GetAnyHttpHEaderOrDefault.cs

/// <summary>
        /// Gets the first header value or default.
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="requestMessage">The <see cref="HttpRequestMessage"/> message.</param>
        /// <param name="headerKey">The header key i.e. the header to read.</param>
        /// <param name="defaultValueSelector">The <see cref="Func{T, TResult}"/> default value selector.</param>
        /// <param name="valueTransform">The <see cref="Func{T, TResult}"/> value transform.</param>
        /// <returns></returns>
        private T GetFirstHeaderValueOrDefault<T>(
            HttpRequestMessage requestMessage,
            string headerKey,
            Func<HttpRequestMessage, string> defaultValueSelector, 
            Func<string, T> valueTransform)
        {
            IEnumerable<string> headerValues;
            HttpRequestMessage httpRequestMessage = requestMessage ?? new HttpRequestMessage();

            // If the header is not found, run the default value selector, transform to T and return
            if (!httpRequestMessage.Headers.TryGetValues(headerKey, out headerValues))
            {
                return valueTransform(defaultValueSelector(httpRequestMessage));
            }

            // header found, check if its value is null
            string headerValue = headerValues.FirstOrDefault() ?? defaultValueSelector(httpRequestMessage);

            return valueTransform(headerValue);
        }