Losing Variable after Async Call

Question

I'm not very familiar with asynchronized calls but I ran into a bit of what I think is a weird issue and not sure why it's happening.

CONCEPT Call a function to retrieve data. If data is stored in our cache, we use Redis as our cache (in case it matters on how the data should be stored as I've read on other threads), then return it. Otherwise, make a call to a third party library (specifically Force.com Toolkit for .Net, but I doubt it matters) which uses async calls and cache the results.

I created a synchronous method which works, but now I want to change it to be asynchronous.

Synchronous

public static Models.Description.ObjectDescription Describe(ForceClient forceClient, string sObject) 
{
    Models.Description.ObjectDescription result;
    var cacheName = "Salesforce_Object_" + sObject;
    if (HttpContext.Current.Cache[cacheName] != null) 
    {
        result = (Models.Description.ObjectDescription) HttpContext.Current.Cache[cacheName];
    } 
    else 
    {
        result = forceClient.DescribeAsync<Models.Description.ObjectDescription>(sObject).Result;

        if (result != null) 
        {
            var expiration = 30; // testing, this will be read from a global variable
            HttpContext.Current.Cache.Insert(
                cacheName, 
                result, 
                null, 
                DateTime.UtcNow.AddSeconds(expiration), 
                Cache.NoSlidingExpiration, 
                CacheItemPriority.Default, 
                null);
        }
    }
    return result;
}

Asynchronous full code

public static class Description {
    public static async Task<Models.Description.ObjectDescription> Describe(ForceClient forceClient, string sObject) 
    {
        Models.Description.ObjectDescription result;
        var cacheName = "Salesforce_Object_" + sObject;
        if (HttpContext.Current.Cache[cacheName] != null) 
        {
            result = (Models.Description.ObjectDescription) HttpContext.Current.Cache[cacheName];
        } 
        else 
        {
            /*
            * only line that changed from above
            */
            result = await forceClient.DescribeAsync<Models.Description.ObjectDescription>(sObject);

            if (result != null) 
            {
                var expiration = 30; // testing, this will be read from a global variable
                HttpContext.Current.Cache.Insert(
                    cacheName, 
                    result, 
                    null, 
                    DateTime.UtcNow.AddSeconds(expiration), 
                    Cache.NoSlidingExpiration, 
                    CacheItemPriority.Default, 
                    null);
            }
        }
        return result;
    }

    public static async Task<IList<Models.Description.PicklistValues>> Picklist(ForceClient forceClient, string sObject, string fieldName) {
        var results = await Describe(forceClient, sObject);

        var field = results.Fields.SingleOrDefault(f => f.Name.Equals(fieldName));

        return field != null ? field.PickListValues : new List<Models.Description.PicklistValues>();
    }
}       
public static class Lead {
    public static async Task<IList<Models.Description.PicklistValues>> Picklist(ForceClient forceClient, string fieldName) {
        return await Description.Picklist(forceClient, "Lead", fieldName);
    }
}

Page

protected void Page_Load(object sender, EventArgs e) {
    /* when I try to `await` this i get an error stating:
    *  The 'await' operator can only be used with an async method.
    *  ...but when I change Page_Load to
    *  protected async Task Page_Load(object sender, EventArgs e) {...}
    *  ... and 'await' the call, My results are blank
    */
    var result = Lead.Picklist(new Authentication.ForceClient(), "Source__c").Result;

    foreach (var value in result) {
        Response.Write("- " + value.Value + "<br />");
    }
}

The Synchronous version works great, but when I convert it to Asynchronous I get an error on the line HttpContext.Current.Cache.Insert(...)

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Currently, this site is a WebForms website so to debug I use Response.Write(). When I try to display the cacheName variable, I get the same results.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Line 30: HttpContext.Current.Response.Write(cacheName + "<br>");

Is the variable cacheName being lost when I make the async call? I highly doubt it, but I'm not sure how to proceed.

Hopefully someone can help.

Answer 1

Web-related frameworks in .NET generally use thread static variables like HttpContext.Current, OperationContext.Current, WebOperationContext.Current, etc. Since execution may continue in a different thread after an async method call, thread static variables are lost.

Here is a console app just to show what I mean:

using System;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication2
{
    class Test
    {
        static void Main()
        {
            MyAsyncMethod().Wait();
            Console.ReadLine();
        }

        [ThreadStatic]
        static int MyContext = 666;

        static async Task MyAsyncMethod()
        {
            MyContext = 555;
            Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " " + MyContext);
            using (var client = new HttpClient())
            {
                var html = await client.GetStringAsync("http://google.com");
            }
            Console.WriteLine(Thread.CurrentThread.ManagedThreadId + " " + MyContext);
        }
    }
}

So, you need to save them before calling an async method:

public static async Task<Models.Description.ObjectDescription> Describe(ForceClient forceClient, string sObject) 
{
    var ctx = HttpContext.Current; //<-- *****
    Models.Description.ObjectDescription result;
    var cacheName = "Salesforce_Object_" + sObject;
    if (ctx.Cache[cacheName] != null) 
    {
        result = (Models.Description.ObjectDescription) ctx.Cache[cacheName];
    } 
    else 
    {
        /*
        * only line that changed from above
        */
        result = await forceClient.DescribeAsync<Models.Description.ObjectDescription>(sObject);

        if (result != null) 
        {
            var expiration = 30; // testing, this will be read from a global variable
            ctx.Cache.Insert(
                cacheName, 
                result, 
                null, 
                DateTime.UtcNow.AddSeconds(expiration), 
                Cache.NoSlidingExpiration, 
                CacheItemPriority.Default, 
                null);
        }
    }
    return result;
}

Answer 2

I needed to add the following to my page in order to get it to work:

<%@ Page Async="true" %>

and change the signature of the Page_Load() method from

protected void Page_Load(object sender, EventArgs e) {...}

to

protected async void Page_Load(object sender, EventArgs e) {...}