SubmissionServiceHttpHandlerStub.cs

namespace Domain.Tests.Unit.Stubs
{
    internal class SubmissionServiceHttpHandlerStub : HttpMessageHandler
    {
        private Dictionary<string, Payload> _requestPayload;

        protected override async Task<HttpResponseMessage> SendAsync(
            HttpRequestMessage request,
            CancellationToken cancellationToken)
        {
            _payload = JsonConvert.DeserializeObject<Dictionary<string,
                Payload>>(
                await request.Content.ReadAsStringAsync());

            // Assume that submission was successful, 
            // construct the response payload
            var result = ConstructResponsePayload(_payload)
            
            return new HttpResponseMessage(HttpStatusCode.OK)
            {
                Content = new StringContent(JsonConvert.SerializeObject(result))
            };
        }

        internal Payload PayloadDto(
            string key) =>
            _payload[key].FirstOrDefault();
    }
}