[Bug]: Multipart requests is not working correctly with Guid and DateOnly

[arthurlomakin11]

Describe the bug 🐞

Using Refit for integration testing for our internal API. When working with it, discovered a problem with Guid processing in request. Guid is added with "..." in it that causes incorrect processing and gives BadRequest in test:

Step to reproduce

1. Go to 'Reproduction repository'
2. Run test
3. I've also added RequestMiddleware and LoggingHandler for easier investigation in debug mode

Reproduction repository

https://github.com/arthurlomakin11/RefitInvalidMultipartGuidTest

Expected behavior

Here's the correct request(used Swagger from 'Reproduction repository'):

As you can see, Guid is shown in plain form, without "..."

Screenshots 🖼️

No response

IDE

Rider Windows

Operating system

No response

Version

No response

Device

No response

Refit Version

8.0.0

Additional information ℹ️

No response

[arturlomakinsparky]

Found the core problem itself and the way to temporary fix it.
When multipart item is processed, it's run through AddMultipartItem method
There it matches types to it's HttpContent and if not processed by regular conditions, fallbacks to serializer.

So, I tried to debug library code of this method in Reproduction repository and got the expected result - Guid is processed through JSON serializer and it serializes 754c7d0c-41bf-406b-bed6-1f35e0bfd895 to "754c7d0c-41bf-406b-bed6-1f35e0bfd895" with quotes.

To fix it, new condition should be added in AddMultipartItem method to support Guid type as Multipart item.

As a temporary fix, we can decorate serializer with our own logic.

public class GuidFriendlyHttpContentSerializer(IHttpContentSerializer inner)
    : IHttpContentSerializer
{
    public HttpContent ToHttpContent<T>(T item)
    {
        if (item is Guid guid)
        {
            return new StringContent(guid.ToString(), Encoding.UTF8, "text/plain");
        }

        return inner.ToHttpContent(item);
    }

    public async Task<T?> FromHttpContentAsync<T>(
        HttpContent content,
        CancellationToken cancellationToken = default)
    {
        return await inner.FromHttpContentAsync<T>(content, cancellationToken);
    }

    public string? GetFieldNameForProperty(PropertyInfo propertyInfo)
    {
        return inner.GetFieldNameForProperty(propertyInfo);
    }
}

And register it like that

_client = RestService.For<IUploadApi>(httpClient, new RefitSettings
    {
        ContentSerializer = new GuidFriendlyHttpContentSerializer(new SystemTextJsonContentSerializer())
    });

Created new branch in reproduction repository with use of those serializer decorator. Now test run correctly with no issues

[arthurlomakin11]

Faced same issue for DateOnly type. Extended reproduction API test and temporary serializer fix to support DateOnly in new branch in reproduction repository.