ValidationProblemDetails.cs

public class ValidationProblemDetails : ProblemDetails
{
    public ValidationProblemDetails() : base()
    {
    }

    public ValidationProblemDetails(
        ModelStateDictionary modelStateDictionary)
        : base()
    {
        this.Errors = modelStateDictionary
            .ToDictionary(x => x.Key,
                y => y.Value.Errors.Select(
                    z => z.Exception.Details()).ToArray());
    }

    [JsonProperty(PropertyName = "errors")]
    public IDictionary<string, string[]> Errors { get; }
}