent = this.Page.GetPostBackClientEvent(this, "");



if (base.HasAttributes)



{



string str3 = base.Attributes["onchange"];



if (str3 != null)



{



postBackClientEvent = str3 + postBackClientEvent;



base.Attributes.Remove("onchange");



}



}



writer.AddAttribute(HtmlTextWriterAttribute.Onchange, postBackClientEvent);



writer.AddAttribute("language", "javascript");



}



base.AddAttributesToRender(writer);



}

注意标黄的部分这里但是multiline的时候是没有做maxlength的控制的。

所以我们必须自己去做这部分工作，其实也很简单，加几个属性就好了，这里需要注意，用键盘输入和鼠标右键粘贴的情况。最简单的做法我是写一个自定义控件，继承于TextBox。
大概代码如下。
public class MulTextBox :TextBox
    {
        protected override void OnPreRender(EventArgs e)
        {
            if (this.Page != null)
            {
                if (this.TextMode == TextBoxMode.MultiLine)
                {
                    this.Page.ClientScript.RegisterClientScriptResource(typeof(MulTextBox), "MulTextBox.CheckFunction.js");
                    this.Attributes.Add("onmousedown", "CheckTextboxLength('" + this.ClientID + "','" + this.MaxLength + "')");
                    this.Attributes.Add("onkeyup", "CheckTextboxLength('" + this.ClientID + "','" + this.MaxLength + "')");
                    this.Attributes.Add("onmouseout", "CheckTextboxLength('" + this.ClientID + "','" + this.MaxLength + "')");
                }
            }
            base.OnPreRender(e);
        }
       
    }

javascript：
function CheckTextboxLength(obj,maxlength)
{
       var textObj = document.getElementById(obj);
       var len = textObj.value.length;
       if(len > maxlength)
       {
            textObj.value = textObj.value.substring(0,maxlength);
       }
}
这里没有使用window.clipboardData.getData("Text")是由于使用此脚本会弹出一个提示，客户体验不是很好。