Serialize content into a ModelStateDictionary.

HttpContentExtensions.cs

public static class HttpContentExtensions
{
    /// <summary>
    /// Serialize the HTTP content to a <see cref="ModelStateDictionary"/> as an asynchronous operation.
    /// </summary>
    /// <returns>A <see cref="ModelStateDictionary"/> that contains validation errors.</returns>
    public static async Task<ModelStateDictionary> ReadAsModelStateAsync(this HttpContent content)
    {
        var fields = await content.ReadAsAsync<IDictionary<string, string[]>>();
        var modelState = new ModelStateDictionary();

        foreach (var field in fields)
        {
            foreach (var errorMessage in field.Value)
            {
                modelState.AddModelError(field.Key, errorMessage);
            }
        }

        return modelState;
    }
}