计算文本框的字算、计算TextBox的字数长度

          <asp:TextBox ID="txt_OUT_Purpose"

runat="server"

Height="54px"
onpaste="return false"

TextBoxMode="MultiLine"

Width="270px"

TextMode="MultiLine"
MaxLength="500"

onkeydown="return LimitLength(this);"

onkeyup="document.getElementById('snInfo').innerHTML=this.value.length+'/500';" >

</asp:TextBox>
<span id='snInfo' style="color: Red"></span>