Nancy caching using Last-Modified/If-Modified-Since header

cacheimplementation.cs

public class Cache : ICache
{
    private ConcurrentDictionary<string, DateTime> cacheLookup = new ConcurrentDictionary<string,DateTime>();

    public Response Get(NancyContext ctx)
    {   
        DateTime lastmodified;
    
        if(ctx.Request.Method == "GET")  //Could be POST as well I guess
        {
            if (!cacheLookup.TryGetValue(ctx.Request.Method + "-" + ctx.Request.Url.ToString(), out lastmodified)
            {
                return null;
            }
            
            if(!ctx.Request.Headers.IfModifiedSince.HasValue)
            {
                return null;
            }
            
            var modifiedsince = ctx.Request.Headers.IfModifiedSince.Value;
            if (modifiedsince < lastmodified)
            {
               return null;
            }
            
            return new Response(){ StatusCode = 304 };
        }
        
        return null;
    }
    
    public AddOrUpdate(NancyContext ctx)
    {
        if(ctx.Request.Method == "GET")  //Could be POST as well I guess
        {
            var id = ctx.Request.Method + "-" + ctx.Request.Url.ToString();//GET - http://mydomain.com/products/1?foo=bar
            var date = DateTime.UtcNow;
            cacheLookup.AddOrUpdate(id,date,(key,value) => date);    
            ctx.Response.Headers.LastModified = date;
        }
    }
}

mainmodule.cs

namespace Nancy.LastModified
{
    using System;
    using Nancy;

    public class MainModule : NancyModule
    {
        public MainModule(ICache cache, ISomeRepoOrServiceOrOtherLayerYouLove someRepoOrServiceOrOtherLayerYouLove)
        {
            Get["/{int:id}"] = x => {
                return someRepoOrServiceOrOtherLayerYouLove.Get(x.id);
            };

            Put["/{int:id}"] = x => {
                var model = this.BindAndValidate<MyModel>();
                someRepoOrServiceOrOtherLayerYouLove.Update(x.id,model);
                return 204;
            };
            
            Delete["/{int:id}"] = x => {
                someRepoOrServiceOrOtherLayerYouLove.Delete(x.id);
                return 204;
            };
            
            Before += ctx => {
                return cache.Get(ctx);
            };
            
            After += ctx => {
                cache.AddOrUpdate(ctx);
            };
        }
    }
}