Q:

Parameter Binding in ASP.NET Web API

I have a DelegatingHandler in my web API for authentification (HMAC). 
I would like to add a GET parameter to the request to return the user's id to my controller. 
In my handler, I tried adding it like so:
public class SecurityHandler : DelegatingHandler
{
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        string apikey = request. Headers. GetValues(AuthConfig. ApiKeyHeader).First();
        UserDTO user = UserRepo. GetUser(apikey);
        if (user == null)
        {
            return SendResponseText("Invalid API key");
        }

        // Validate signature ...

        // Add user Id to the URI
        request. RequestUri = new Uri(request. RequestUri. OriginalString + "&UserId=" + user. Id);
        return base. SendAsync(request, cancellationToken);
    }
}

In my controller, I'm able to get the newly added parameter from the request uri, however the parameter binding is not working
public class MyModel
{
    public int UserId { get; set; }
    ...
}

public string Get([FromUri] MyModel model)
{
    // model. UserId has not been set (= 0)
    // Request. RequestUri contains "&UserId=5"
}

Update
I'm guessing the binding is being done from the Params in the HttpContext.  I tried something like this, but Params collection is readonly. 
var newContext = request. Properties["MS_HttpContext"] as HttpContextWrapper;
newContext. Request. Params. Add("UserId", "8");
request. Properties. Remove("MS_HttpContext");
request. Properties. Add("MS_HttpContext", newContext);

A:

I have tried at my side and it working. 
Here is my sample url. 
http://localhost:50382/api/values?UserId=10

Here is controller action. 
public class ValueController : ApiController
{
    public IEnumerable<string> Get([FromUri]Model my)
            {
                return new string[] { "value1", "value2" , my. UserId. ToString() };
            }
}

As per your comment here I created delegate handler. 
  public class MyMessageHandler : DelegatingHandler
  {
    protected override Task<HttpResponseMessage> SendAsync(
            HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var myTempObj = new { id = 20 };
      /* You have to check this and remove this key.  If key present that FromUri use cached properties*/
        if (request. Properties. ContainsKey("MS_QueryNameValuePairs"))
        {
            request. Properties. Remove("MS_QueryNameValuePairs");                
        }

        // Now prepare or update uri over here.  It will surely work now. 
            if (! string. IsNullOrEmpty(request. RequestUri. Query) && request. RequestUri. Query. Contains('?')) 
            {
                request. RequestUri = new Uri(request. RequestUri + "&UserID=" + myTempObj.id);
            }
            else
            {
                request. RequestUri = new Uri(request. RequestUri + "? UserID=" + myTempObj.id);
            }    

        return base. SendAsync(request, cancellationToken);
    }
}

It is work as expected. 
If your original request contain userid then it get duplicated and it will not work.  It return 0. 
I also have doubt that you are using request. RequestUri. OriginalString.  Just try to use request. RequestUri.  It might possible that OriginalString has encoded value.