sample message handler for managing etags in web api

EtagMessageHandler.cs

public class EtagMessageHandler : DelegatingChannel
{
    readonly IETagStore _eTagStore;

    public EtagMessageHandler(HttpMessageChannel innerChannel) : base(innerChannel) {
        Trace.WriteLine("EtagMessageHandler - Ctor");

        _eTagStore = new InMemoryETagStore();
    }

    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,
                                                            CancellationToken cancellationToken) {
            
        Trace.WriteLine("EtagMessageHandler - SendAsync");

        var taskFactory = new TaskFactory<HttpResponseMessage>();

        if (request.Method == HttpMethod.Get) {
            //if this is not a conditional get
            if (request.Headers.IfNoneMatch.Count == 0)
                return base.SendAsync(request, cancellationToken).ContinueWith(task => {
                    Trace.WriteLine("EtagMessageHandler - Task - Not a conditional GET");

                    var resp = task.Result;
                    resp.Headers.ETag = new EntityTagHeaderValue(_eTagStore.Fetch(request.RequestUri));
                    return resp;
                });

            //if this is not modified, stop processing the message and return a 304; make sure the etag is also set
            if (IfNoneMatchContainsStoredEtagValue(request))
                    
                return taskFactory.StartNew(() =>
                {
                    Trace.WriteLine("EtagMessageHandler - Task - Conditional GET match");

                    var resp = new HttpResponseMessage(HttpStatusCode.NotModified, "Not Modified");
                    resp.Headers.ETag = new EntityTagHeaderValue(_eTagStore.Fetch(request.RequestUri));
                    return resp;
                });

            //if this resource has been updated, let the request proceed to the operation and set a new etag on the return
            return base.SendAsync(request, cancellationToken).ContinueWith(task => { 
                Trace.WriteLine("EtagMessageHandler - Task - Conditional GET not matched");

                var resp = task.Result;
                resp.Headers.ETag = new EntityTagHeaderValue(_eTagStore.Fetch(request.RequestUri));
                return resp;
            });
        }

        if (request.Method == HttpMethod.Put || request.Method == HttpMethod.Post)
            //let the request processing continue; new etag value for resource; update response header
            if (IfMatchContainsStoredEtagValue(request))
                return base.SendAsync(request, cancellationToken).ContinueWith(task => {
                    Trace.WriteLine("EtagMessageHandler - Task - Conditional update matched");

                    var resp = task.Result;
                    resp.Headers.ETag = new EntityTagHeaderValue(_eTagStore.UpdateETagFor(request.RequestUri));
                    return resp;
                });

            //stop processing and return a 412/precondition failed; update response header
            else 
                return taskFactory.StartNew(() => {
                    Trace.WriteLine("EtagMessageHandler - Task - Conditional update not matched");

                    var resp = new HttpResponseMessage(HttpStatusCode.PreconditionFailed, "Precondition Failed");
                    resp.Headers.ETag = new EntityTagHeaderValue(_eTagStore.Fetch(request.RequestUri));
                    return resp;
                });

        //by default, let the request keep moving up the message handler stack
        return base.SendAsync(request, cancellationToken);
    }

    bool IfNoneMatchContainsStoredEtagValue(HttpRequestMessage request) {
        //we're going to assume that this handler will only care about 1 entity tag value for the specified resource
        if (request.Headers.IfNoneMatch.Count == 0)
            return false;

        return request.Headers.IfNoneMatch.Select(v => v.Tag).Contains(_eTagStore.Fetch(request.RequestUri));
    }

    bool IfMatchContainsStoredEtagValue(HttpRequestMessage request) {
        if (request.Headers.IfMatch.Count == 0)
            return false;

        return request.Headers.IfMatch.Select(v => v.Tag).Contains(_eTagStore.Fetch(request.RequestUri));
    }
}