Thanks to the built-in model binder feature in ASP.NET Core, uploading files is easy. You only need to specify an IFormFile as an action method parameter and the framework does all the hard work for you. All very handy and easy, until it doesn't work...
Today I had an issue when I tried to upload multiple files at once. This is certainly supported and should work with any of the following collections that represent several files:
Here is a code example:
Nothing wrong with the code above I would think. But unfortunately it didn’t work…
To make it even stranger, although the List<IFormFile> remained empty, the uploaded files where available when I directly accessed the HttpContext and took a look at the Request.Forms.Files property.
The problem turned out to be related in the way I uploaded the files. I had created a small helper library to construct the MultipartFormDataContent object:
After a lot of debugging I could trace the cause of the issue to the following line:
The problem was with the second argument. When I always passed the same value for the second argument, the model binding worked but when I passed a different value(like the document name in the example above) the collection remained empty.
A look at the documentation for this method didn’t help me to understand why this should be the case.
I updated my helper to always use the same value for the second parameter:
Hope that helps!