How to upload a file (or files) with ASP.NET MVC

8 months ago

To upload a file or set of files with ASP.NET MVC we first need to understand what happens under the hood. First, ASP.NET MVC is different since we don't get to use ASP.NET Server Controls as we're used to them. There are no "server controls" in the way that we're used to them.

Second, it'd be important to write Unit Tests for something like File Upload, ASP.NET MVC didn't do any special work for File Upload support. It uses whatever stuff is built into ASP.NET itself. This may or not be helpful or interesting or even easy to test.

Why do we have to add enctype="multipart/form=data" on our forms that include file uploads? Because the form will now be POSTed in multiple parts.

Uploading a single file

Let’s start with the view. Here’s a form that will post back to the current action.

 

<form action="" method="post" enctype="multipart/form-data">

<label for="file">Filename:</label>

<input type="file" name="file" id="file"/>

<input type="submit"/>

</form>

 

Here’s the action method that this view will post to, which saves the file into a directory in the App_Data folder named “uploads”.

 

[HttpPost]

public ActionResult Index(HttpPostedFileBase file){

if(file.ContentLength>0){

var fileName = Path.GetFileName(file.FileName);

var path = Path.Combine(Server.MapPath("~/App_Data/uploads"), fileName);

file.SaveAs(path);

}

return RedirectToAction("Index");

}

 

Notice that the argument to the action method is an instance of HttpPostedFileBase. ASP.NET MVC introduces a new value providers features.

Whereas model binders are used to bind incoming data to an object model, value providers provide an abstraction for the incoming data itself.

In this case, there’s a default value provider called the HttpFileCollectionValueProvider which supplies the uploaded files to the model binder. Also notice that the argument name, file, is the same name as the name of the file input. This is important for the model binder to match up the uploaded file to the action method argument.

Uploading multiple files

In this scenario, we want to upload a set of files. We can simply have multiple file inputs all with the same name.

 

<form action="" method="post" enctype="multipart/form-data">

<label for="file1">Filename:</label>

<input type="file" name="files" id="file1"/>

<label for="file2">Filename:</label>

<input type="file" name="files" id="file2"/>

<input type="submit" />

</form>

 

Now, we just tweak our controller action to accept an IEnumerable of HttpPostedFileBase instances. Once again, notice that the argument name matches the name of the file inputs.

 

[HttpPost]

public ActionResult Index(IEnumerable<HttpPostedFileBase>files){

foreach(var file in files){

if (file.ContentLength > 0){

var fileName = Path.GetFileName(file.FileName);

var path = Path.Combine(Server.MapPath("~/App_Data/uploads"), fileName);

file.SaveAs(path);

}

}

return RedirectToAction("Index");}

 

Yes, it’s that easy. :)

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