I have an interface that I'm using with a couple different concrete classes. What I wish is that there was something like this...
_kernel.GetMock<ISerializeToFile>().Named("MyRegisteredName")
.Setup(x => x.Read<ObservableCollection<PointCtTestDataInput>>(
It.IsAny<string>()));
The project I'm working on uses the service locator pattern - anti-pattern which I'm getting less fond of all the time...
Originally I tried..
[ClassInitialize]
public static void ClassInitialize(TestContext testContext)
{
_kernel = new MoqMockingKernel();
}
[TestInitialize]
public void TestInitialize()
{
_kernel.Reset();
ServiceLocator.SetLocatorProvider(
() => new NinjectServiceLocator(_kernel));
_kernel.Bind<ISerializeToFile>().ToMock()
.InSingletonScope().Named("ObjectToFile");
_kernel.GetMock<ISerializeToFile>()
.Setup(x => x.Read<ObservableCollection<PointCtTestDataInput>>(
It.IsAny<string>()));
_kernel.GetMock<ISerializeToFile>()
.Setup(x => x.Save<ObservableCollection<PointCtTestDataInput>>(
It.IsAny<ObservableCollection<PointCtTestDataInput>>(),
It.IsAny<string>()));
}
I got the standard Ninject error stating that more than one matching binding is available. So, I moved _kernel = new MoqMockingKernel(); into the TestInitialize, and then that error went away... Perhaps I'm incorrectly guess at what _kernel.Reset() does?
Reset removes any instance from the cache. It does not delete existing bindings. So the second test will have the ISerializeToFile twice.
Related
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.
At the current moment all default Startup.cs flow executed on every db related operation like droping db, adding migration, updating db to migrations, etc.
I have heavy app specific code in Startup which need to be invoked only if application run for real. So how could I detect that Startup class run from migration or other database related dotnet command.
Well, as it was already noticed in comment to a question there is a IDesignTimeDbContextFactory interface which need to be implemented to resolve DbContext at design time.
It could look somewhat like this:
public static class Programm{
...
public static IWebHost BuildWebHostDuringGen(string[] args)
{
return WebHost.CreateDefaultBuilder(args)
.UseStartup<StartupGen>() // <--- I'm just using different Startup child there where could be less complex code
.UseDefaultServiceProvider(options => options.ValidateScopes = false).Build();
}
}
public class DbContextFactory : IDesignTimeDbContextFactory<MyDbContext>
{
public MyDbContex CreateDbContext(string[] args)
{
return Program.BuildWebHostDuringGen(args).Services.GetRequiredService<MyDbContext>();
}
}
However, due to some unclear reasons (I asked guys from Microsoft, but they don't explain this to me) dotnet currently on every operation implicitly call Programm.BuildWebHost even if it's private - that's the reason why standard flow executed each time for the question's author. Workaround for that - Rename Programm.BuildWebHost to something else, like InitWebHost
There is an issue created for that, so maybe it will be resolved in 2.1 release on in future.
The documentation is still a bit unclear as to why this occurs. I've yet to find any concrete answer as to why it runs Startup.Configure. In 2.0 it's recommend to move any migration/seeding code to Program.Main. Here's an example by bricelam on Github.
public static IWebHost MigrateDatabase(this IWebHost webHost)
{
using (var scope = webHost.Services.CreateScope())
{
var services = scope.ServiceProvider;
try
{
var db = services.GetRequiredService<ApplicationDbContext>();
db.Database.Migrate();
}
catch (Exception ex)
{
var logger = services.GetRequiredService<ILogger<Program>>();
logger.LogError(ex, "An error occurred while migrating the database.");
}
}
return webHost;
}
public static void Main(string[] args)
{
BuildWebHost(args)
.MigrateDatabase()
.Run();
}
We're using MvvmCross in our app, and using the MvxSimpleIoCContainer
In the app startup, we register all of our Migrations.
it's easy do do since all migrations inherit from IMigration
typeof (IMigration)
.Assembly
.CreatableTypes()
.Inherits<IMigration>()
.AsTypes()
.RegisterAsLazySingleton();
After the migrations are registered, we need to run them consecutively, and therefore the MigrationRunner looks a little something like this.
Mvx.Resolve<IMigrationRunner>().RunAll(SystemRole.Client, new List<IMigration>
{
Mvx.IocConstruct<Migration001>(),
Mvx.IocConstruct<Migration002>()
});
as you can see, I'm explicitely constructing each Migration using Mvx. This get's tedious and is prone to mistakes when a bunch of migrations end up in the app.
What I'd prefer to be able to do is resolve the entire collection in one fell swoop, and not have to touch it every time I create a new Migration.
Is there a way to do this via MvvmCross?
Pseudo Code
Mvx.Resolve<IMigrationRunner>()
.RunAll(SystemRole.Client, Mvx.ResolveAll<IMigration>());
I would use LINQ to get the list of types. Unfortunately there's no way to get a list of registered types, so you'll have to enumerate the types again like you do for registration. You can even sort by type name. Now that you have a list of types, you can create a new list of instantiated/resolved types to pass into RunAll(). Something like:
var migrationTypes = typeof (IMigration)
.Assembly
.CreatableTypes()
.Inherits<IMigration>()
.AsTypes()
.OrderBy(t => t.Name)
.ToList();
Mvx.Resolve<IMigrationRunner>()
.RunAll(SystemRole.Client,
migrationTypes.Select(t => Mvx.Resolve(t)).ToList());
This is "browser" code, so no guarantees, but you get the gist.
