How to extract custom header value in Web API message handler?

Question

Try something like this:

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

There's also a TryGetValues method on Headers you can use if you're not always guaranteed to have access to the header.

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

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

Safe solution using TryGetValues:

Include System.Linq;

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

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

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.

Create a new method - 'Returns an individual HTTP Header value' and call this method with key value everytime when you need to access multiple key Values from HttpRequestMessage.

public static string GetHeader(this HttpRequestMessage request, string key)
        {
            IEnumerable<string> keys = null;
            if (!request.Headers.TryGetValues(key, out keys))
                return null;

            return keys.First();
        }

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");