I've been assigned to upscale a project built by a former coworker. I'm not a .NET Core specialist, I understand most of it as is similar to any other language, but I'm having trouble understanding the Fluent configuration made at startup.
At the Startup.cs, there is this function declared:
public void Configure(IApplicationBuilder app)
At some point, there is an initialzation of a service that listens for something. I can manage that from the already initialized class/service, but I'd like to understand what is this:
app.UseRawRequestRequestBodyHandler(options => options
.Handlers
.AddRange(new[] {
new RawRequestHandler
{
ContentType = NotificationSubscriber.ContentType,
StartSegments = NotificationSubscriber.StartSegments,
Response = "[OK]",
Endpoint = new Uri(_configManager.Client.BaseAddress, "v1/payments").ToString(),
ModifyRequestBodyAsyncFunc = async (handler, context, bodyContent) =>
{
using (var scope = app.ApplicationServices.CreateScope())
{
var subscriber = scope.ServiceProvider
.GetRequiredService<INotificationSubscriber>();
await subscriber.QueueAndAkcknowledgeAsync(handler, context, bodyContent);
}
return bodyContent;
}
},
I'm having special trobule with the ModifyRequestBodyAsyncFunc function, that is declared (in the interface) like this:
public Func<RawRequestHandler, HttpContext, string, Task<string>> ModifyRequestBodyAsyncFunc { set; get; }
Also, I don't get how or where are initialized handler, context and bodyContent (RawRequestHandler handler, HttpContext context, string bodyContent as declared in the NotificationSubscriber class). I pressume these are loaded by Dependency Injection, but It would be different for other DI implementations I've seen.
Any help would be appreciated; also, I take reading recommendations.
Thank you very much!
I'm having special trobule with the ModifyRequestBodyAsyncFunc function
This is a special C# type, called a delegate. The delegate in question is a function that accepts RawRequestHeader, HttpContext, string and returns a Task<string>, which tells us that it's asynchronous.
Next, this is a syntax to create an anonymous async function and assign it to the delegate property:
/* SomeProp */ = async (handler, context, bodyContent) =>
{
// ...
return bodyContent;
}
Also, I don't get how or where are initialized handler, context and bodyContent
The .UseRawRequestRequestBodyHandler(...) registers a middleware which is basically a piece of code which runs for every request. So, somewhere inside that middleware, there is code that has access to said parameters and probably passes them like that:
// the params are not necessarily named exactly like this, only the types must match
string content = await rawRequestHeader.ModifyRequestBodyAsyncFunc(handler, context, bodyContent);
Notice the await keyword (we must await asynchronous functions) and also the fact that the delegate is invoked just like a normal method.
My goal
I want to create a new IdentityUser and show all the users already created through the same Blazor page. This page has:
a form through you will create an IdentityUser
a third-party's grid component (DevExpress Blazor DxDataGrid) that shows all users using UserManager.Users property. This component accepts an IQueryable as a data source.
Problem
When I create a new user through the form (1) I will get the following concurrency error:
InvalidOperationException: A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread-safe.
I think the problem is related to the fact that CreateAsync(IdentityUser user) and UserManager.Users are referring the same DbContext
The problem isn't related to the third-party's component because I reproduce the same problem replacing it with a simple list.
Step to reproduce the problem
create a new Blazor server-side project with authentication
change Index.razor with the following code:
#page "/"
<h1>Hello, world!</h1>
number of users: #Users.Count()
<button #onclick="#(async () => await Add())">click me</button>
<ul>
#foreach(var user in Users)
{
<li>#user.UserName</li>
}
</ul>
#code {
[Inject] UserManager<IdentityUser> UserManager { get; set; }
IQueryable<IdentityUser> Users;
protected override void OnInitialized()
{
Users = UserManager.Users;
}
public async Task Add()
{
await UserManager.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" });
}
}
What I noticed
If I change Entity Framework provider from SqlServer to Sqlite then the error will never show.
System info
ASP.NET Core 3.1.0 Blazor Server-side
Entity Framework Core 3.1.0 based on SqlServer provider
What I have already seen
Blazor A second operation started on this context before a previous operation completed: the solution proposed doesn't work for me because even if I change my DbContext scope from Scoped to Transient I still using the same instance of UserManager and its contains the same instance of DbContext
other guys on StackOverflow suggests creating a new instance of DbContext per request. I don't like this solution because it is against Dependency Injection principles. Anyway, I can't apply this solution because DbContext is wrapped inside UserManager
Create a generator of DbContext: this solution is pretty like the previous one.
