The exception explains clearly that there is only one asynchronous operation per context allowed at a time.

So, you either have to `await` them one at a time as the error message suggests:

    var banner = await context.Banners.ToListAsync();
    var newsGroup = await context.NewsGroups.ToListAsync();

Or you can use multiple contexts:

    var banner = context1.Banners.ToListAsync();
    var newsGroup = context2.NewsGroups.ToListAsync();
    await Task.WhenAll(banner, newsGroup);

If you are using IoC container for your Data Provider injection, consider to use &quot;transient&quot; or &quot;PerWebRequest&quot; type for your lifecycle.

For example: https://github.com/castleproject/Windsor/blob/master/docs/lifestyles.md

If you use Unity for dependency injection with for example repository pattern you will get the following error using two or more contexts with create/update/delete:

&gt; The relationship between the two objects cannot be defined because
&gt; they are attached to different ObjectContext objects.

This can be solved using `PerRequestLifetimeManager`. More info here:

https://stackoverflow.com/questions/45146126/c-sharp-ef6-make-multiple-async-calls-to-one-context-using-unity-asp-net-web-a/45162704#45162704

    container.RegisterType&lt;DbContext&gt;(new PerRequestLifetimeManager());
    container.RegisterType&lt;ISupplierRepository, SupplierRepository&gt;();
    container.RegisterType&lt;IContactRepository, ContactRepository&gt;();

I&#39;m getting this error while loading a Puppet image from a Oracle virtual box. How can I fix it?

    Failed to open a session for the virtual machine learn-puppet-centos-6.4-pe-3.1.0.

    VT-x is disabled in the BIOS. (VERR_VMX_MSR_VMXON_DISABLED).

    Result Code: E_FAIL (0x80004005)
    Component: Console
    Interface: IConsole {8ab7c520-2442-4b66-8d74-4ff1e195d2b6}
In addition, I see that the `acellerate`-tab in Virtual Box is disabled.

VERR_VMX_MSR_VMXON_DISABLED when starting an image from Oracle virtual box

As you may know, in JavaScript `&#39;&#39; + null = &quot;null&quot;` and `&#39;&#39; + undefined = &quot;undefined&quot;` (in most browsers I can test: Firefox, Chrome and IE). I would like to know the origin of this oddity (what the heck was in the head on Brendan Eich?!) and if there is any aim for changing it in a future version of ECMA. It&#39;s indeed pretty frustrating having to do `&#39;sthg&#39; + (var || &#39;&#39;) ` for concatenating Strings with variables and using a third party framework like *Underscore* or other for that is using a hammer for jelly nail pounding.

**Edit:**

To meet the criteria required by StackOverflow and clarify my question, it is a threefold one:

* What is the history behind the oddity that makes JS converting `null` or `undefined` to their string value in `String` concatenation?
* Is there any chance for a change in this behavior in future ECMAScript versions?
* What is the prettiest way to concatenate `String` with potential `null` or `undefined` object without falling into this problem (getting some `&quot;undefined&quot;` of `&quot;null&quot;` in the middle of the String)? By the subjective criteria *prettiest*, I mean: short, clean and effective. No need to say that `&#39;&#39; + (obj ? obj : &#39;&#39;)` is not really pretty…

JavaScript String concatenation behavior with null or undefined values

This is my code:

    var banner = context.Banners.ToListAsync()
    var newsGroup = context.NewsGroups.ToListAsync()
    await Task.WhenAll(banner, newsGroup);

But when i called the function from controller. It showed error

&gt;A second operation started on this context before a previous asynchronous operation completed. Use &#39;await&#39; to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe.


Please help me solve this issue.



Multi-async in Entity Framework 6?

This is my code:
<pre><code class="hljs language-ini">var banner = context.Banners.ToListAsync()
var newsGroup = context.NewsGroups.ToListAsync()
await Task.WhenAll(banner, newsGroup);
</code></pre>
But when i called the function from controller. It showed error
>A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe.
Please help me solve this issue.

I need to integrate Asp.Net latest MVC version with an *existing* database which has an additional column `String Address` to table `dbo.AspNetUsers`


I need to create an instance `ApplicationUser` which has property Address.


Any idea how to do it?




Using Asp.Net Identity DataBase first approach

I am getting into the Entity Framework, but I am unsure if I am missing a critical point in the code-first approach.

I am using a generic repository pattern based on the code from https://genericunitofworkandrepositories.codeplex.com/ and have created my entities.

But when I try to access or modify the entity I run into the following:

&gt; System.InvalidOperationException: The entity type Estate is not part
&gt; of the model for the current context.

