ASP.NET ProblemDetails application/problem+json

example.cs

services.AddControllers(configure =>
{
    configure.Filters.Add<ProblemResultFilter>();
});

[UsedImplicitly]
public class ProblemResultFilter : IResultFilter
{
    [SuppressMessage("ReSharper", "MergeIntoPattern")]
    public void OnResultExecuting(ResultExecutingContext context)
    {
        if (context.Result is ObjectResult objectResult
            && objectResult.StatusCode == StatusCodes.Status400BadRequest)
        {
            objectResult.ContentTypes.Add(KnownMediaTypes.Problem);
        }
    }
    public void OnResultExecuted(ResultExecutedContext context)
    {
    }
}

public static class KnownMediaTypes
{
    public const string Problem = "application/problem+json";
}