Mvc: Unable to use IFormFile with ApiController in 2.1

[Eilon]

_From @mitchellj on Thursday, 09 August 2018 20:06:10_

When we are trying to use IFormFile it works perfectly when we are not using [ApiController], as soon as we introduce ApiController one of two things happens

1) If we do not prefix with [FromForm] we get a Serialization exception which generates a Bad Request response.
2) If we do prefix FromForm to the IFormFile then it becomes null

In order to get IFormFile to work we need to remove ApiController completely in which case the IFormFile becomes completely propagated correctly.

_Copied from original issue: aspnet/Home#3403_

[mkArtakMSFT]

Thanks for contacting us, @mitchellj.
This is because we do not special case the IFormFile type and treat it as a complex type. ApiController conventions for complex types are that by default they're treated as FromBody.
@pranavkm, seems this is a good candidate to be special-cased.

[pranavkm]

1) Do you have your compatibility version set to 2.1? See https://docs.microsoft.com/en-us/aspnet/core/migration/20_21?view=aspnetcore-2.1#changes-to-startup
2) We had an issue specifically with collection of IFormFile - https://github.com/aspnet/Mvc/issues/7770 that was fixed in 2.2

[Schmaga]

I ran into the same problem. And strangely, when not using any controller parameter and trying to access or iterate Request.Form.Files the thread would just die without ever responding to the browser or even entering any exception handling blocks that are around the code. Feels kind of weird. My ASP.NET Core version was 2.1.2.

Until this is solved, I was able to help myself with a workaround by explicitly stating the name of the form parameter using the [FromForm] attribute like this:

public async Task<IActionResult> UploadLogo([FromForm(Name = "file")] IFormFile file)        
{
     ...
}

After that for some reason the binding worked. I discovered this behavior by accident after having a hard time getting the file upload from an angular 6 application to work. Hope it helps someone else. The link to the corresponding stackoverflow answer is: https://stackoverflow.com/a/52017637/8007572

[pranavkm]

@Schmaga did the compat switch work for you?

[mitchellj]

Sorry I was out of the country for a while, I will be back in a couple of days and try the compat switches and report back. Compatibility was NOT set previously so that is most likely the issue will confirm though.

[pranavkm]

@mitchellj feel free to reopen this issue if you still see it after changing the compat version.

[Schmaga]

I am also still on vacation. Will check as soon as I am back home.

Am 28.08.2018 23:26 schrieb Pranav K [e-mail]:

@Schmagahttps://github.com/Schmaga did the compat switch work for you?

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[Schmaga]

@pranavkm I returned from vacation and checked with the compat switch set to 2.1.

It now works, but I also have to remove the [FromForm] Attribute completely from the controller's IFormFile parameter, otherwise it is still null.

I think it might make sense to better document what exactly the compat switch affects, because I would not have thought it would produce such strange behavior as described in my above post.

[pranavkm]

It now works, but I also have to remove the [FromForm] Attribute completely from the controller's IFormFile parameter, otherwise it is still null.

That isn't intended. Do you have a repro app for this?

[Schmaga]

Not yet, but I could extract it from our project for you. What format is best? Will it be enough to paste the relevant snippets here or do you need a zip file with a project or something?

This is my first .NET OSS issue, so sorry if the answer might be obvious. And I didn't find anything specific about providing example code in the contribution guidelines :)

[pranavkm]

A sample repo on github works really well to share code. If that's difficult, any of your suggestions work fine.

[Eilon]

Moved to preview3 for now.

[mitchellj]

Sorry back now, I can confirm as @pranavkm has found as well that

• Decorating the controller with [ApiController] and the IFormFile with [FromForm] gives us a null IFormFile.
• Decorating the controller with [ApiController] and not decorating the IFormFile parameter binds the IFormFile correctly

[Schmaga]

Thanks @pranavkm. Will provide a repo then, hopefully in the next few days.

[pranavkm]

Looks like yet another case that we need to special case in https://github.com/aspnet/Mvc/blob/release/2.2/src/Microsoft.AspNetCore.Mvc.Core/Internal/ApiBehaviorApplicationModelProvider.cs#L215-L233. FormFileModelBinder requires the model name to match which the provider messes with.

For 2.1, I'd recommend not specifying the attribute or if you do specify the attribute, also specify the name .e.g [FromForm("file")].

[Schmaga]

@pranavkm here is the link to the repositry: https://github.com/Schmaga/aspnet_issue_8311

You can play around with removing the compat switch and/or the attribute a bit.

A bit late, but I hope it still helps.

[ranouf]

I have made a repo to reproduce the problem and thanks to @dougbu we have found the solution. Let take a look there: https://github.com/ranouf/TestMultipleUpload