It happens when I am trying to access it from my repository:

    public virtual void Insert(TEntity entity)
    {
        ((IObjectState)entity).ObjectState = ObjectState.Added;
        _dbSet.Attach(entity); // &lt;-- The error occurs here
        _context.SyncObjectState(entity);
    }

The database (./SQLEXPRESS) is created just fine, but the entities (tables) is just not created on startup. 

I am wondering if I need to explicit set the mapping of the entities? Is EF not able to this by its own? 

My Entity is:

    public class Estate : EntityBase
    {
        public int EstateId { get; set; }
        public string Name { get; set; }
    } 


My context is as so:

    public partial class DimensionWebDbContext : DbContextBase // DbContextBase inherits DbContext
    {
        public DimensionWebDbContext() :
            base(&quot;DimensionWebContext&quot;)
        {
            Database.SetInitializer&lt;DimensionWebDbContext&gt;(new CreateDatabaseIfNotExists&lt;DimensionWebDbContext&gt;());
            Configuration.ProxyCreationEnabled = false;
        }
    
        public new IDbSet&lt;T&gt; Set&lt;T&gt;() where T : class
        {
            return base.Set&lt;T&gt;();
        }
        
    }



Is there any specific reason why this error occurs? I have tried enable migrations and enable automatic migrations without any help either.

The entity type &lt;type&gt; is not part of the model for the current context

I wrote such class:

    class Test
    {
        [Key]
        [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
        public int Id { get; set; }
        [Required]
        public List&lt;String&gt; Strings { get; set; }

        public Test()
        {
            Strings = new List&lt;string&gt;
            {
                &quot;test&quot;,
                &quot;test2&quot;,
                &quot;test3&quot;,
                &quot;test4&quot;
            };
        }
    }

and

    internal class DataContext : DbContext
    {
        public DbSet&lt;Test&gt; Tests { get; set; }
    }

After run code:

    var db = new DataContext();
    db.Tests.Add(new Test());
    db.SaveChanges();

my data is getting saved but just the `Id`. I don&#39;t have any tables nor relationships applying to **Strings** list. 

What am I doing wrong? I tried also to make **Strings** `virtual` but it didn&#39;t change anything.

Thank you for your help.


Entity Framework - Code First - Can&#39;t Store List&lt;String&gt;

I had seen some books(e.g *programming entity framework code first Julia Lerman*) define their domain classes (POCO) with no initialization of the navigation properties like:

&lt;!-- language: c# --&gt;

    public class User
    {
        public int Id { get; set; }
        public string UserName { get; set; }
    
        public virtual ICollection&lt;Address&gt; Address { get; set; }
        public virtual License License { get; set; }
    }
some other books or tools (e.g *Entity Framework Power Tools*) when generates POCOs initializes the navigation properties of the the class, like:

&lt;!-- language: c# --&gt;

    public class User
    {
        public User()
        {
            this.Addresses = new IList&lt;Address&gt;();
            this.License = new License();
        }
        public int Id { get; set; }
        public string UserName { get; set; }
    
        public virtual ICollection&lt;Address&gt; Addresses { get; set; }
        public virtual License License { get; set; }
    }

&lt;h3&gt;Q1: Which one is better? why? Pros and Cons?&lt;/h3&gt;

**Edit:**

&lt;!-- language: c# --&gt;

    public class License
    {
        public License()
        {
            this.User = new User();
        }
        public int Id { get; set; }
        public string Key { get; set; }
        public DateTime Expirtion { get; set; }

        public virtual User User { get; set; }
    }

&lt;h3&gt;Q2: In second approach there would be stack overflow if the `License` class has a reference to `User` class too. It means we should have one-way reference.(?) How we should decide which one of the navigation properties should be removed?&lt;/h3&gt;

EF codefirst : Should I initialize navigation properties?

I need to set a value in a table for a subset of rows. In SQL, I would do this:

    UPDATE dbo.Person SET is_default = 0 WHERE person_id = 5

Is there a way to do this in LINQ?

I currently use the:

    var result = (from p in Context.People....)

notation.

Is there an update method I can use? Or do I have to get all the records, then update them one-by-one in a Foreach?

Is this the most efficient way, if this is even possible?

    (from p in Context.person_account_portfolio where p.person_id == personId select p)
       .ToList()
       .ForEach(
    	   x =&gt;
    	   x.is_default =
    	   false);

Update records using LINQ

I seem to be a little bit confused when comparing Clojure&#39;s **core.async** to the so called **Reactive Extensions** (Rx) and **FRP** in general. They seem to tackle similar problem of async-hronicity, so I wonder what are the principal differences and in what cases is one preferred over the other. Can someone please explain?

EDIT: To encourage more in-depth answers I want to make the question more specific:

1. Core.async allows me to write synchronous-looking code. However as I understand it FRP only needs one level of nested callbacks (all the function that handle logic are passed as arguments to FRP API). This seems that both approaches make the callback *pyramids* unnecessary. It is true that in JS I have to write `function() {...}` many times, but the main problem, the *nested* callbacks, is gone in FRP also. Do I get it right?

