问关于如何更新DOM但如何使用代码隐藏检查数据的问题

EN

$("someButton").click(function () {
    //Get the image name
    var imageToCheck = $("#imgFileName").val();
    //construct the data to send to the handler
    var dataToSend = {
        fileName: imageToCheck
    };
    $.post("/somePath/ValidateImage.ashx", dataToSend, function (data) {
        if (data === "valid") {
            //Do something
        } else {
            //Handle error
        }
    }, "html");
})

public class Handler1 : IHttpHandler
{

    public void ProcessRequest(HttpContext context)
    {
        var fileName = context.Request["fileName"];
        var fullPath = Path.Combine("SomeLocalPath", fileName);

        //Do something to validate the file
        if (File.Exists(fullPath))
        {
            context.Response.Write("valid");
        }
        else
        {
            context.Response.Write("invalid");
        }

    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}

$(document).ready(function () {
    $('#selectBoxName').change(function (event) {

        var image_name = 'calculated image name';

        $.ajax({
            type: "POST",
            url: 'SomePage.aspx/CheckImageName',
            data: "{'imageName': '" + image_name + "'}",
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: function (msg) {
                alert(msg);
            },
            error: function (a, b, c) {
                alert("The image could not be loaded.");
            }
        });

    });
});

    [System.Web.Services.WebMethod(true)]
    [System.Web.Script.Services.ScriptMethod(ResponseFormat = System.Web.Script.Services.ResponseFormat.Json)]
    public static bool CheckImageName(string imageName)
    {
        /*
         * Do some logic to check the file


        if (file exists)
        return true;

        return false;
         */
    }

Like this:
http://stackoverflow.com/questions/333634/http-head-request-in-javascript-ajax



<script type="text/javascript">
    function UrlExists(url) {
        var http = new XMLHttpRequest();
        http.open('HEAD', url, false);
        http.send();
        return http.status != 404;
    }

    function ConstructImage() {
        var e = document.getElementById("opt");
        var url = '[yourpath]/' + e.value + '.jpg';
        if (!UrlExists(url)) {
            alert('doesnt exists');
            //do stuff if doesnt exist
        } else {
            alert('exists');
            //change img if it does
        }
    }

</script>
<select id="opt" onchange="ConstructImage()">
    <option value="red">Red</option>
    <option value="blue">Blue</option>
    <option value="green">Green</option>
</select>