Using Entity Framework Core with Blazor
Why I want to use IQueryable
I want to pass an IQueryable as a data source for my third-party's component because its can apply pagination and filtering directly to the Query. Furthermore IQueryable is sensitive to CUD
operations.
UPDATE (08/19/2020)
Here you can find the documentation about how to use Blazor and EFCore together
UPDATE (07/22/2020)
EFCore team introduces DbContextFactory inside Entity Framework Core .NET 5 Preview 7
[...] This decoupling is very useful for Blazor applications, where using IDbContextFactory is recommended, but may also be useful in other scenarios.
If you are interested you can read more at Announcing Entity Framework Core EF Core 5.0 Preview 7
UPDATE (07/06/2020)
Microsoft released a new interesting video about Blazor (both models) and Entity Framework Core. Please take a look at 19:20, they are talking about how to manage concurrency problem with EFCore
General solution
I asked Daniel Roth BlazorDeskShow - 2:24:20 about this problem and it seems to be a Blazor Server-Side problem by design.
DbContext default lifetime is set to Scoped. So if you have at least two components in the same page which are trying to execute an async query then we will encounter the exception:
InvalidOperationException: A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread-safe.
There are two workaround about this problem:
(A) set DbContext's lifetime to Transient
services.AddDbContext<ApplicationDbContext>(opt =>
opt.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")), ServiceLifetime.Transient);
(B) as Carl Franklin suggested (after my question): create a singleton service with a static method which returns a new instance of DbContext.
anyway, each solution works because they create a new instance of DbContext.
About my problem
My problem wasn't strictly related to DbContext but with UserManager<TUser> which has a Scoped lifetime. Set DbContext's lifetime to Transient didn't solve my problem because ASP.NET Core creates a new instance of UserManager<TUser> when I open the session for the first time and it lives until I don't close it. This UserManager<TUser> is inside two components on the same page. Then we have the same problem described before:
two components that own the same UserManager<TUser> instance which contains a transient DbContext.
Currently, I solved this problem with another workaround:
I don't use UserManager<TUser> directly instead, I create a new instance of it through IServiceProvider and then it works. I am still looking for a method to change the UserManager's lifetime instead of using IServiceProvider.
tips: pay attention to services' lifetime
This is what I learned. I don't know if it is all correct or not.
I downloaded your sample and was able to reproduce your problem. The problem is caused because Blazor will re-render the component as soon as you await in code called from EventCallback (i.e. your Add method).
public async Task Add()
{
await UserManager.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" });
}
If you add a System.Diagnostics.WriteLine to the start of Add and to the end of Add, and then also add one at the top of your Razor page and one at the bottom, you will see the following output when you click your button.
//First render
Start: BuildRenderTree
End: BuildRenderTree
//Button clicked
Start: Add
(This is where the `await` occurs`)
Start: BuildRenderTree
Exception thrown
You can prevent this mid-method rerender like so....
protected override bool ShouldRender() => MayRender;
public async Task Add()
{
MayRender = false;
try
{
await UserManager.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" });
}
finally
{
MayRender = true;
}
}
This will prevent re-rendering whilst your method is running. Note that if you define Users as IdentityUser[] Users you will not see this problem because the array is not set until after the await has completed and is not lazy evaluated, so you don't get this reentrancy problem.
I believe you want to use IQueryable<T> because you need to pass it to 3rd party components. The problem is, different components can be rendered on different threads, so if you pass IQueryable<T> to other components then
They might render on different threads and cause the same problem.
They most likely will have an await in the code that consumes the IQueryable<T> and you'll have the same problem again.
Ideally, what you need is for the 3rd party component to have an event that asks you for data, giving you some kind of query definition (page number etc). I know Telerik Grid does this, as do others.