2. &quot;**FRP** complects the communication of messages with flow of control&quot; Can you (someone) please give a more specific explanation?

3. Can&#39;t I pass around FRP&#39;s observable endpoints the same way as i pass channels?

In general I understand where both approaches historically come from and I have tried few tutorials in both of them. However I seem to be &quot;paralyzed&quot; by the non-obviousness of differences. Is there some **example of a code that would be hard to write in one of these and easy using the other**? And what is the architectural reason of that?

Comparing core.async and Functional Reactive Programming (+Rx)

I am trying to figure out what the `async` &amp; `await` keywords are all about, however the output isn&#39;t what I&#39;m expecting.

The console application is as follows:

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(&quot;Foo called&quot;);
            var result = Foo(5);

            while (result.Status != TaskStatus.RanToCompletion)
            {
                Console.WriteLine(&quot;Thread ID: {0}, Status: {1}&quot;, Thread.CurrentThread.ManagedThreadId, result.Status);
                Task.Delay(100).Wait();
            }

            Console.WriteLine(&quot;Result: {0}&quot;, result.Result);
            Console.WriteLine(&quot;Finished.&quot;);
            Console.ReadKey(true);
        }

        private static async Task&lt;string&gt; Foo(int seconds)
        {
            return await Task.Run(() =&gt;
                {
                    for (int i = 0; i &lt; seconds; i++)
                    {
                        Console.WriteLine(&quot;Thread ID: {0}, second {1}.&quot;, Thread.CurrentThread.ManagedThreadId, i);
                        Task.Delay(TimeSpan.FromSeconds(1)).Wait();
                    }

                    return &quot;Foo Completed.&quot;;
                });
        }
    }

The output is:

    Foo called
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 6, second 0.
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 6, second 1.
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 6, second 2.
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 6, second 3.
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 6, second 4.
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Thread ID: 10, Status: WaitingForActivation
    Result: Foo Completed.
    Finished..

I was expecting to see the status changing from `WaitingForActivation` once the method is started.

How can it stay in this state and be active?


Async always WaitingForActivation

I am testing a method for a service that makes a Web `API` call. Using a normal `HttpClient` works fine for unit tests if I also run the web service (located in another project in the solution) locally. 

However when I check in my changes the build server won&#39;t have access to the web service so the tests will fail.

I&#39;ve devised a way around this for my unit tests by creating an `IHttpClient` interface and implementing a version that I use in my application. For unit tests, I make a mocked version complete with a mocked asynchronous post method. Here&#39;s where I have run into problems. I want to return an OK `HttpStatusResult` for this particular test. For another similar test I will be returning a bad result. 

The test will run but will never complete. It hangs at the await. I am new to asynchronous programming, delegates, and Moq itself and I&#39;ve been searching SO and google for a while learning new things but I still can&#39;t seem to get past this problem.

Here is the method I am trying to test:

    public async Task&lt;bool&gt; QueueNotificationAsync(IHttpClient client, Email email)
    {
        // do stuff
        try
        {
            // The test hangs here, never returning
            HttpResponseMessage response = await client.PostAsync(uri, content);

            // more logic here
        }
        // more stuff
    }

Here&#39;s my unit test method:

    [TestMethod]
    public async Task QueueNotificationAsync_Completes_With_ValidEmail()
    {
        Email email = new Email()
        {
            FromAddress = &quot;bob@example.com&quot;,
            ToAddress = &quot;bill@example.com&quot;,
            CCAddress = &quot;brian@example.com&quot;,
            BCCAddress = &quot;ben@example.com&quot;,
            Subject = &quot;Hello&quot;,
            Body = &quot;Hello World.&quot;
        };
        var mockClient = new Mock&lt;IHttpClient&gt;();
        mockClient.Setup(c =&gt; c.PostAsync(
            It.IsAny&lt;Uri&gt;(),
            It.IsAny&lt;HttpContent&gt;()
            )).Returns(() =&gt; new Task&lt;HttpResponseMessage&gt;(() =&gt; new HttpResponseMessage(System.Net.HttpStatusCode.OK)));

        bool result = await _notificationRequestService.QueueNotificationAsync(mockClient.Object, email);

        Assert.IsTrue(result, &quot;Queue failed.&quot;);
    }

What am I doing wrong?

Thank you for your help.

Using Moq to mock an asynchronous method for a unit test

Does anyone know why `async` methods are not allowed to have `ref` and `out` arguments? I&#39;ve done a bit of research on it but the only thing I could find was that it has to do with the stack unrolling.













ref and out arguments in async method

Has anyone come across a scenario for using `ConfigureAwait(true)`? Since `true` is the default option I cannot see when would you ever use it.

