How to extract custom header value in Web API mess

Question

I currently have a message handler in my Web API service that overrides 'SendAsync' as follows:

protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
  //implementation
}

Within this code I need to inspect a custom added request header value named MyCustomID. The problem is when I do the following:

if (request.Headers.Contains("MyCustomID"))  //OK
    var id = request.Headers["MyCustomID"];  //build error - not OK

...I get the following error message:

Cannot apply indexing with [] to an expression of type 'System.Net.Http.Headers.HttpRequestHeaders'

How can I access a single custom request header via the HttpRequestMessage (MSDN Documentation) instance passed into this overridden method?

Answer 1

To expand on Youssef's answer, I wrote this method based Drew's concerns about the header not existing, because I ran into this situation during unit testing.

private T GetFirstHeaderValueOrDefault<T>(string headerKey, 
   Func<HttpRequestMessage, string> defaultValue, 
   Func<string,T> valueTransform)
    {
        IEnumerable<string> headerValues;
        HttpRequestMessage message = Request ?? new HttpRequestMessage();
        if (!message.Headers.TryGetValues(headerKey, out headerValues))
            return valueTransform(defaultValue(message));
        string firstHeaderValue = headerValues.FirstOrDefault() ?? defaultValue(message);
        return valueTransform(firstHeaderValue);
    }

Here's an example usage:

GetFirstHeaderValueOrDefault("X-MyGuid", h => Guid.NewGuid().ToString(), Guid.Parse);

Also have a look at @doguhan-uluca 's answer for a more general solution.

Answer 2

The line below throws exception if the key does not exists.

IEnumerable<string> headerValues = request.Headers.GetValues("MyCustomID");

Work around :

Include System.Linq;

IEnumerable<string> headerValues;
var userId = string.Empty;

     if (request.Headers.TryGetValues("MyCustomID", out headerValues))
     {
         userId = headerValues.FirstOrDefault();
     }           

Answer 3

To further expand on @neontapir's solution, here's a more generic solution that can apply to HttpRequestMessage or HttpResponseMessage equally and doesn't require hand coded expressions or functions.

using System.Net.Http;
using System.Collections.Generic;
using System.Linq;

public static class HttpResponseMessageExtensions
{
    public static T GetFirstHeaderValueOrDefault<T>(
        this HttpResponseMessage response,
        string headerKey)
    {
        var toReturn = default(T);

        IEnumerable<string> headerValues;

        if (response.Content.Headers.TryGetValues(headerKey, out headerValues))
        {
            var valueString = headerValues.FirstOrDefault();
            if (valueString != null)
            {
                return (T)Convert.ChangeType(valueString, typeof(T));
            }
        }

        return toReturn;
    }
}

Sample usage:

var myValue = response.GetFirstHeaderValueOrDefault<int>("MyValue");