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Asked: 2026-05-26T06:09:02+00:00

I am using ASP.NET C# 4.0, my web form consist of input type file

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I am using ASP.NET C# 4.0, my web form consist of input type file to upload images.

i need to validate the image on client side, before saving it to my SQL database

since i am using input type= file to upload images, i do not want to post back the page for validating the image size, dimensions.

Needs your assistance
Thanks

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1 Answer

  1. Editorial Team

    you can do something like this…

    You can do this on browsers that support the new File API from the W3C, using the readAsDataURL function on the FileReader interface and assigning the data URL to the src of an img (after which you can read the height and width of the image). Currently Firefox 3.6 supports the File API, and I think Chrome and Safari either already do or are about to.

    So your logic during the transitional phase would be something like this:

    1. Detect whether the browser supports the File API (which is easy: if (typeof window.FileReader === 'function')).

    2. If it does, great, read the data locally and insert it in an image to find the dimensions.

    3. If not, upload the file to the server (probably submitting the form from an iframe to avoid leaving the page), and then poll the server asking how big the image is (or just asking for the uploaded image, if you prefer).

    Edit I’ve been meaning to work up an example of the File API for some time; here’s one:

    <!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-type" content="text/html;charset=UTF-8">
<title>Show Image Dimensions Locally</title>
<style type='text/css'>
body {
    font-family: sans-serif;
}
</style>
<script type='text/javascript'>

    function loadImage() {
        var input, file, fr, img;

        if (typeof window.FileReader !== 'function') {
            write("The file API isn't supported on this browser yet.");
            return;
        }

        input = document.getElementById('imgfile');
        if (!input) {
            write("Um, couldn't find the imgfile element.");
        }
        else if (!input.files) {
            write("This browser doesn't seem to support the `files` property of file inputs.");
        }
        else if (!input.files[0]) {
            write("Please select a file before clicking 'Load'");
        }
        else {
            file = input.files[0];
            fr = new FileReader();
            fr.onload = createImage;
            fr.readAsDataURL(file);
        }

        function createImage() {
            img = document.createElement('img');
            img.onload = imageLoaded;
            img.style.display = 'none'; // If you don't want it showing
            img.src = fr.result;
            document.body.appendChild(img);
        }

        function imageLoaded() {
            write(img.width + "x" + img.height);
            // This next bit removes the image, which is obviously optional -- perhaps you want
            // to do something with it!
            img.parentNode.removeChild(img);
            img = undefined;
        }

        function write(msg) {
            var p = document.createElement('p');
            p.innerHTML = msg;
            document.body.appendChild(p);
        }
    }

</script>
</head>
<body>
<form action='#' onsubmit="return false;">
<input type='file' id='imgfile'>
<input type='button' id='btnLoad' value='Load' onclick='loadImage();'>
</form>
</body>
</html>

    Works great on Firefox 3.6. I avoided using any library there, so apologies for the attribute (DOM0) style event handlers and such.

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