js 统制显示加减号

<script type="text/javascript">
var SwitchPicArr = new Array("../Images/black_minus.jpg","../Images/black_plus.jpg")
   
function setDetailMsgRow(rowID, btn) {
        var row = document.getElementById(rowID);
        if (row != null) {
            if (row.style.display == (document.all ? "block" : "table-row")) {
                row.style.display = "none";

                 if (btn != null) {
                     btn.style.backgroundImage = "url(" + SwitchPicArr[1] + ")";
                }
            }
            else {
                row.style.display = (document.all ? "block" : "table-row");
                if (btn != null) {
                    btn.style.backgroundImage = "url(" + SwitchPicArr[0] + ")";
                }
            }
        }

//        if (btn != null) {
////            if (btn.value != "Hide Detail") {
////                btn.value = "Hide Detail";
////            }
////            else {
////                btn.value = "Display Detail";
////            }

//        }

    }

</script> 


.cshtml 部分：
  @if (item.MessageDetail == "")
                                { @Html.Raw(" ")}
                                else { 
                                     <input type=button value="" onclick="setDetailMsgRow('@sTmpRowID',this)" style="background:url(../Images/black_plus.jpg);width:40px;height:40px"/>
                                }