How do I do file upload using ASP.NET Core 6 minimal api?

Question

I want to create a simple file upload endpoint in ASP.NET Core 6 and thought it would be as easy as described here https://dotnetthoughts.net/handling-file-uploads-in-openapi-with-aspnet-core/.

When I have an endpoint defined like:

app.MapPost("/upload", (IFormFile file) =>
{
    //Do something with the file
    return Results.Ok();
}).Accepts<IFormFile>("multipart/form-data").Produces(200);

I get a 415 back when I call the endpoint. The message I get back is something like:

Expected a supported JSON media type but got "multipart/form-data; ...

Not sure why it expected a supported json when I say that the endpoint should accept multipart/form-data.

Any ideas or thoughts on what to do here?

Answer 1

Currently out of the box support for binding in Minimal APIs is quite limited. Supported binding sources:

• Route values
• Query string
• Header
• Body (as JSON)
• Services provided by dependency injection
• Custom

NOTE: Binding from forms is not natively supported in .NET 6

You can either leverage custom binding or use special types handling:

app.MapPost("/upload", (HttpRequest request) =>
{
    //Do something with the file
    var files = request.Form.Files;
    return Results.Ok();
})
.Accepts("multipart/form-data")
.Produces(200);

UPD

Since .net-7.0 Minimal APIs should be able to bind IFormFile/IFormFileCollection directly:

app.MapPost("/upload", async (IFormFile file) =>
{
    // ...
});

app.MapPost("/upload_many", async (IFormFileCollection myFiles) =>
{
    foreach (var file in myFiles)
    {
        // ...
    }
});

Answer 2

This has been addressed with .NET 7. Support for IFormFile and IFormFileCollection has been added. You should be able to use it in your MediatR pipelines.

Ref: .NET 7 Minimal API Support for File Upload Bindings

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.MapPost("/upload", async (IFormFile file) =>
{
    var tempFile = Path.GetTempFileName();
    app.Logger.LogInformation(tempFile);
    using var stream = File.OpenWrite(tempFile);
    await file.CopyToAsync(stream);
});

app.MapPost("/upload_many", async (IFormFileCollection myFiles) =>
{
    foreach (var file in myFiles)
    {
        var tempFile = Path.GetTempFileName();
        app.Logger.LogInformation(tempFile);
        using var stream = File.OpenWrite(tempFile);
        await file.CopyToAsync(stream);
    }
});

app.Run();

Answer 3

app.MapPost("/upload",
    async Task<IResult> (HttpRequest request) =>
    {
        if (!request.HasFormContentType)
            return Results.BadRequest();

        var form = await request.ReadFormAsync();

        if (form.Files.Any() == false)
            return Results.BadRequest("There are no files");

        var file = form.Files.FirstOrDefault();

        if (file is null || file.Length == 0)
            return Results.BadRequest("File cannot be empty");

        using var stream = file.OpenReadStream();

        var reader = new StreamReader(stream);
        var text = await reader.ReadToEndAsync();

        return Results.Ok(text);
    }).Accepts<IFormFile>("multipart/form-data"); 

Answer 4

Just noting here, you can upload a file with any ContentType like the following sample code.

In this example I chose a text/plain ContentType, but you can choose your own by editing the .Accepts<IFormFile>("text/plain"); line to your desired ContentType.

app.MapPost("/upload",
    async (HttpRequest request) =>
    {
        using (var reader = new StreamReader(request.Body, System.Text.Encoding.UTF8))
        {

            // Read the raw file as a `string`.
            string fileContent = await reader.ReadToEndAsync();

            // Do something with `fileContent`...
    
            return "File Was Processed Sucessfully!";
        }
    }).Accepts<IFormFile>("text/plain");

It is also well supported by Swagger UI: