How to extract custom header value in Web API message handler

Question

I currently have a message handler in my Web API service that overrides  SendAsync  as follows   protected override Task lt HttpResponseMessage gt  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

User · Answer

For ASP Net Core there is an easy solution if want to use the param directly in the controller method  Use the  FromHeader  annotation           public JsonResult SendAsync  FromHeader  string myParam                    if myParam    null     Param not set in request header                      return null                    return doSomething               Additional Info  In my case the  myParam  had to be a string  int was always 0

User · Answer

request Headers FirstOrDefault  x   gt  x Key     MyCustomID    Value FirstOrDefault     modern variant

User · Answer

var token   string Empty  if  Request Headers TryGetValue  quot MyKey quot    out headerValues         token   headerValues FirstOrDefault

User · Answer

Try something like this   IEnumerable lt string gt  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

User · Answer

The line below throws exception if the key does not exists    IEnumerable lt string gt  headerValues   request Headers GetValues  MyCustomID      Work around    Include System Linq   IEnumerable lt string gt  headerValues  var userId   string Empty        if  request Headers TryGetValues  MyCustomID   out headerValues                   userId   headerValues FirstOrDefault

User · Answer

One line solution  var id   request Headers GetValues  MyCustomID   FirstOrDefault

User · Answer

For ASP NET you can get the header directly from parameter in controller method using this simple library package  It provides a  FromHeader  attribute just like you have in ASP NET Core     For example               using RazHeaderAttribute Attributes        Route  api  controller         public class RandomController   ApiController                               GET api random          HttpGet          public IEnumerable lt string gt  Get  FromHeader  pages    int page   FromHeader  string rows                           Print in the debug window to be sure our bound stuff are passed                Debug WriteLine   Rows  rows   Page  page

User · Answer

This may sound obvious  but make sure the Controller where you are reading the headers in  is the first Controller where the request goes through  I had two WebAPI projects communicating with each other  The first one was a proxy  the second contained the logic  Silly me  I tried reading the custom headers in the second Controller  instead of the first one

User · Answer

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 lt string gt  keys   null              if   request Headers TryGetValues key  out keys                   return null               return keys First

User · Answer

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 lt T gt           this HttpResponseMessage response          string headerKey                var toReturn   default T            IEnumerable lt string gt  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 lt int gt   MyValue

User · Answer

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 lt T gt  string headerKey      Func lt HttpRequestMessage  string gt  defaultValue      Func lt string T gt  valueTransform                IEnumerable lt string gt  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   gt  Guid NewGuid   ToString    Guid Parse     Also have a look at  doguhan-uluca  s answer for a more general solution

[c#] How to extract custom header value in Web API message handler?

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