Ok, so reflection is the answer to this problem for now, and eventually, I'd like to either extend our custom MvxServiceLocator : IServiceLocator to include something like
public IEnumerable<object> GetAllInstances(Type serviceType){...}
but for now I've just got a RunMigrations() method in the app
private void RunMigrations()
{
var migrationType = typeof (IMigration); // IMigration is in a separate assembly
var migrations = GetType().Assembly
.GetTypes()
.Where(
t => migrationType.IsAssignableFrom(t) && !t.IsAbstract)
.OrderBy(t => t.Name)
.Select(m => _serviceLocator.GetInstance(m) as IMigration)
.ToList();
var migrationRunner = new MigrationRunner(Mvx.Resolve<IDbProvider>());
migrationRunner.RunAll(SystemRole.Client, migrations);
}
where _serviceLocator.GetInstance(m) just lives in our custom MvxServiceLocator
public object GetInstance(Type serviceType)
{
return _ioCProvider.Resolve(serviceType);
}
Edit: here's how I extended our service locator wrapper.
public class MvxServiceLocator : IServiceLocator
{
private readonly IMvxIoCProvider _ioCProvider;
public MvxServiceLocator(IMvxIoCProvider ioCProvider)
{
_ioCProvider = ioCProvider;
}
public IEnumerable<TService> GetAllInstances<TService>()
{
var serviceType = typeof(TService);
var registrations = GetType().Assembly
.GetTypes()
.Where(
t => serviceType.IsAssignableFrom(t) && !t.IsAbstract)
.Select(m => (TService)_ioCProvider.Resolve(m));
return registrations;
}
}
I've written an Xtext-based plugin for some language. I'm now interested in creating a new independent view (as a separate plugin, though it requires my first plugin), which will interact with the currently-active DSL document - and specifically, interact with the model Xtext parsed (I think it's called the Ecore model?). How do I approach this?
I saw I can get an instance of XtextEditor if I do something like this when initializing my view:
getSite().getPage().addPartListener(new MyListener());
And then, in MyListener, override partActivated and partInputChanged to get an IWorkbenchPartReference, which is a reference to the XtextEditor. But what do I do from here? Is this even the right approach to this problem? Should I instead use some notification functionality from the Xtext side?
Found it out! First, you need an actual document:
IXtextDocument doc = editor.getDocument();
Then, if you want to access the model:
doc.modify(new IUnitOfWork.Void<XtextResource>() { // Can also use just IUnitOfWork
#Override public void process(XtextResource state) throws Exception {
...
}
});
And if you want to get live updates whenever it changes:
doc.addModelListener(new IXtextModelListener() {
#Override public void modelChanged(XtextResource resource) {
for (EObject model : resource.getContent()) {
...
}
}
});
Following line of code gives me an error saying "The underlying connection was closed".
return this.repository.GetQuery<Countries>().Include(g => g.Cities).AsEnumerable().ToList();
But if I remove .Include(g => g.cities) it works fine.
this code is written in one of the operation in my WCF service, and I try to test it using WCF test client. I tried by calling this operation from MVC application also, and the same issue was occurring there too.
Also, i am using generic repository with entity framework
Repository code (only few important extract)
Constructor:
public GenericRepository(DbContext objectContext)
{
if (objectContext == null)
throw new ArgumentNullException("objectContext");
this._dbContext = objectContext;
this._dbContext.Configuration.LazyLoadingEnabled = false;
this._dbContext.Configuration.ProxyCreationEnabled = false;
}
GetQuery method:
public IQueryable<TEntity> GetQuery<TEntity>() where TEntity : class
{
var entityName = GetEntityName<TEntity>();
return ((IObjectContextAdapter)DbContext).ObjectContext.CreateQuery<TEntity>(entityName);
}
Attempt#1
Created following overloads in repository code:
public IQueryable<TEntity> GetQuery<TEntity>(params string[] includes) where TEntity : class
{
var entityName = GetEntityName<TEntity>();
IQueryable<TEntity> query = ((IObjectContextAdapter)DbContext).ObjectContext.CreateQuery<TEntity>(entityName);
foreach(string include in includes)
{
query = query.Include(include);
}
return query;
}
public IQueryable<TEntity> GetQuery<TEntity>(Expression<Func<TEntity, bool>> predicate, params string[] includes) where TEntity : class
{
return GetQuery<TEntity>(includes).Where(predicate);
}
WCF is now trying to execute following line of code:
return this.repository.GetQuery<Countries>("Cities").AsEnumerable().ToList()
But it still gives the same error of "The underlying connection was closed". I tested it in WCF test client. However, when I debug the repository code it shows the navigation object getting included in result, but the issue seems occurring while trying to pass the output to client (WCF test client, or any other client)
After looking at the code you've now posted, I can conclude that, indeed, your DbContext is being closed at the end of the GetQuery method, and is thus failing when you try to use include. What you might want to do to solve it is to have an optional params variable for the GetQuery method that will take in some properties to be included, and just do the include right in the GetQuery method itself.