Minimal API: Dictionary<string, string> always null when sending data with IFormFile via multipart/form-data

Question

I’m using ASP.NET Core Minimal API. I have an endpoint where I want to receive an IFormFile along with a Dictionary<string, string> property in the same request. I set the Accepts<TestDictionaryRequest>("multipart/form-data") and use [FromForm] on the handler parameter. Uploading the file works correctly, but the TestDictionary property is always null, even when I set values in the Swagger UI.

Here is the code:

public class TestDictionaryRequest
{
    public Dictionary<string, string> TestDictionary { get; set; }
    public IFormFile TestFile { get; set; }
}

public class TestDictionaryEndpoint : IEndpoint
{
    public void MapEndpoint(IEndpointRouteBuilder app)
    {
        app.MapPost("test/dictionary", Handler)
           .DisableAntiforgery()
           .WithTags("Test")
           .Accepts<TestDictionaryRequest>("multipart/form-data");
    }

    public static IResult Handler([FromForm] TestDictionaryRequest request)
    {
        return Results.Ok(new
        {
            Dictionary = request.TestDictionary,
            Count = request.TestDictionary?.Count ?? 0,
            IsNull = request.TestDictionary == null
        });
    }
}

What additional steps or configuration are needed so that the Dictionary<string, string> TestDictionary is bound correctly (not null) when using multipart/form-data with IFormFile in a Minimal API endpoint?

How are you trying the API call? With Postman, or directly with cUrl? — Tasos K.
– Tasos K., Commented Sep 15 at 14:16
Using swagger. And now I can that is a reason of error. Swagger attempts to send TestDictionary = {"additionalProp1":"string","additionalProp2":"string","additionalProp3":"string"} instead of sending each key-value like TestDictionary["additionalProp1"] = "string", etc. — Dmytro Kotenko
– Dmytro Kotenko, Commented Sep 22 at 18:14

Guru Stron · Accepted Answer · 2025-10-02 08:50:06Z

0

To bind form to dictionary the form keys should be in a specific format containing keys enclosed in [] (see for example tests in the ASP.NET Core github repo), i.e. for your model they should look like TestDictionary[some_key_here] (i.e. TestDictionary[hello]).

For example if posting via Postman:

Which will result in the data successfully parsed:

If you move the dictionary to the "root" handler:

static IResult Handler([FromForm] IDictionary<string, string> request, IFormFile testFile) =>

Then form keys should be like [some_key_here].

If the default binding convention is not working for you - you can always handle the raw form:

static IResult Handler(HttpRequest r)
{
    var files = r.Form.Files;
    var data = r.Form;
    return Results.Ok(...);
}

Or do some custom binding for example via introducing BindAsync method to your model.

edited Oct 2 at 8:50

answered Sep 15 at 17:11

Guru Stron

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