ASP.NET如何在TEXTBOX光标定位处插入字符？哪位高手做过

ASP.NET怎么在TEXTBOX光标定位处插入字符？谁做过？
我想用个按钮比如它是“＋”　如果光标定位在TEXBOX里的字符串的中间位置就插入

[解决办法]
mark
[解决办法]

HTML code

<script language="javascript">function addString(str){     obj=document.getElementById("txt");     obj.focus();     if(document.selection==null)     {         var iStart = obj.selectionStart;         var iEnd = obj.selectionEnd;         obj.value = obj.value.substring(0,iEnd) + str + obj.value.substring(iEnd, obj.value.length);     }  else     {         var range =  document.selection.createRange();         range.text += str;     }     }</script><asp:TextBox ID="txt" runat="server" Text="oooooo"></asp:TextBox><input type="button" value="+" onClick="addString('k')">