When should I use ConfigureAwait(true)?

I can&#39;t believe I couldn&#39;t find a working solution to this after an hour of searching. I&#39;m following [this article](http://www.dotnetcurry.com/showarticle.aspx?ID=941) on Entity Framework 6.0 which gives a simple walk-through on Code First. I created the project and installed the latest [EF Nuget package](https://www.nuget.org/packages/EntityFramework/6.0.2) for the project to compile. I also verified that I have Microsoft SQL Server 2012 Express LocalDB installed which came with Visual Studio 2013. I don&#39;t have any other instances of SQL installed on my local computer. The program runs and entries are added to the database and outputted in the console. But when the article says &quot;check your localdb&quot; it doesn&#39;t say how! I don&#39;t see any &#39;.mdf&#39; or &#39;.ldf&#39; files created under the project folder. I tried every way to connect Visual Studio&#39;s Server Explorer to LocalDB. The wizard cannot locate `(localdb)` or cannot find any provider in Server Explorer to accept connection string like `(localdb)\v11.0;Integrated Security=true;` I&#39;ve seen this asked several places in StackOverflow but no answer works or marked as answer. Please help, this doesn&#39;t have to be this frustrating!

What are the steps to connect Visual Studio Server Explorer to LocalDB?

How to connect to LocalDB in Visual Studio Server Explorer?

I&#39;m trying to build an EF Entity with Code First, and an `EntityTypeConfiguration` using fluent API. creating primary keys is easy but not so with a Unique Constraint. I was seeing old posts that suggested executing native SQL commands for this, but that seem to defeat the purpose. is this possible with EF6?

Setting unique Constraint with fluent API?

What is the difference between `System.Transactions.TransactionScope` and EF6&#39;s `Database.BeginTransaction`?

Could someone give a small example or just explain which one to use when with a clear difference?

P.S: In my project, I&#39;m using EF6. I&#39;ve already read the documentation but it didn&#39;t help much. Also looked up the examples but they are rather using `SqlConnection.BeginTransaction` and now MS has introduced this new `Database.BeginTransaction` in EF6.

Database.BeginTransaction vs Transactions.TransactionScope

With EF6 you have a new transaction which can be used like:

    using (var context = new PostEntityContainer())
            {
                using (var dbcxtransaction = context.Database.BeginTransaction())
                {
                    try
                    {
                        PostInformation NewPost = new PostInformation()
                        {
                            PostId = 101,
                            Content = &quot;This is my first Post related to Entity Model&quot;,
                            Title = &quot;Transaction in EF 6 beta&quot;
                        };
                        context.Post_Details.Add(NewPost);
                        context.SaveChanges();
                        PostAdditionalInformation PostInformation = new PostAdditionalInformation()
                        {
                            PostId = (101),
                            PostName = &quot;Working With Transaction in Entity Model 6 Beta Version&quot;
                        };

                        context.PostAddtional_Details.Add(PostInformation);
                        context.SaveChanges();

                        dbcxtransaction.Commit();
                    }
                    catch
                    {
                        dbcxtransaction.Rollback();
                    }
                }
            }

Is rollback actually needed when things go sideways? I&#39;m curious because the Commit description says: &quot;Commits the underlying store transaction.&quot;

Whereas the Rollback description says: &quot;Rolls back the underlying store transaction.&quot;

This makes me curious, because it looks to me that if Commit isn&#39;t called, the previously executed commands will not be stored (which seems logical to me). But if that is the case, what would the reason be to call the Rollback function? In EF5 I used TransactionScope which didn&#39;t have a Rollback function (only a Complete) which seemed logical to me. Due to MS DTC reasons I cannot use the TransactionScope anymore, but I also cannot use a try catch like the example above (i.e., I only need the Commit).



Entity Framework 6 transaction rollback

Is there a way to create an index on a property/column using code-first, instead of using the new [`IndexAttribute`](https://msdn.microsoft.com/en-us/library/jj591583(v=vs.113).aspx#Anchor_10) ?

Content Type	Original Author	Original Content on Stackoverflow
Question	An Hv	View Question on Stackoverflow
Solution 1 - Entity Framework	Stephen Cleary	View Answer on Stackoverflow
Solution 2 - Entity Framework	Teoman shipahi	View Answer on Stackoverflow
Solution 3 - Entity Framework	Ogglas	View Answer on Stackoverflow

Multi-async in Entity Framework 6?

Entity Framework Problem Overview

Entity Framework Solutions

Solution 1 - Entity Framework

Solution 2 - Entity Framework

Solution 3 - Entity Framework

JavaScript String concatenation behavior with null or undefined values

VERR_VMX_MSR_VMXON_DISABLED when starting an image from Oracle virtual box

Attributions