That way you can do the following
Acquire a lock
Run the query with the filter applied
Release the lock
Pass the results to the component
You cannot use lock() in async code, so you'd need to use something like SpinLock to lock a resource.
private SpinLock Lock = new SpinLock();
private async Task<WhatTelerikNeeds> ReadData(SomeFilterFromTelerik filter)
{
bool gotLock = false;
while (!gotLock) Lock.Enter(ref gotLock);
try
{
IUserIdentity result = await ApplyFilter(MyDbContext.Users, filter).ToArrayAsync().ConfigureAwait(false);
return new WhatTelerikNeeds(result);
}
finally
{
Lock.Exit();
}
}
Perhaps not the best approach but rewriting async method as non-async fixes the problem:
public void Add()
{
Task.Run(async () =>
await UserManager.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" }))
.Wait();
}
It ensures that UI is updated only after the new user is created.
The whole code for Index.razor
#page "/"
#inherits OwningComponentBase<UserManager<IdentityUser>>
<h1>Hello, world!</h1>
number of users: #Users.Count()
<button #onclick="#Add">click me. I work if you use Sqlite</button>
<ul>
#foreach(var user in Users.ToList())
{
<li>#user.UserName</li>
}
</ul>
#code {
IQueryable<IdentityUser> Users;
protected override void OnInitialized()
{
Users = Service.Users;
}
public void Add()
{
Task.Run(async () => await Service.CreateAsync(new IdentityUser { UserName = $"test_{Guid.NewGuid().ToString()}" })).Wait();
}
}
I found your question looking for answers about the same error message you had.
My concurrency issue appears to have been due to a change that triggered a re-rendering of the visual tree to occur at the same time as (or due to the fact that) I was trying to call DbContext.SaveChangesAsync().
I solved this by overriding my component's ShouldRender() method like this:
protected override bool ShouldRender()
{
if (_updatingDb)
{
return false;
}
else
{
return base.ShouldRender();
}
}
I then wrapped my SaveChangesAsync() call in code that set a private bool field _updatingDb appropriately:
try
{
_updatingDb = true;
await DbContext.SaveChangesAsync();
}
finally
{
_updatingDb = false;
StateHasChanged();
}
The call to StateHasChanged() may or may not be necessary, but I've included it just in case.
This fixed my issue, which was related to selectively rendering a bound input tag or just text depending on if the data field was being edited. Other readers may find that their concurrency issue is also related to something triggering a re-render. If so, this technique may be helpful.
Well, I have a quite similar scenario with this, and I 'solve' mine is to move everything from OnInitializedAsync() to
protected override async Task OnAfterRenderAsync(bool firstRender)
{
if(firstRender)
{
//Your code in OnInitializedAsync()
StateHasChanged();
}
{
It seems solved, but I had no idea to find out the proves. I guess just skip from the initialization to let the component success build, then we can go further.
/******************************Update********************************/
I'm still facing the problem, seems I'm giving a wrong solution to go. When I checked with this Blazor A second operation started on this context before a previous operation completed I got my problem clear. Cause I'm actually dealing with a lot of components initialization with dbContext operations. According to #dani_herrera mention that if you have more than 1 component execute Init at a time, probably the problem appears.
As I took his advise to change my dbContext Service to Transient, and I get away from the problem.
#Leonardo Lurci Had covered conceptually. If you guys are not yet wanting to move to .NET 5.0 preview, i would recommend looking at Nuget package 'EFCore.DbContextFactory', documentation is pretty neat. Essential it emulates AddDbContextFactory. Ofcourse, it creates a context per component.
So far, this is working fine for me so far without any problems...
I ensure single-threaded access by only interacting with my DbContext via a new DbContext.InvokeAsync method, which uses a SemaphoreSlim to ensure only a single operation is performed at a time.
I chose SemaphoreSlim because you can await it.
Instead of this
return Db.Users.FirstOrDefaultAsync(x => x.EmailAddress == emailAddress);
do this
return Db.InvokeAsync(() => ...the query above...);
// Add the following methods to your DbContext
private SemaphoreSlim Semaphore { get; } = new SemaphoreSlim(1);
public TResult Invoke<TResult>(Func<TResult> action)
{
Semaphore.Wait();
try
{
return action();
}
finally
{
Semaphore.Release();
}
}
public async Task<TResult> InvokeAsync<TResult>(Func<Task<TResult>> action)
{
await Semaphore.WaitAsync();
try
{
return await action();
}
finally
{
Semaphore.Release();
}
}
public Task InvokeAsync(Func<Task> action) =>
InvokeAsync<object>(async () =>
{
await action();
return null;
});
public void InvokeAsync(Action action) =>
InvokeAsync(() =>
{
action();
return Task.CompletedTask;
});
#Leonardo Lurci has a great answer with multiple solutions to the problem. I will give my opinion about every solution and which I think it is the best one.
