Most websites use HTML5 and jQuery plugins to upload files without requiring the user to send a POST request to the server but that is not a productive way to upload files.
Prior way to upload files
In the old days, people would create a simple HTML form and assign the enctype to it. That would let them select a file and then they will click on the Submit button that would then take them to a new page, where they will be shown the message, “Your photos have been uploaded”, or an error message telling them that there was an error. This is a bit irritating and awkward, because the user uploaded more like 5 photos among them with one getting an error and the remaining 4 were also discarded and the user was told after 2 minutes to upload the 4 photos only and not that one specific image. Sometimes, the connection is lost and other stuff!
New way to upload files
The web is changing and there are many new ways for a user to upload an image or file to the server, without having to submit the form as he did back in 90s.
Using iframes
Well before HTML5, people used hidden iframes to target the form to be uploaded from. The iframe is just another HTML document embedded in your main HTML document that if even navigated to any other web page it doesn't trigger a navigation event in the main page. Which means that if the form is submitted using an iframe then the main page the user is at is never submitted. But he can still view the results and other associated data, such as form elements and the result window.
HTML5 and JavaScript
But these are also old now, JavaScript is strong enough to upload your files and using HTML5 you can easily work your way with the validations of files too. You can manage which file to upload and which one to reject. This option also minimizes the chances for error for any file that was triggered in the old methods and the user was informed of those errors after he uploaded all of the HTTP request data.
In JavaScript, we can shorten the code for creating an Ajax request. An Ajax request is an asynchronous request to the server to get some resources from the server and display it in the web page without needing to reload the web page. This technology (Ajax) enables us to get data to the web server without having to use the POST requests and reload the web page for the new content and similarly it also enables us to send any data to the server, such as files, images and other data, to the web server. This makes the uploading of the content easy.
jQuery code
In this article I will be using jQuery; why? Because I love jQuery, it is shorter in syntax and looks cool, but remember it is slower than pure JavaScript because it is just a library that runs over JavaScript but the difference is barely noticeable. Ajax sends requests to the server, along with the file data that we want to be uploaded.
The first stage is to create the HTML form that will be capturing any of the file that the user wants to upload to the server. For example, the following code would be an example of the HTML form that will accept the files from the user and contain a simple button from which he will upload the files. I know, a button is an old method, but in this method this button won't perform the built-in function, it will be captured by JavaScript and the page will stay where it is all using the JavaScript. Watch the jQuery code for more on this section.
- <form method="post" id="form" enctype="multipart/form-data">
- <input type="file" name="file" />
- <input type="submit" value="Upload" />
- </form>
The preceding is the example of the form. Now once we look into the jQuery code, we will be able to omit the enctype=”multipart/form-data” part too. Until then, this form, as it stands, will accept a single (multiple files are not allowed yet) file and the Upload button will be used to trigger the main function.
The following is the JavaScript (jQuery) code that will capture the event and send a request along with the file.
- $(document).ready(function () {
- // On the document ready
- $('input[type=submit]').click(function () {
- // Before the request starts, show the 'Loading message...'
- $('.result').text('File is being uploaded...');
- event.preventDefault();
- // On the click even,
- var formData = new FormData($('#form')[0]);
- $.ajax({
- type: 'POST',
- processData: false,
- contentType: false,
- data: formData,
- success: function (data) {
- // The file was uploaded successfully...
- $('.result').text('File was uploaded.');
- },
- error: function (data) {
- // there was an error.
- $('.result').text('Whoops! There was an error in the request.');
- }
- });
- });
- });
In the preceding code, there is an element where the result is being rendered. That was like this:
In this element you will be showing the process that is currently going on.
- First of all, it will show in this paragraph a message that a file is being uploaded.
- After the upload, the success message will be displayed in the paragraph.
If the file was uploaded, a success note will be shown otherwise an error message will be displayed to the user.
How will a button not submit the form using a POST request? That is overridden by the code, in the second line inside the event handler. event.preventDefault(); in this code line, the default behavior that will be triggered is overridden. Meaning that the page won't be loaded again by a POST request, instead only that code will execute that we want, the Ajax code in the code block.
ASP.NET method to capture and make sure the request has a file
You can use the following code of ASP.NET to ensure the file is attached with the request and then follow on to the saving process and so on. For example this code:
- files = Request.Files.Count;
- if(files > 0) {
- // Files are sent!
- for (int i = 0; i < files; i++) {
- var file = Request.Files[i];
- // Got the image...
- string fileName = Path.GetFileName(file.FileName);
- // Save the file...
- file.SaveAs(Server.MapPath("~/" + fileName));
- }
- }
The preceding code will work for any image or any file type. There is no validation until now, you can attach your own validations depending on the MIME type or the size of the file that was attached to the request.
Omitting the enctypeOh well, I said that after the jQuery code I will explain how you can omit the enctype attribute, or if you even miss writing the enctype the result will be valid to upload the file. When you're using the FormData object to send the data, it causes the form to behave as if the enctype of the form was set to the multipart/form-data (that is required to encode the files and transfer them on HTTP).
That is why, even if you miss this part and are using the FormData, you will see that the images are being uploaded to the server.
Points of InterestHTML5 and jQuery are widely used frameworks to upload the files using an Ajax request to the web servers making it easier to control the file types to be uploaded, validating the maximum file size to be uploaded and some other specific validation can be handled easily on the client-side making it easier for the user to do uploading tasks quickly.
FormData is an object that, once used, makes it easy to encode the files and other data as if the form's enctype was set to the multipart/form-data. This is used (and is required) to upload files on the server using HTTP requests.
You can prevent any default method to be executed upon any element or control, using the JavaScript's preventDefault() method.
Sr KarthigaPosted Feb 14, 2016, 8:01 AM
Good one sir
Sam HobbsPosted Jan 28, 2015, 12:26 PM
I have heard of something called Java. It existed before JavaScript, right? Could Java be used?
Suraj SahooPosted Jan 28, 2015, 1:20 AM
Great Afzaal.. welcome :)
Dinesh BeniwalPosted Jan 27, 2015, 11:52 PM
Good work, Welcome to C# Corner Afzaal.