Making DBContext transient - it is a solution but it is not optimized for this cases..
Carl Franklin suggestion - the singleton service will not be able to control the lifetime of the context and will depend on the service requester to dispose the context after use.
Microsoft documentation they talk about injecting DBContext Factory into a component with the IDisposable interface to Dispose the context when the component is destroied. This is not a very good solution, because a lot of problems happen with it, like: performing a context operation and leaving the component before it finishes that operation, will dispose the context and throw exception..
Finally. The best solution so far is to inject the DBContext Factory in the component yes, but whenever you need it, you create a new instance with using statement like bellow:
public async Task GetSomething()
{
using var context = DBFactory.CreateDBContext();
return await context.Something.ToListAsync();
}
Since DbFactory is optimazed when creating new context instances, there is no significante overhead, making it a better choice and better performing than Transient context, it also disposes the context at the end of the method because of "using" statement.
Hope it was useful.
I need to secure my json responses from hijacking in aspnet core. I used to have a working solution in beta 8, but I cannot get that to work properly in 1.1.
What I want to do is to check whether a Json response is an enumerable or not. If it is, I wrap the enumerable in an object.
This is what I had in an override of JsonOutputFormatter:
public override Task WriteResponseBodyAsync(OutputFormatterContext context)
{
if (context.Object is IEnumerable)
{
context.Object = new { result = context.Object };
}
return base.WriteResponseBodyAsync(context);
}
This no longer works due to changes in the framework.
Does anyone have a good suggestion on how to wrap enumerable responses into an object to avoid hijacking?
Given an async action method on an ASP MVC Controller and an async method that gets some data:
public async Task<ActionResult> Index()
{
var apiClient = new apiClient();
var data = await apiClient.GetDataAsync(id);
I want GetDataAsync to store the result as a Cache item, so it doesn't have to request from an external api next time.
I can sometimes get this to work by passing in a reference to the cache object, (often inexplicably null even though passed from what I thought was the starting thread. Must not be the starting thread I guess?).
var data = await apiClient.GetDataAsync(id, System.Web.HttpContext.Current.Cache);
But I seem to remember that this is not a good idea (something to do with thread safety).
What is the recommended way to handle caching in an async method? Should I move cache handling into the controllers? Seems a shame as it made sense for the cache management to happen closer to the data source.
I'm implementing a call to NServiceBus.Send() inside a MVC 4 Web API Service call. I'm doing this in .Net 4, so I don't have access to C# 5 features (async, etc.).
I'm very new to Moq, so I'd appreciate some advice creating the correct way to Setup Moq for this. I did look at [Mocking a delegate wrapper using Moq][1] and [Testing Bus Send in an application using NServiceBus][2] but my understanding of Moq is still to primitive to adapt for my testing needs, especially with the Tasks involved to deal with the asynchronous calls:
public Task<HttpResponseMessage> Post(Command command)
{
var commandCompletion = new TaskCompletionSource<HttpResponseMessage>();
var commandCompletionTask = commandCompletion.Task;
Bus.Send(command).Register<CommandResult>(status =>
{
var response = new HttpResponseMessage { StatusCode = HttpStatusCode(status) };
commandCompletion.SetResult(response);
});
return commandCompletionTask;
}
I run into trouble because I don't know how to reference the commandCompletionTask within the mockedBus... My failed attempt looks like this:
_commandController = new CommandController(MockBus.Object);
MockBus.Setup(b => b.Send(It.IsAny<ICommand>()).Register<CommandResult>((Action<CommandResult> s) =>
{
var response = new HttpResponseMessage { StatusCode = HttpStatusCode(s) };
commandCompletion.SetResult(response);
}));
Writing this allowed my brain to come up with a couple of ideas...
To Moq TaskCompletionSource, but that doesn't have an interface, so can't be Moq'd apparently.
To use a real TaskCompletionTask, (which allows me to test for the Task returned from CommandController.Post()). Sadly this didn't work any better, as I get an error from LINQ as it tries to process the delegate.
Ideas/suggestions?